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December 31, 2007

HMT Forecast Made 1930Z M 31 Dec 07

Models remain consistent in indicating a major pcpn event for the American River Basin Th through Sunday 3-6 Jan. Details, however, are, in my opinion still up for grabs, due partly to a complicated upper level situation near and northwest of HI that the models may or may not be capturing appropriately.

Here are the main aspects of the event as affecting the ARB, as I see it from here.
1. Deep and slowly progressive upper trough in the westerlies will be over the eastern Pacific by W - Th 2 and 3 Jan and shift inland by M 7 Jan.

2. The pcpn event still seems likely to occur in 3 parts. Models have been quite consistent about this aspect for the past few days. First part will be Th as a deepening low moves enewd toward the Northwest out of the eastern Pacific. The associated cold front will approach the northern CA coast by late Th but may not pass the ARB as an identifiable feature, as another surface low pressure advances eastward hard on the heels of the first. This one may be more intense than the first one, though I'm inclined to think that the GFSs with a 960 mb low center is too deep. This second part will bring pcpn on Friday, with probably no break between parts 1 and 2. It seems likely that F 4 Jan will be the wettest day. The final part will be on Sunday, as a short-wave trough traverses the Central Pacific ridge and into the larger-scale cyclonic circulation over the eastern Pacific. The GFS has this feature coming in fartehr south, over the Bay Area. As previous forecasts, this looks like a colder system for the ARB, with less orographic forcing.

3. It now appears likely that significant pcpn over the ARB will not commence in earnest till after daybreak on Th 3 Jan.

4. I'm still concerned about the potential for strong wind gusts due to combined effects of strong synoptic-scale pressure gradient and barrier jet. Friday 4 Jan looks like the highest risk for this, but Th could also see strong winds.

5. Both the ECMWF and GFS are forecasting 5" plus melted pcpn vicinity Blue Canyon for the Th - Sun period. I think that's still a good bet. Models now both have heaviest pcpn over the Sierra near Yosemite.

6. I'm sticking with ARB snow levels between 4,000 and 5,500 ft for Th and F, 3 and 4 Jan, with snow levels down to 3,000ft on Sun.

Looking farther ahead, M and Tu look to be drier days without significant (> 1" melted) pcpn at Blue Canyon. The ECMWF is indicating another significant storm on W 9 Jan, but the GFS takes this farther north, bringing the worthwhile pcpn into the Northwest.

John B.

December 30, 2007

HMT forecast made 1930Z Sun 30 Dec 07

Still looks like the new year will start with great IOP potential! Conditions remain on track for a major event (could well be the best of this La Nina year) coming up Th 3 thru Sun 6 Jan 08! Models continue to be consistent in calling for this event coming in 4 parts, the first one to affect mainly the Northwest and coast ranges of northern CA on W, and the remaining 3 to affect all of CA. Total pcpn amounts for the Th thru Sunday period are virtually guaranteed to exceed 5" melted at Blue Canyon in my opinion, and could well approach twice that. This is due to a combination of adequate moisture, good "balanced dynamics" (e.g., quasi-geostrophic forcing of upward motion) and very strong orographic forcing due to the overall strength of the onshore flow. There could well be periods of good warm advection, not only in the earlier stages of the Th 3 Jan event, but also in the 2 to follow. Consensus of ensembles is that there will be no American River Basin pcpn before sundown Wednesday 2 Jan; the first batch of heavier pcpn will occur mainly during the day Th (although could well commence before daybreak Th 3 Jan), followed closely by another on Friday, and then perhaps a slight break before the last batch late Saturday and ending daytime Sunday. As indicated yesterday, snow levels are likely to vary between 4,000 and 5,500ft, with Blue Canyon getting mostly snow Th 3 Jan and F 4 Jan, with snow levels coming down on Saturday to between 3,000 and 4,000ft, so Blue Canyon will get all snow in the last event.

Be advised that strong winds (combination strong synoptic pressure gradient and barrier jet) will also accompany this event, particularly Th and F 3 and 4 Jan, with gusts of up to 50kt possible KSCK and KSAC, so tippable equipment should be well secured.

Beware of dense fog potential for New Years Eve.

Beyond Sunday, the liklihood of further multi-inch (melted) events at Blue Canyon goes down substantially.
Ensembles begin to diverge markedly on Sunday, some (as the operational GFS initialized 12Z this morning) showing a trough digging into AZ and others keeping conditions more zonal. So, although the potential for events after Sunday 6 Jan isn't zero, it certainly reduces to be closer to climatology.

John B.

December 29, 2007

HMT Forecast made 1930Z Sat 29 Dec 07

A weak system is passing over the area at present, with smartly rising barometers on bouys off the Northwest coast behind a wind shift from south to west near shore. Light pcpn is likely to continue, over the higher terrain particularly, through noon tomorrow (Sunday 30th), as strong onshore W to WNW flow at mountain-top level continues. Cold air aloft over the Northwest will favor convection that area beginning later today, but I think convection farther south will be shallower and not an important pcpn producer (< 0.4" melted at Blue Canyon) over the ARB. By afternoon Sunday conditions will begin to improve.

Early next week should see calm, dry conditions over the American River Basin, with weak offshore flow as high pressure builds inland over the Northern Rockies and Great Basin and the Pacific High weakens. This will promote a mid-level (~700mb) flow of dry, relatively cool air from the east and, later southeast, over the area, and will be favorable for fog formation in the Valley under a weak subsidence inversion.

Situation still on track for a good event beginning Wednesday night or early Th 2 - 3 Jan, continuing through Saturday night the 5th and perhaps longer. As moist ascending air coming in from the SW aloft during the initial phases of the event experiences cloud and pcpn formation, there will be a period of evaporating pcpn (i.e., virga) aloft prior to pcpn reaching the surface. It remains to be seen how much of the high moisture near HI (up to 2.3" IPW) will work into this system, but the GFS is keeping 500-1000 mb thickness values mostly below 550dm in the ARB area, leading me to think that high snow levels (say, above 6,500ft) will not occur. Given that there seem to be only 5-6" snow-water equivalent over the higher elevations at present, it seems unlikely that this event will produce a major snow-melt enhancement of runoff. The HPC notes that GFS moisture-flux-magnitude anomalies are +4-6 standard deviations above the mean. GFS is indicating up to 8" water equivalent for the 5 days ending M 7 Jan, and with much of this coming as snow at KBLU, appropriate preparations are called for. GFS operational calls for 2-3 pulses of pcpn durg this period and ensemble members are in agreement with the overall timing of the event, in particular the onset as being after sundown on W 2 Jan. At higher elevations, it is likely that pcpn will be continuous for 48-72h beginning probably early Th 3 Jan, but there will be periods when the pcpn will become quite light. Snow level is anticipated to time-vary between 4,000 and 5,500 feet, probably near 4,000ft at start of the event, and dropping below 4,000 ft by Saturday. Details on the GFS operational from 12z this mrng suggest that there may be one or 2 good warm advection events during this period, including the initial event beginning W night or Th, 2nd or 3rd Jan.

Following this, it aprs likely, based on the GFS ensembles, there will be a quieter period beginning Sunday -Monday the 6th, and 7th. However, this is far enough ahead that confidence is only medium.

December 28, 2007

HMT Forecast made 1930Z F 28 Dec 07

Ridge off W Coast surface and aloft is bringing NW flow aloft over the American River Basin (ARB) with weak s/w trofs embedded. These have little vigor at the sfc and as John M. indicated yesterday, are expected to produce total pcpn in the 1/4 to 1/2 inch range (water equivalent) at best. The last of this series is forecast to cross the coast midday Sunday with confluent flow aloft behind it.

Overall, trend is for a fairly zonal pattern across the Pacific next few days with progressive features. Upper ridge is xpctd to amplify off W Coast Sunday and move over the coast Monday, and then gradually shift inland ahead of trough now in extreme western Pacific. This is a major change in the longwave pattern, at least in its implications for the West, and shows good promise of bringing storms into the W Coast. The first of these will affect mainly the Pacific NW on Wednesday, but then by Thursday the GFS (Global Forecast System) indicates a good possibility of a rain/snow event into the Nrn Sierra. I expect there will be considerable cool, dry air near the surface getting into the circulation of this first system to affect the area, making for tricky prediction of snow levels and also for good low-level flow blocking by the Sierra and a barrier jet. This mrng's GFS is indicating 2 other potential events immediately behind this one, so midweek travel in preparation for a few days stay in CA should be considered. These later events, if they materialize, are likely to be warmer than the initial event with higher snow levels. Uncertain at this time is whether these will tap into the extensive subtropical moisture vicinity of HI, but this seems likely.

It should be noted that during the telcon there was some opinion expressed that these events will not be extremely warm (though warmer than the current weak events, in which a little light snow has fallen at Auburn), and that snow levels are likely to get to 6,500 to 7,000ft briefly, if at all, even after the first event.

Regarding the first event (anticipated Thursday next week) precipitation may be relatively heavier in the coast ranges than the Sierra, as it likely will be weakening as it approaches the coast and encounters the residual shallow cold, relatively dry air and high static stability at low levels noted above.

HMT folks in CA should be aware of Valley travel hazards due to fog likely developing over the weekend and continuing into early next week.

John B.

HMT Forecast made 1930Z F 28 Dec 07

Ridge off W Coast surface and aloft is bringing NW flow aloft over the
American River Basin (ARB) with weak s/w trofs embedded.
These have little vigor at the sfc and as John M. indicated yda,
are xpectd to produce total pcpn in the 1/4 to 1/2 inch range
at best. The last of this series is forecast to cross the coast midday Sunday with confluent flow aloft behind it.

Overall, trend is for a fairly zonal pattern across the Pacific next few days with progressive features
Upper ridge is xpctd to amplify off W Coast Sunday and move over the coast Monday,
and then gradually shift inland ahead of trough now in extreme western Pacific.
This is a major change in the longwave pattern, at least in its implications for the West, and shows
good promise of bringing storms into the W Coast.
The first of these will affect mainly the Pacific NW on Wednesday,
but then by Thursday the GFS indicates a good possibility
of a rain/snow event into the Nrn Sierra.
I expect there will be considerable cool,
dry air near the surface getting into the circulation of this
first system to affect the area, making for tricky prediction
of snow levels and also for good low-level flow blocking by
the Sierra and a barrier jet.
This mrng's GFS is indicating 2 other potential events
immediately behind this one, so midweek travel in preparation
for a few days stay in CA should be considered.
These later events, if they materialize,
are likely to be warmer than the initial event with higher
snow levels. Uncertain at this time is whether these will tap into the extensive subtropical moisture vicinity of HI, but this seems likely.

HMT folks in CA should be aware of Valley travel hazards
due to fog likely developing over the weekend and continuing into
early next week.

John B.

December 27, 2007

HMT Forecast for 27 Dec 2008

Synopsis: ESRL and NWS forecasters pretty much in agreement. Broad ridge in the east Pacific centered about 155W is creating WNW flow on the West Coast. In this flow are two imbedded short wave troughs one due to influence the HMT area on 28 Dec 06GMT and the second close on its heels moving in on the 29th/00GMT. With the ridge shifting to the east both waves have warm advection signatures in the time-height cross-sections over the HMT area. The moisture associated with these two systems have weak coupling to the subtropics over the Philippines, but IPW values in the eastern extremity are quite low ranging from 16-24mm. The moist plume in the eastern Pacific is also quite far north (above 40N) so the moist advection is coming in from the WNW.

Precipitation outlook: With the low water content, weakness of the dynamics, and speed of these waves, precipitation will be light, 24 hour totals amounting to only about 0.3-0.5 for the first one and 0.4-0.6 for the second.

Operational recommendation: Neither event qualifies for IOP criteria for amount of precipitation, even though there is a weak warm advection signature and connection to a weak plume.

Long Range Outlook: Convection over the equatorial west Pacific is shifting eastward and this may help to move the ridge over the continental western US creating troughy conditions in the east Pacific by the middle part of next week. A trough of good strength is evident in the GFS at about 145 W on Wednesday (2 Jan) and this is predicted to move steadily eastward. The southerly flow ahead of the trough taps into subtropical moisture southeast of Hawaii so a significant moist plume or river should be associated with the warm advection sector of the trough. Based on the GFS this first wave should move in on Thursday (3 Jan) and have significant precipitation (2-3 inches)with high IPW values (30-40mm). After this first system the door will be open for a couple of additional events Friday into Saturday (4-5 Jan), and the following week.


December 26, 2007

HMT forecast discussion for 26 Dec 2007

Happy Holidays to all....plenty white out here and more on the way. Not the 2006 version, but nice. As for HMT, a fast-moving wave on Friday followed by another Saturday, then ridging for awhile until potentially things get interesting late next week, perhaps by Thu/3 Jan, with more of a sw flow system. This could then usher in an interesting period of several waves through the following ~7 days.

Current set up has a mean trough over the eastern Rockies with an upper level ridge off the West Coast. Shortwave troughs digging down the east side of the ridge have been amplifying over CO/NM and this will be the case with the next couple of waves. But they do affect the ARB area as they quickly move by. The next more moist one Friday with another close behind, coming out of the northwest. These do not look as strong as the previous IOPs, and so are not expected to produce nearly the amount of precip. Still, over 48-72h period ending Saturday night (29 Dec) suppose one could certainly see more than an inch of precip at Blue Canyon. Some models (the Canadian Global, for example) are more exuberant with this event, but most roughly similar to the GFS/NAM. Don't think these are worthy of an IOP, especially since 3 better events are in the books. Also, more interesting potential system exists for later next week.

Decent agreement in the GFS ensembles from 00z on the event beginning on Thu of next week (3 Jan). This is then followed by other waves in the GFS as the overall mean trough retrogrades and a more zonal-looking flow sets up into the West Coast. Canadian Global ensembles have more spread, with tendency towards a more prolonged event in several members, perhaps beginning a day or so later than in the GFS. Latest 12z ECMWF supports the overall GFS 12z deterministic solution with a high-amplitude trough approaching the ARB on Thu/3 Jan then weakening as it passes by Fri/4 Jan, but followed by a strong zonal flow into the area. One wonders if beginning later next week we set up a prolonged period of several events, similar to what we had just before Christmas, but potentially (at least with the first event) with more sw flow ahead of it. Such a prolonged period of wet weather is seen in the NAEFS (North American Forecast System, 00z runs) meteogram for Sacramento, which has precip in the mean every day beginning 3-4 Jan and continuing through 10 Jan.

ed szoke NOAA/CIRA/GSD

December 20, 2007

HMT Discussion for Thursday 20 December 2007

HMT 12/20/2007 Forecast Discussion

Current Conditions
PW plume passage occurred this morning at 06 UTC at Bodega Bay and at 08 UTC at Truckee. As of 14 UTC, the ARB is on the northern edge of the PW Plume. Light radar echoes cover the region with a band of moderate echo in the valley. The moderate echoes are pushing southward signaling an slow end to the precipitation in the ARB. Expect another 0.25" by 18 UTC and complete shut down of precipitation around 18 UTC.

The short-wave trough did not fracture as it approached the California Coast, and it is positioned just offshore and just south of the ARB. This puts the ARB on the cold side of the thickness gradient, just along the northern edge of the warm advection region.

Long-range guidance continues to emphasize the Pacific NW as a landing point for strong waves. Occasionally a few ensemble members push a front into the ARB. One such situation occurs on Christmas Day. With the forecast of a high-amplitude ridge in place, the PW plume does originate near Hawaii in this forecast rather than the central Pacific; however, precipitation amounts are modest as the front becomes disconnected from the high-speed upper-air flow. This pattern is forecast to continue through about Dec 28.

Followup to IOP3: Forecast and observed precipitation
Precipitation forecasts and amounts for the 18-hour period 00 UTC 12/20 to 18 UTC 12/21.
36-hour forecast 1.6-2.1" 1.80" 1.95" 56%>1", 41%>2" 1.8"
12-hour forecast 1.5-2.0" 1.75" 1.55-1.80" 84%>1", 56%>2" 1.65-2.25"
OBS, Sloughhouse: 0.4-0.45"
Colfax: 1.65-1.7"
Alta: 0.8-0.85"
Blue Canyon: Missing obs
Big Bend: 2.35-2.50"
Norden: 2.35-2.50"
Truckee: 0.25-0.30"

December 19, 2007

HMT Discussion for Wednesday 19 December 2007

HMT 12/19/2007 Forecast Discussion

Executive Summary
Model guidance begins precipitation between 00 and 06 UTC 12/20 and ends it between 18 UTC 12/20 and 00 UTC 12/21. The PW plume passes the ARB shortly after 18 UTC 12/20. Forecast amounts for the 24-hour period 00 UTC 12/20 to 00 UTC 12/21 range 1.5"-2.0" with 2.5+" possible at Blue Canyon, though much of the precipitation falls within the 12-hour period between 06 and 18 UTC 12/20. Freezing levels should be higher than in the previous storm during the early stages; the NWS is thinking freezing levels will drop around 12 UTC 12/21 to perhaps below 4000 ft.

Current Conditions
Water vapor imagery shows a short-wave trough coming on shore at WA. A slug of moisture is evident in the warm sector coming on shore in OR. SSM/I estimates of PW are just under 2 cm with the initial moisture push but exceed 3 cm in a large pool extending from 130W to 160W.

A second short-wave trough is positioned on the latitude parallel to OR at 145W.

0-72 hour & 3-5 day forecast
Model guidance is nearly unanimous in the speed of propogation of the short-wave trough positioned at 145W and PW values associated with it. The warm advection sector is over N CA by 06 UTC 12/20. The trough makes landfall in central CA around 12 UTC 12/20 and moves into Nevada by 00 UTC 12/21. Onset of precipitation occurs between 00 and 06 UTC 12/20, though ~0.01" has accumulated by 00 UTC. The NAM produces the highest QPF, as it did with the previous system. Twenty-four hour totals in the 00 UTC 12/20 to 00 UTC 12/21 period are 2.5-3.0" (NAM), 1.4-1.9" (GSD), 1.75-2.25" (GFS), 2.25"-2.50" (CNRFC), and probability of >1" and >2" of 84% and 56% in the PSD Analogue ensemble. The period of heaviest precipitation is 06-18 UTC 12/20 when the PW plume is approaching and impenging on the ARB with 700 mb flow as high as 50 kts.

My biggest concern is that the trough is fairly broad in the water vapor imagery, and it might be a little too potent in the analysis for the model initialization. Everyone has seen troughs fracture as they approach the Coast. Most likely this is just a problem with sparse measurements in the central Pacific. If this trough does fracture, it seems more likely that the warm advection period could be longer than forecasted. I don't see anyway that significant precipitation rates occur prior to 00 UTC 12/20, even if the trough is not accurately depicted in the model initial state.

5-10 day forecast
Very little hope in the model guidance for precipitation south of extreme N CA.

10 day and beyond forecast
The flow becomes more zonal by the New Year.

Followup to IOP2: Forecast and observed precipitation
Precipitation forecasts and amounts for the 24-hour period 00 UTC 12/18 to 00 UTC 12/19.
36-hour forecast 1.5-2.0" 2.0-2.5" 1.0-1.5" 80%>1", 58%>2"
12-hour forecast 1.35-2.25" 2.4-3.25" 1.4-1.75" 92%>1", 84%>2" 2-2.25"
Sloughhouse: 0.6"
Alta: 1.8-1.9"
Colfax: 2.0"
Blue Canyon: 2.75-2.9"

Chris Anderson

December 18, 2007

HMT Discussion for Tuesday 18 December 2007

HMT 12/18/2007 Forecast Discussion

Executive Summary
Forecasted precipitation accumulation in the 18 UTC 12/18 to 00 UTC 12/19 period is about 0.25" less than yesterday. Expect about 1.0" to accumuluate during this period. Light precipitation should persist through 06 UTC 12/19.

Consistent with yesterday's forecast, model guidance pegs the next heavy precipitation event to begin around 00 UTC 12/20 and to shut off abruptly after 00 UTC 12/21. Forecast amounts for the 24-hour period range 1.5"-2.0", with 2.0+" in isolated areas along ridge lines. The NAM continues to be wetter than all other models with amounts in the 2"-3" range.

Current Conditions
Precipitation began about 08 UTC.
Blue Canyon 00-12 UTC accumulation is ~0.5" and 12-16 UTC accumulation is ~0.25".
Sloughhouse 00-12 UTC accumulation is ~0.4" and 12-16 UTC accumulation is ~0.2"; freezing level is 5500 ft 12-14 UTC.
Bodega Bay freezing level is 7000 ft; PW is 1" or a tick higher.

The short-wave trough took a slightly more southern route than expected, and the base of the trough at 12 UTC was located parallel to SF Bay. At 12 UTC, it had a bit of a fractured appearance in the Canadian analysis, with stronger winds moving inland in N CA and S OR.

Water vapor imagery shows a moisture plume moved through the area between 08 and 17 UTC; another skinnier plume is moving into the area. Moderate radar echo is evident just north of ARB. SSM/I PW estimates are 2+ cm offshore.

A short-wave is positioned near 150W parallel Washington. A slug of moist air with SSM/I PW 3+ cm is associated with this trough.

0-72 hour & 3-5 day forecast
Model guidance continues to pinpoint the 18 UTC 12/18 to 00 UTC 12/19 period as the heaviest precipitation period. NAM has the highest totals during that period with values approaching 2". All other guidance puts the 6-hr total in the .5"-1.0" range, which is about 0.25" below yesterday's projection. Also, NAM has the precipitation lingering to 12 UTC 12/19, whereas other guidance suggests a light precipitation through 06 UTC 12/19 and nearly complete shutoff after that. The region has seen ~0.75", and expect another ~0.75" this afternoon.

Period of heaviest precipitation: 12 UTC 12/18 - 00 UTC 12/19
Most likely precipitation amount: 1.25"-1.75" (slightly down from yesterday's 2.0"-2.5" forecast)

Another potent wave is forecasted to hit the CA coast around 00 UTC 12/20. Model guidance has not reached a consensus on the timing of this wave. The CMC ensemble brings it onshore prior to 00 UTC; whereas, the GFS ensemble has precipitation onset after 00 UTC. Models are in agreement about an abrupt shutoff shortly after 00 UTC 12/21. Precipitation totals during this 24-hour period will likely to range 1.5-2.0" with the possibility of 2.0+" along the ridge lines.

5-10 day forecast
Most of ensemble members are focusing the short-wave track on the Pacific NW. Some ensemble members continue to have a trough pass over ARB on Monday with a PW plume extending across most of the central Pacific containing PW values >3 cm.

10 day and beyond forecast
Ridging expected by middle of next week in the PSD reforecast ensemble.

Chris Anderson

December 17, 2007

HMT Discussion for Monday 17 December 2007

HMT 12/17/2007 Forecast Discussion

Executive Summary
Model guidance is nearly in complete agreement that a heavy precipitation period will occur between 12 UTC 12/18 to 00 UTC 12/19, though the NAM is a bit slower at ending the event. The heaviest precipitation period is 18 UTC 12/18 to 00 UTC 12/19 with all guidance producing at least 1" of precipitation. The total precipitation between 12 UTC 12/18 and 06 UTC 12/19 ranges 1.5" to 3.5", with most guidance in the 2.0" to 2.5" range.

NWS forecast offices pointed out that precipitation rate would increase prior to 12 UTC 12/18. I concur. QPF from CNRFC and GSD Ensemble in the 6-hour periods ending 12 UTC, 18 UTC, and 00 UTC are 0.25", 0.75", and 1.15" and 0.25", 0.5" and 1.25", respectively.

Model guidance is coming to a consensus on a second heavy precipitation period beginning 00 UTC 12/20 and ending 00 UTC 12/21. The ingredients are in place for larger precipitation amounts than in the current IOP.

Current Conditions
Over the past 24 hours ending 12 UTC, Huysink reports 0.36", Sac reports 0.16".
At 16 UTC, 30F at BLU with light snow and 46F at SAC with fog.

Bodega Bay PW is 0.6"; freezing level is 5000 ft at 6 UTC
Sloughhouse freezing level is 5000 ft at 6-12 UTC
Chico freezing level is 4500 ft at 2-5 UTC

A short-wave trough is evident in NRL water vapor imagery along the latitude of N CA at 130W. The wave has slightly higher amplitude in the GFS compared to Canadian analyses. SSM/I imagery suggests the PW is topping out at 2 cm. The GFS initialization is a bit higher, pushing 23 mm.

Another more potent short-wave trough is apparent along the latitude of OR at 145W. This wave is also apparent in both the Canadian and GFS analysis as a sharp trough. PW values associated with this wave are quite a bit higher into the 3-3.5 cm range.

0-72 hour & 3-5 day forecast
Model guidance is nearly unanimous on the timing of the next short-wave and taking the short-wave right over the ARB. PW values are a bit higher than in previous forecasts, pushing to just over 1", as well. Thus, precipitation amounts from nearly all model forecasts are higher than in previous cycles.

The NAM continues to have the highest precipitation totals, exceeding 3" for the 12-hour period. Also, it also allows the precipitation to linger. My first thought is to discount the NAM precipitation. However, the PSD seminar that described the mesoscale effects of the mountain chain make me hesitant to throw out the high-end NAM results. And, it appears from the PSD Analogue ensemble that >3" has occurred in the past as the probability of >2" has 84% probability.
As a result, I'm bumping up the precipitation forecast from yesterday.

Period of heaviest precipitation: 12 UTC 12/18 - 00 UTC 12/19
Most likely precipitation amount: 2.0"-2.5" (can't rule out 3+")

A second short-wave is forecasted to occur later in the week. The model guidance on this wave is coming into focus. All GFS ensemble members, 9 out of 12 CMC ensemble members and the NAM have pegged the onset of precipitation to be around 00 UTC 12/20 and have heavy precipitation through 00 UTC 12/21. This system is forecasted to have higher PW values, and, so, has a chance to produce more precipitation than the current system. It also has a very distinct frontal passage marking the end of the precipitation period.

5-10 day forecast
GFS continues to have one more event in the long-range, though the current cycle pushes it back another day to Monday and takes the short-wave trough into the Pacific NW and northward. The trailing front that pushes through the ARB has a far less interesting precipitation scenario than in previous cycles.

10 day and beyond forecast
Ridging expected by middle of next week in the PSD reforecast ensemble.

December 16, 2007

HMT Discussion for Sunday 16 December 2007

HMT 12/16/2007 Forecast Discussion
The general forecast is similar to yesterday's discussion, but the details of the storm structures are quite different. Forecasted peak precipitation period remains the 00 UTC 12/18 through 00 UTC 12/19. A wetter system may follow sometime in the 12 UTC 12/19 to 00 UTC 12/21 period.

Best guess precipitation onset: no earlier than 02 UTC 12/17
Conservative guess precipitation onset: no earlier than 00 UTC 12/17
First heavy precipitation period: 12 UTC 12/18 (Tuesday) through 00 UTC 12/19, expect 1.25-1.75" with > 2.5" possible
Second heavy precipitation period: sometime after 12 UTC 12/19 and before 00 UTC 12/21

Current Conditions
The most critical aspects of this forecast is identifying where and how much water vapor is available and whether the short-wave trough is a coherent mass or is becomeing shredded.

The 14 UTC water vapor image showed three bands of high levels of water vapor. The eastern most plume appears to be left over from the trough passage yesterday, while the other two plumes are associated with a potent short-wave trough located near 135W at the latitude of N CA. This position is a bit south of the position valid at this time in yesterday's forecast. SSM/I at 13 UTC suggests PW values do not exceed 2 cm.

The short-wave trough over the Pacific has more structure than in yesterday's model runs. One vort max is headed into Canada with a 60-70 kt wind max at 500 mb. The vort max on a latitude parallel to N CA does not appear to have any wind observations around it. The Canadian and GFS analyses depict it as a coherent mass, but I'm skeptical with the fractured appearance in the IR baroclinic leaf and the multiple water vapor plumes.

0-72 hour & 3-5 day forecast
There is some discrepancy in the onset of precipitation between the GFS and NAM, as the GFS keeps the wave more coherent but the NAM strings it out. The NAM has a slightly later onset due to smaller values of PW and a less potent short-wave trough. All models have accumulation by 06 UTC; however, the GFS ensemble and CMC ensemble amounts suggest an onset not long after 00 UTC. The GSD ensemble uses the NAM as boundary conditions, so that the timing and intensity of synoptic waves is tied closely to the NAM evolution. Despite this, precipitation onset is sooner and accumulation greater by 06 UTC in the GSD ensemble. This may reflect a bias in the NAM onset introduced by the cloud parameterization.

Best guess of precipitation onset is around 03 UTC 12/17; A conservative guess would be 00 UTC 12/17.

Once precipitation has started, it likely won't have a complete break until Friday. This is illustrated nicely by the PSD Analogue forecast PQPF in which the probability of measureable 24-hour precipitation accumulation exceeds 70% through 00 UTC 12/22 (Saturday). The reason for this is that the wind field never turns to a northerly component, even though moisture plumes (not exactly atmospheric rivers) move through the region.

The models are in agreement that a period of heavy precipitation is expected between 12 UTC 12/18 (Tuesday) and 00 UTC 12/19 (Wednesday). Precipitation amounts range 2-4" in the NAM forecasts and a couple of GFS members but only 1-2" in the most GFS ensemble members and the GSD ensemble. NWS forecast offices call for snow levels to be fairly low, 4000-4500 ft and perhaps as low as 3500 ft, at the beginning of the precipitation period and to rise above 4500 ft during the period of heaviest precipitation.

A second period of heavy precipitation is expected sometime in the period of 12 UTC 12/19 (Wednesday) through 12 UTC 12/21 (Friday). The timing of this system is quite variable in the ensembles, though it appears that it will have a more moist moisture plume emanating from the central Pacific and that it will be a warmer event.

5-10 day forecast
Long range guidance continues to suggest a more classic atmospheric river event could happen late in the weekend or early next week, during the Christmas downtime.

10 day and beyond forecast
PSD reforecast ensemble continues to indicate ridging will develop along the West Coast beginning the middle of next week.

Chris Anderson

December 15, 2007

HMT Discussion for Saturday 15 December 2007

HMT 12/15/2007 Forecast Discussion
Current Conditions
At 16 UTC, clear skies over the area with some fog in the valley. The important feature are upper-level waves west of Oregon. A sharp-amplitude, large and fast moving trough was positioned at 12 UTC at approximately 150W, due west of Oregon. A smaller short-wave trough was positioned along the Oregon Coast as of 12 UTC.

The 11 UTC SSM/I estimates of IWV is less than 2 cm with the first short-wave. A slightly more moist pool is evident with the larger trough.

0-72 hour forecast
The weak short-wave trough hitting the Oregon Coast at this time is forecast to move quickly inland with a trailing front that briefly pushes into N CA before retreating as the Big Dog approaches.

The second trough is also forecasted to take make landfall on the OR coast and move eastward. However, the size of this system will create upslope flow into the ARB. Since the track of the trough is so far north, upper-level forcing of ascent will remain north and precipitation onset is unlikely to begin before upslope flow is established. 12 UTC model forecasts are becoming more similar with the propogation speed, but the NAM has lower PW values and the GFS has higher LI values; hence the GFS has an earlier onset to precipitation.
About half of 09 UTC SREF members has precipitation by 06 UTC 12/17 (Monday), similar to GFS. NAM has 12 UTC precipitation onset. Even in the 09 UTC SREF the NAM members have a later onset than the RSM members: after 12UTC compared to before 06 UTC. Monitoring of the accuaracy of the PW initialization is critical to determining the timing.

The high-resolution ensemble from GSD, which is initialized with NAM has a slightly earlier onset than the NAM with precipitation beginning between 06 UTC and 12 UTC 12/17 (Monday). So, only the NAM of all 12 UTC and 09 UTC forecasts has precipitation onset after 12 UTC.

In all cases, the early stages of the storm are forecasted to be the least intense periods of precipitation. Precipitation accumulation by 00 UTC 18 Dec (Tuesday) is forecasted to be at most 1", but much more frequently in the 0.3-0.5" range. Most of the model guidance has the heaviest precipitation occurring between 09 UTC 12/18 and 00 UTC 12/19 with as much as 3.5" but forecasts in the range of 1.5-2.5" are more common.

The PSG Analgue forecasts have 12% of measureable precipitation in the 24-hr period ending 00 UTC 12/17 (Monday), 78% ending 00 UTC 12/18, 88% ending 00 UTC 12/19, 76% ending 00 UTC 12/20, and 78% ending 00 UTC 12/21. The periods of highest likelihood of heavy precipitation are the 24-hour periods ending 00UTC 12/28 and 00 UTC 12/19 with approximately 58% chance of >1.0" and approximately 30% chance of >2.0" in both periods.

Taking into account all of this guidance, storm totals by 00 UTC 12/19 appear almost certain to exceed 1" and very likely to exceed 2". NWS forecasters are expecting snow levels to be lower with the first system than the second system. In the first system, the snow levels could begin as low as 3500 ft.

3-5 day forecast
Another very large trough is forecasted to move into the NW after 12 UTC 12/19 (Wednesday). There is less agreement about the timing, orientation of the height field, and PW values. The timing of this system is difficult to pin down. It is possible the main system could be preceeded by small short-waves that were shredded from the main short-wave, resulting in essentially no break in precipitation, especially at high elevation. Also, this system could have slightly lower snow levels if a bit of cold air advection sneaks in behind the first system.

5-10 day forecast
Another potent wave is forecasted to hit the ARB area Friday night into Saturday. The GFS forecast of this event has more of an appearance of a "Pineapple Express" configuration.

10 day and beyond forecast
PSD Reforecasts indicate a ridge building along the West Coast, but the flow remains very zonal and strong throughout the central Pacific. This appears to be linked to intensification of a tropical 150 mb streamfunction ridge in the Western Pacific above a region of heavy, widespread precipitation.

Chris Anderson

December 14, 2007

HMT forecast Discussion for 12/14/2007

HMT 12/14/2007 Forecast Discussion
Current Conditions
At 16 UTC, clear skies and chilly. Northwest flow is present through the entire depth of the atmosphere as a large ridge is positioned off the Pacific NW coast. The ARB is embedded within a temperature gradient associated with the NW geostrophic flow.

0-72 hour forecast
A sharp amplitude short-wave trough is forecast to move onshore over the Pacific NW overnight tonight. Model guidance remains consistent among the many current forecasts and past forecasts with the wave staying north of the ARB, the surface front becoming diffuse as it approaches the ARB, and PW values less than 1" offshore of CA. Add it all up, and precipitation is expected to be light and confined to N CA.

3-5 day forecast
This is when the fun begins. 00 UTC prognosis from GFS ensemble, CMC ensemble, PSD retrospective ensemble, and 09 UTC NCEP SREF show a potent short wave crashing into the Pacific NW overnight Sunday into Monday. Despite a forecasted short-wave track similar to the current scenario, much more precipitation is forecasted for the ARB region. The differences between the forecast and current scenario are higher PW values (in excess of 1" in the forecast) and the possibility of multiple waves passing over for while the high PW values are in place off the CA shore.

Timing of the onset of precipitation is still murky. Most (about 2/3) forecasts suggest an event that begins after 12 UTC on Monday. Precipitation appears to be nearly continuous in most forecasts through 00 UTC Thursday, with the heaviest amount forecast for the 00 UTC Tuesday to 00 UTC Wednesday period. QPF in a few GFS members exceeds 4" in this 24-hour period. Similar amounts are evident in some members of the Canadian ensemble. In all, approximately 1/3 of the global ensemble forecasts (CMC + GFS) have 24-hour amounts exceeding 4" for the 00 UTC Tuesday to 00 UTC Wednesday period. The PSD Analogue ensemble, based upon the GFS ensemble mean precipitation, forecasts 15-20% chance of exceeding 2" in that period and 80% chance of exceeding 1".

In my experience over the past two HMT field projects, large waves like these often come through with a number of smaller waves, as if the large wave were torn into smaller pieces. It's probably premature to say the period of heaviest precipitation is highly likely to be 00 UTC Tuesday to 00 UTC Wednesday given the tendencies for waves to split apart. With the pool of high PW air offshore, a burst of heavy precipitation could occur in any 12-hour period after 12 UTC on Monday and before 00 UTC on Thurday.

Reno and Sacramento feel the snow levels will be 5-6K ft at the beginning and drop to 4500 ft during the event, though Reno thinks it is possible it could drop a little lower due to strong, synoptic scale ascent within the system. Their general impression of onset and heaviest period of pr
ecip is in line with the model discussion above, ie heaviest period Monday afternoon through Tues
day afernoon.

5-10 day forecast
Beyond Thursday, global model guidance indicates the potential for more quick moving waves coming through the ARB area through the weekend. The ensemble trend after the weekend is to build ridging in the Gulf of Alaska. In fact, the PSD reforecast ensemble mean generates intense convection in the Tropical western Pacific that creates zonal flow with high cyclone activity over the western Pacific that cause ridging as they approach the Gulf of Alaska.

10 day and beyond forecast
Something is bubbling in the Indian Ocean. Perhaps we'll hear about another MJO very soon, though the PSD reforecast ensemble mean indicates a ridging pattern and has active convection in the western rather thean central Pacific which is more in line with what is expected of La Nina conditions.

Chris Anderson, NOAA/GSD

December 13, 2007

HMT discussion for Thursday 13 December 2007

Active pattern in store with a series of fast-moving waves across the Pacific, beneath a quasi-stationary upper low over Alaska, that will be affecting the ARB beginning late Sunday and continuing probably right up to the Holiday break. First glance at the 00z models raised the question of which of these systems, if any, would be worthy of an IOP. Further review and the trend in the latest models, however, and consensus from the conference call, indicates a likely event with the main questions the timing of the start and the duration. As might be expected with such a pattern, there are model differences in timing and strength of the systems as they hit the West Coast. There are two main waves of note in this fast flow. The first of these arrives on late Sun into Mon/16-17 Dec and the second right behind it Tue-Wed/18-19 Dec. The first wave at this point looks to be fast-moving with limited amplitude, but potentially decent precip, while the second is stronger. An important question is whether there will be much of a true separation between these two in terms of precip ending/starting. Trend in the models is that there would be a break between the systems late Monday perhaps into Tuesday, but probably no real end to the precip. Another question is timing for the start of significant precipitation with the first wave. Latest indications favor an early beginning to the first wave, with the potential for precip amounts Sunday night ending 12z/Mon approaching an inch in the ARB in the latest ECMWF run.

Currently wave #1 is a potent system near the Dateline at ~50N, and it rapidly cruises across the Pacific and moves into the ARB on Mon/17 Dec. Wave #2 is just entering the western Pacific. The latest GFS forecast starts precip with the first system Mon ~15z, with close to an inch at BLU by 12z/Tue/18 Dec. There is a very limited tap to some moisture that is pooled way in the sw Pacific at this time but the moisture is limited, however orographic flow should be pretty favorable. The 00z runs were variable in their moisture amount with this first wave, with the Canadian Global model appearing to be the wettest. The 00z ECMWF run is faster with the first wave but also rather moist, producing up to 0.65" in the ARB by 12z/Mon, with not a lot more by 12z/Tue. The 12z Canadian Global model has quite a bit of precip by 12z Mon with this first wave. In the 12z GFS ensembles there is good consensus of a pretty decent amount of precip with this first wave. Timing has it beginning before 12z/Mon/17 Dec, with no real break before the next system sets in. Most of the members have ~0.5" of precip by 12z/Tue, with max amounts up to about an inch. The latest 12z ECMWF just in supports the earlier onset of precip shown in the Canadian and some of the ensemble members of the GFS. Precip in the ECMWF begins about 00z/Mon, with a max of 0.82" in the ARB by 12z/Mon, then precip diminishing by ~18z/Mon. Although a fast-moving wave, there is rather strong sw flow with it of 30-40 kts at 700 mb. Snow levels look to be 4500 feet or so with this wave.

The latest 12z GFS is stronger with the second wave, and stronger than the 00z GFS, trending towards the 00z ECMWF and a good number of the 00z GFS ensemble members, although then actually dropping farther south than all other models. There is no real break in the precip in the 12z GFS, with precip picking up Tue afternoon and about another inch at BLU by 12z/Wed. More to the south as the GFS takes a more southerly track with this stronger 2nd wave, as noted more south than any of the other models. A look at the U of Hawaii site indicates a better tap into the sw Pacific moisture with this system. In the 00z GFS precip also starts up again on Tue, with another ~2" by 12z/Wed. This is more of an open wave than the latest GFS. The latest NOGAPS has similar timing to what is noted above, but the system is farther north than the GFS. The 12z Canadian Global model is also an open wave, and considerably farther north than the GFS but with overall similar timing. The latest UKMET run is also an open wave farther to the north, and a little faster than most of the other models. The 12z GFS ensembles just in support more of an open wave that does not dive as far south as the operational GFS run. By 12z/Wed as the overall event winds down the accumulated precip for both waves ranges from over 2" to closer to a half an inch in the ensembles, with most members exceeding an inch. The 12z ECMWF comes in also as an open wave but very moist, and with sw to wsw flow at 700 mb reaching 40 kts by 12z/Tue produces a good amount of precip. Snow levels probably again in the 4500 foot range, perhaps rising a tad as the wave approaches then falling by early Wed. The ECMWF 12z run is faster than the GFS is picking up the precip in earnest with the second wave, with a half-inch new in the ARB in the 12 h ending 12z/Tue. By 00z/Wed the second wave has produced up to ~1.8" max in the ARB in the latest ECMWF, with a trough passage near 06z/Wed. Almost 3" in the EC for the combined waves by the time we reach 12z/Wed.

A third but weaker wave in all the models arrives Thu-Fri/20-21 Dec. In the 12z GFS this wave does not amplify and only drops another third of an inch or so in the ARB. In the 00z ECMWF this third wave has similar timing to the latest GFS but more moisture, with about 0.75" at BLU. The latest ECMWF run is more moist as well, with ~1.4" max in the ARB between 15z/Thu and 12z/Fri, with the trough axis in central Nevada by 12z/Fri. A few of the ensemble members in the 12z GFS ensemble are a little deeper and slower with this third wave, with a trough passage near 12z/Fri/21 Dec, but in all the members it is a weaker system than the second wave. We'll have time to watch this but currently there would be a break in the precip later Wed into Thu. Spread in the longer term but overall consensus favors ridging on the West Coast after the third wave passes into Christmas week.

ed szoke NOAA/CIRA/GSD

December 12, 2007

HMT forecast Discussion 12/12/2007

HMT 12/12/2007 Forecast Discussion
Current Conditions
At 17 UTC, Low clouds are hanging around the ARB and eastward, but are not production precipitation. They are an indicator of a short-wave trough that at 12 UTC was positioned off the CA coast directly west of ARB. Associated with the trough are high mid-level relative humidity values. The cyclonic vorticity advection and most of the forcing for ascent have passed by the ARB by now. The main impact of this trough will be to continue cold air advection into the ARB region.Chances of precipitation events of interest to HMT over the next 48-hours are nil.

0-72 hour forecast
ARB will be in a pattern of anticyclonic advection throughout this period. Additionally, cold air advection will occur during most of the period, with a bit of warming at the very end of the period ahead of the next short-wave trough. Chance of HMT precipitation events is zero.

3-5 day forecast
A potent short-wave trough is forecasted to hit the Pacific NW shoreline in the 3-4 day period. GFS, CMC, and UK model guidance, including ensembles, take the short-wave track Idaho, draping behind a diffuse frontal zone that makes it to the ARB in some projections but not others. Adding more gloom to the forecast are PW values not exceeding 1" along the so called front. The chance that this period will have an precipitation event of interest to HMT is no more than 5%.

5-10 day forecast
Good news for HMT during this period perhaps. A series of potent short-wave troughs are evident in GFS and CMC ensembles. The first of these waves may affect the ARB by late Monday. The good news is that a pool of PW vales greater than 1" develops offshore and remains entact as multiple waves come through the area. It appears there could be two strong waves separated by 2 days, though my experience looking at GFS/CMC over the past couple of years would suggest the waves will be more fragmented. The chance for an HMT precipitation event is very high. I would almost call it a certainty, so give it a 95% chance.

10 day and beyond forecast
Something is bubbling in the Indian Ocean. Perhaps we'll hear about another MJO very soon, though the PSD reforecast ensemble mean indicates a ridging pattern and has active convection in the western rather thean central Pacific which is more in line with what is expected of La Nina conditions.

December 10, 2007

HMT Forecast Discussion for Monday, 10 December 2007

Guidance continues to point to the next 5 days being dry over HMT domain, but big changes are in store starting the weekend as a more favorable large-scale flow is firmly established by no later than 2007121600, and it should last until the Holiday shutdown around 2007122200 (1600 PST Friday DEC 21)

Latest 2007121012 NAM-GFS-ECMWF runs keep the ARB dry through 2007121512. Interestingly, the GFS digs the 500 mb trof, which has consistently looked innocuous in its fcst arrival over ARB this Wednesday, well offshore and closes it off; systems that close-off has been the scenario the past ~10 days. Again, no precip over HMT domain with the mid-week system.

The GFS has precip reaching BBY-CZC btw 2007121600-2007121606 (1600-2200 PST Saturday), and ARB 6 hours after that. It is considerably faster than yesterday's run which was 24 h slower, and it was a tad faster than the prior day's run. I smell trend. Precipitation lasts 18 h over the coast, and longer over ARB with max GFS accumulations around 1.0 in. there. There is little break before the next impulse, as the GFS brings a second juicier system onshore 24 h later 2007121706 (2200 PST Sunday) which deposits a comparable amount over ARB. Finally, ARB finds itself at the southern end of a third event toward the end of the run (2007121912-2007122012).

The ECMWF has precip reaching BBY-CZC btw 2007121500-2007121512, or faster than 2007120912 cycle, so the run-to-run trend continues. Amounts look light, with 0.10-0.20 in. near BBY-CZC and around 0.50 in. over ARB. The ECMWF's next system arrives onshore 2007171200 (0400 PST Monday) and is wetter, with 3/4 in. on coast and 0.5 in. over ARB. It appears ARB never really clears out before a third system comes over basin.

The latest 2007121012 GFS ens. guidance (http://www.meteo.psu.edu/~gadomski/ewallmref.html) has the majority of members with 500 mb S/W troughs hitting the HMT domain starting around 20071211600 and continuing up to the holiday break. The S/W troughs are embedded in WNW flow whose westerly component should be faster than normal according to the ERSL/PSD EPO refcst teleconnection (http://www.cdc.noaa.gov/map/wx/images/epo.gif). The NAESF plumes for the latest 2007121012 run continue to show the period 2007121712-2007122012 as the most likely window for rain over KSMF, but 1 or 2 outliers appear to start the precip as early as 2007121512, consistent with the fast 2007121012 ECMWF solution. The 2007121012 CMC calibrated PQPF's suggest the 24 h period 2007121800-2007121900 will be the wettest.

The overall message from the latest GFS, ECMWF, NAEFS guidance, and ESRL/PSD refcts ensembles and ESRL/PSD teleconnections seems to be a wet pattern beginning no later than Saturday PST. The pattern will probably stay wet thereafter, with rapidly moving systems hitting the HMT domain every 24-36 h and little clearing likely btw systems over the ARB. Their timing is impossible to pinpoint 5+ days out, however. There is one BIG caveat, however: HMT is at the southern end of the significant precip associated with some of the systems, so a subtle poleward shift in the jet axis shunts the rain to the north, so the situation will need to be updated.

Steve Mullen

December 9, 2007

HMT Forecast Discussion for Sunday 9 December 2007

All models point to the next 6 days being dry over HMT domain, but a more favorable large-sacle should set up starting approximately 2007121600, and it should last until the Holidays shutdown around 2007122200 (1600 PST Friday DEC 21)

The most current 2007120912 NAM, 2007120912 GFS and ECMWF runs remain adamant about keeping the ARB dry through 2007121612; the CMC calibrated probabilities and ESRL refcst analogs support the dry interpretation. Although the HMT domain will remains under dry N-NW flow aloft during the next 6 days, the flow becomes more W-SW in week two as a ridge develops near 170W, and a downstream trof around 140W depending upon model and ens member. The 170W ridge, early in week 2, has been a consistent signal in all models runs the past 3-4 days; it is the evolution of the downstream trof and associated sfc cyclones that varies considerably.

The 1200 GFS has precip reaching BBY-CZC btw 2007121700-2007121712, and ARB 12 hours after that; its primary sfc cyclone tracks NE into Van. Island. MAX 12 h precip over ARB is btw 1.0-1.5 in. starting 2007121712. The 2007120912 ECMWF is faster (a significant change from the 2007120900 run that was later) and starts precip over BBY-CZC 12 h earlier. The EC run deepens a secondary 995 mb low at 39N 128W at 200712172 that tracks ESE, and 12 h precip max approaches 2 in. over ARB.

The NAESF plumes continue to show the period 2007121712-2007122012 as wetter than normal over KSMF. The calibrated CMC ens precipitation fcsts suggest the 5-7 day period starting 2007121600 as the most likely to be the wettest over HMT domain, with the 24 period 2007121712-2007121812 looking the most promising; thus the CMC runs support the GFS and ECMWF deterministic solutions early next week.

In summary, the 3-4 days starting 2007121612 appear very promising. A candidate IOP could begin as early as 2007121618-2007121700 (Sun 1000-1600 PST). The situation will become clearer in the next 2-3 days as the models and ensemble should begin to converge somewhat in their synoptic details.

Steve Mullen

December 8, 2007

HMT Forecast Discussion for Saturday, 8 December 2007

The HMT/ARB region is currently between two 500 mb S/W trofs, underneath N steering flow aloft. IOP-1 was associated with first trof, located over KLAS at 15Z. The upwind trof is over the CA-OR border at 15Z, and it is fcst to be over ARB domain later this afternoon. The system will be associated with scattered instability showers, mostly over the foothills and higher terrain of ARB. The 2007120812 NAM shows only isolated showers, while the GFS is once again more aggressive in coverage and amounts. I think the NAM offers the best solution for ARB, as there is little available moisture. MAX QPF for the ARB, if any precip occurs, should be less than 0.05-0.10 in. range at the wettest locations.

Both the NAM and GFS are adamant about keeping ARB dry from 2007120912 to 200712120 (end of NAM run). No arguments here, as the HMT domain remains under N-NNW flow aloft and low-level offshore geostrophic flow, effectively shutting off any Pacific moisture into ARB. Confidence is very high for dry early Sunday (tomorrow) through early Wednesday LST.

On the longer range, the 2007120812 GFS shows the 500 mb steering flow becoming NW starting Wed (2007121212) as a modest S/W trof comes onshore over WA-OR. A more vigorous systems crashes into the PNW on 2007121412, as the offshore UA ridge continues to flatten. By the end of the run (2007121712), the GFS shows a vigorous 500 mb closed circulation off the CA coast with a good moisture tap from low latitudes. The 200712012 ECMWF run (The preliminary discussion erroneously referred to the 00Z EC run as the 12 Z run.) has the same broad message as the GFS in terms of dry through the end of next week, but it erodes the offshore ridge 1-2 days faster. The EC has precip starting over HMT-BBY/CZC by 2007121700 (1600 PST Sunday) and ARB by 2007121800; most importantly, it shows signs of a moisture tap down to 30N, generally consistent with the GFS idea.

The 2007120812 GFS ensemble (http://www.meteo.psu.edu/~gadomski/ewallmref.html) for 500 mb height suggests dry through 2007121312 (0400 PST Thursday). The 2007120812 CMC ensembles
(http://www.meteo.psu.edu/~gadomski/ewallmref.html) now have no members that suggest wet, but so it too screams dry through 2007121312 (0400 PST Thursday). The NAEFS plumes from 2007120812 IC's (http://www.meteo.gc.ca/ensemble/EPSgrams_e.html) suggest the period 20071217-20071221 has the highest probability of being wet for KSMF. Calibrated probabilities from the CMC system (http://www.meteo.gc.ca/ensemble/charts_e.html?Hour=192&Day=0&RunTime=12&Type=prob)
begin to raise the likelihood of 12 h precip to near 20 percent by 2007121612. The ESRL ens refcst-analogs (http://www.cdc.noaa.gov/reforecast/narr/) from 2007120800 IC's indicate an approx 40 percent chance of >50.0mm over ARB during the 5-day period 2007121300-2007121800, and approx 60 percent chance of >50.0mm over ARB during the week two period 2007121500-200712200; the upswing in probability seems consistent with the latest GFS-EC runs and conservative CMC calibration.

The overall picture that I piece together from the GFS-EC runs, MREF-CMC-NAEFS members and ESRL refcst-analogs is that next 6-7 days will be dry, period, with a minimal (2-5 percent) chance of precip btw 200712091200-200712130000, but the week of 15-22 DEC looks far more promising for a candidate IOP system.

Steve Mullen

December 7, 2007

HMT Forecast Discussion for Friday, 7 December 2007

The HMT domain is beginning to experience strong CAA and drying as the UA trof for IOP-1 moves out of the HMT domain and the UA flow is transitions to N-NNW. Continuous precip for KBLU has stopped by 18Z; 24h total at KBLU for 18-18Z period was 2.96 in. Yesterday's forecast was too dry (up to 2.0 in.).

The 12Z DEC 7 NAM and GFS runs show a 500mb trof that is over BC this morning will move S, embedded in the northerly flow. It will be over HMT by late Saturday (00Z DEC 10). This system could be prodigious precip producer over the mtns of SoCal and the Baja the following day, but it looks harmless over HMT/ARB. The NAM shows widely scatter instability showers over the ARB, and while the GFS is a tad more aggressive in coverage and amounts (~0.05-0.10 in over ARB), it is not encouraging. Moist, low-level flow from the Pacific will have problems making it into the ARB domain. I am downplaying the system.

For the remainder of week one, the 12Z DEC 7 GFS shows zero precip for HMT thru Tues. DEC 11 as CA is dominated by N-NW flow aloft. By Wed. DEC 12, the system discussed yesterday shows minimal promise. Its track and strength bears a "some" resemblance to IOP-1, but the question is moisture; there is little evidence of significant moisture transport into CA as has the system's trajectory hugs the West Coast before it digs offshore to west of Baja by Friday 12Z DEC 14th. The 00Z DEC 7 ECMWF T799 deterministic run agrees with the GFS solution from fresher IC's. Paradoxically, the ESRL/CPC ens. refcsts from 00Z DEC 7 IC's (http://www.cpc.ncep.noaa.gov/products/predictions/short_range/tools/cdc.pub.html) show day 6-10 (DEC 13-17) with a slightly above normal signal (50-60 percent prob of being in the upper tercile). The ESRL/CPC wet outlook holds into week 2 (days 8-14, DEC 15-21). However, the NAEFS plumes from 00Z DEC 7 runs for KSMF now show DEC 18-20 as the dates with the most promise (http://www.meteo.gc.ca/ensemble/EPSgrams_e.html), or 1-2 days later than yesterday's 00Z runs. Survey of NAEFS ensemble members suggests little real hope over the next 9-10 days.

Of course, the situation will be monitored over the weekend just in case the progs begin to hint at an unforeseen (at this time) regime transition.

Steve Mullen
19:00 UTC 7 December 2007

December 6, 2007

HMT forecast discussion for Thursday 6 December 2007

Operations associated with IOP-1 are well underway. As of 19Z, mostly light-moderate amounts (0.30 in. at BLU) are indicated by the radar estimates and gage reports to the north and west of ARB. Stronger echoes (>30 dBz) moving from the west over the ARB at this time, which suggests the ~18 h period of most intense precipitation has begun.

The UA flow has moderate westerly jet with 250 mb winds of ~120kts over the HMT region at 15Z (+3 h NAM). The westerly flow extends down to ~700 mb, turns to WSW at 850 mb suggesting WAA, and is SSW over the Central Valley as the flow impinges against the Sierras. The westerly flow is ahead of a sharp tough that is intensifying as it digs down the West Coast to a NAM position over SAN by 12Z Sun Dec 9th. 500 mb trough axis crosses over ARB 12-15Z 7 DEC, with FROPA 09-12Z 7 DEC. GFS is the fastest, and I tend to favor GFS on timing for West Coast systems (better numerics, better handling downstream energy propagation from Pacific). With FROPA and commencement of CAA, precip will become more showery and snow levels should drop to 4.5K.

Look at the 12Z 6 DEC NAM and GFS and 09Z 6DEC SREF (www.spc.noaa.gov/exper/sref) runs suggests a shutdown of ARB precip around 15Z 7 DEC 2007. NAM has MAX ARB storm total of just under 2.00 inches, and the coarser GFS has only a tad less. MAX SREF uncalibrated 24h Prob>1.00 in. = ~0.70 (Prob>2.00in. =0) ending 15Z 7 DEC, suggesting a Prob>1.50-2.00 in. ~0.70 in favored orographic regions. The HMT 3km, lagged ensemble for the 24 h of 18Z 6 DEC to 18Z 7 DEC shows Prob>1.00in. > 0.50 on favored ridges. I leave SREF downscaling to the local hydromet experts; ARB is not an area where I can claim experience.

On the longer range, the NAEFS plumes for SAC suggest zero precip for HMT thru Tues. DEC 11 as CA is dominated by N-NW flow aloft. A few members show a weak S/W trough embedded in WNW steering flow on Wed. DEC 12, but precip chances look nil as the system propagates into difluent steering over CA. The outlook for week two suggests that after the weekend of DEC 15-16, especially starting Mon-Tues DEC 17-18, COULD bode well as a few outlier NAEFS members show ppt >25 mm and a strong 500 mb low off the CA coast. The time frame is several days beyond the predictability limit for synoptic systems. That caveat in mind, the NCEP MREF members for 12Z DEC 16 (+day 10) (www.meteo.psu.edu/~gadomski/ENSHGTNH_12z/ensloopmref.html) are consistent with building a 500 mb LW ridge just east of the dateline. The pattern deepens a significant trough near 160W in a few members (say ~20 percent), which progresses toward CA by day 12 (12Z DEC 18) and has strong SW flow ahead of it. The scenario has the potential for a more classic Atmo. River set up. The CMC ensemble (http://www.meteo.gc.ca/ensemble/) from 00Z 6 DEC is in a general agreement (~15 percent of members look favorable). The situation should come into better focus by the end of my shift (DEC 12th).

Steve Mullen
20:00 UTC 6 December 2007

December 5, 2007

HMT forecast discussion for Wednesday 5 December 2007

Our approaching shortwave trough is near 45N/150W this morning, with the separate southern wave now a nice Kona low sitting just north of Hawaii. The upper low that was over Alaska has retrograded to join with one near Japan to form a large circulation in the northern Pacific, beneath which is an elongated strong jet with max speed near 170 kts near 35N/155W. Another jet extends north on the east side of the Kona low and joins with the jet that is still present from the big plume into the Pacific Northwest earlier this week. This jet is also quite strong also with a 170 kt max near 44N/140W, with the plume of moisture just south of this jet and then extending back to the Kona low. This configuration is feeding some moisture into the approaching northern shortwave trough, and at least some connection, albeit increasingly bent and certainly nothing like what we saw in the Pacific Northwest, is forecast to be maintained until the system hits the coast later Thu. It would seem that how much of this moisture gets entrained into the approaching system is one of the wild cards. SSMI imagery shows some pretty hefty PW values still present in the plume and the trend in the latest satellite imagery loop shows some of this moisture is pumping northward just ahead of the northern shortwave. The models are coming into better agreement overall, both for 0000 UTC and 1200 UTC, and the system can be characterized as a fast-mover, not particularly large, but taking a pretty ideal track to give the most precip into the HMT area. The chance of the major precip and the system passing too far south seems unlikely based on the current jet configuration which will be driving the shortwave more eastward than southward until fairly late in the game, when the upper level ridge then builds behind the system off the West Coast, plunging the wave into the Southwest after it moves past the HMT area on Friday.

As for the model details. Good overall consensus in the 0000 UTC runs with the exception of the NAM, which was rather far south. The 500 mb upper low passes right over the HMT area in most of the runs near 1200 UTC/Fri/7 Dec, with the 700 mb low either near the HMT or to the north of the HMT. As long as the upper low is not too far north a slightly more northern position actually brings a bit stronger maximum 700 mb flow, up to 40 kts briefly in the latest 1200 UTC GFS at 0600 UTC on Friday. Pretty much everything is over by 1200 UTC Fri except for some light precip. Start time right now looks like 1500 UTC/Thu with light precip but most models have hefty precip from ~1800/Thu through ~0800 UTC/Fri. Some numbers from the 0000 UTC runs include maximum in the HMT area of 1.8" in the ECMWF, 0.7" Global Canadian (which has more of an open wave farther to the north), ~1.4" GFS. The 1200 UTC runs are looking roughly similar to the 0000 UTC runs, and are all in good agreement, even now the NAM. Timing is about the same, total values include 1.75" for the GFS, ~an inch for the Canadian Global, 1.2" but not too far away up to 4" in the Canadian Regional, over an inch in the NOGAPS, and 1.2 at BLU in the NAM but just to the south of HMT 1.7", while the UKMET has 1.2" max in the HMT, and the ECMWF just in has 1.24 at SAC, 1.32 at BLU, and a max close to 2" in the HMT area. One note on the latest ECMWF run compared to the GFS is that the 700 mb low in the GFS passes just north of the HMT, while it passes just south in the ECMWF. This brings the strongest wsw-sw flow into the HMT in the GFS, but a bit south of the HMT area in the ECMWF, which has its overall precip max of close to 3" to the south of the HMT. It would be interesting to see the NCEP window runs from the WRF at higher resolution but they have stalled out at 24 h. A look at the 0900 UTC SREF indicates a min of 0.75" and most members higher, with the Eta members most generous and ranging up to 2-3". SREF mean comes in at 1-1.25" for max precip in the HMT area. Just saw the 1500 UTC SREF and it is a bit higher in the mean, with a max more in the 1.25-1.5" range for the HMT area, and the Eta members still the wettest. Our GSD high-resolution 3 km runs are partially in. The 1200 UTC run with Schultz microphysics and simulated radar reflectivity output shows a few echoes in the higher terrain showing up by 1200 UTC, moreso by 1800 UTC, then echoes really increasing thereafter through the end of the available run at 0000 UTC/Fri, by which time max precip amounts are in the 0.2-0.5" range. Freezing levels start out pretty high, dropping to a min of 4500' by ~0600 UTC/Fri as the trough/upper low nears. Consensus on the conference call was rain shower activity in the intial phase at BLU, with a changeover to snow by 0000 UTC or perhaps a tad earlier. There was not great consensus on when the heavier precipitation will start, with some thought that it could be later than 1800 UTC. Pretty good agreement that most will be over by 0900-1200 UTC.

Longer range about as thought before. A break for a few days over the weekend into next week as the upper level ridge builds off the West Coast and Thursday's storm system digs into the Southwest and then fiinally lifts out by about Tue/11 Dec. Then a fairly active pattern with a mean trough into the Intermountain West and a threat from waves progressing across the Pacific and then diving into this trough. The first threat is by midweek next week though consensus keeps this too far north. A wild card though with this is that the Kona Low hangs out near Hawaii and in the GFS and ECMWF and then lifts north on the backside of the building ridge to become entrained into the system that hits the Coast midweek. By the end of the week another stronger wave digs more off the West Coast so bears watching. Other threats then into the following week as well, with an increasing number now of the members in the latest GFS ensemble from 1200 UTC having a potential deep and slower moving trough off the West Coast in the Wed/19 Dec time frame...certainly a long way off, but it would be a good way to end the last HMT week before Christmas!

ed szoke NOAA/CIRA/GSD

December 4, 2007

HMT forecast discussion for Tuesday 4 December 2007

There was some juice left in the plume as it finally drifted south over the HMT area this morning. The 1200 UTC sounding at SFO jumped to 1.5" of precipitable water with RNO up to 0.70". The Blue Canyon METAR had 0.50" as of 1800 UTC this morning. Lots more to the north and northwest over northern California. Radar imagery indicates the heaviest of the precip continues to drift to the south of the area so most of the rain should have occurred already. As for the next event, some conflicting model forecasts from 0000 UTC with the GFS the least favorable, driving the next wave too far south. But the 1200 UTC GFS is farther north, in agreement with other 0000 UTC runs that continued to maintain a stronger system more favorable to an event.

Right now there is an elongated upper level low in the Gulf of Alaska with the upper level ridge now elongated and positioned to the north of Alaska. The next system is a shortwave trough now located near 45N/165W and moving ese, ahead of a strong wave to its west that will lift to the northeast to a position west of Alaska, building the ridge behind our approaching shortwave for Thu-Fri. There is a southern wave near 30N/170W that is heading off to the south and will form a closed low north of Hawaii by Wed. Model forecasts show a brief tap into the moisture plume that will push to the north of this upper low by the progressive northern wave but this then gets pinched off by late Thu/6 Dec. Some details about the model forecasts...as noted above the 0000 GFS plunged the approaching wave to the south with not much sw flow at 700 mb, and at most about an inch of precip but more split around the HMT, falling mostly between 1800 UTC Thu and 1800 UTC Fri (6-7 Dec). But the ECMWF maintained a more northerly track, with 30 kts of wsw 700 mb flow at 0000 UTC Fri before the 700 mb low ends up pretty much right over the HMT area by 1200 UTC Fri, then some wrap-around as it heads to the east. Max precip through the event is ~1.5", with the heaviest falling from about 2200 UTC/Thu to 1500 UTC/Fri. The Canadian Global model from 0000 UTC is like the ECMWF, and quite generous with the precip, with up to 2.5". The UKMET from 0000 UTC has similar timing but is not as deep as the Canadian and ECMWF models. The 0000 UTC NOGAPS looks more like the ECMWF. The position of the approaching shortwave is very similar in the GFS and ECMWF all the way through 1200 UTC/Thu, then the GFS drops the wave much farther south. The 1200 UTC runs coming in are similar for the UKMET and Canadian, though a little faster in pushing the system east in the Canadian model so max precip is down to ~1-1.5". The big change is with the GFS, which is now farther north and deeper with the upper low, resulting in much better sw flow, reaching 25-30 kt at 700 mb at 0000 UTC/Fri, with the 700 mb low then passing just to the south of the HMT area by 1200 UTC/Fri, after which a period of wraparound moisture occurs in n to ne flow with snow levels droping. The max precip is more in the valley than into the higher terrain of the HMT, ranging from ~1.75" near SAC to ~1.5" in the higher terrain. The period of precip is rather long, beginning with light precip ~1200 UTC/Thu, heavier late Thu and overnight, then wraparound on Friday before ending ~0300 UTC/Sat/8 Dec. The 1200 UTC NOGAPS is similar to the 0000 UTC run, and close to the latest GFS, with an estimated 1-1.5" total precip. The 1200 UTC ECMWF run just in is still favorable, with a little farther northern position for the upper low that is not quite as strong as the GFS, but owing to its slightly more northern position gives about the same amount of precip as the GFS, with up to 2" just to the south of the HMT area. The 1200 UTC NAM is both weak and far south, with maybe 0.20" of precip in the area, but my faith in this would be quite a bit less than any of the global runs. There is a good deal of spread in the 0900 UTC SREF ensembles, but the ensemble mean does have a pretty decent event, beginning about 1500 UTC/Thu and ending late Fri, with ~0.5" of precip. The 0000 UTC GFS ensembles also have a lot of spread in the precip amounts for the storm, from very little to about what the operational run produced. On the other hand, the latest 1200 UTC GFS ensembles are more in agreement on a wetter system, with only one (of 12) member having very little for the event. Bottom line is this is not a huge system, it may be rather compact as it hits onshore, but if it takes the right path has potential for interesting amounts of precip, especially if it picks up something from the Hawain closed low. Another potential for more precip is that the system could linger in terms of wraparound longer than Friday as it deepens to the south and east. However, Its relatively size as it hits the coast implies that potential track changes could result in vastly different amounts of precip. Consensus from the conference call suggests that while precip could start as early as 1500 UTC/Thu the main precip would get going later, perhaps 2100-0000 UTC, with a trough passage 0600-1200 UTC/Fri, and then most of the precip ending. Expected snow levels will initially be quite high, around 8000 ft, but fall below ~4500 ft as the trough/upper low nears around 0600 UTC/Fri and continue some lowering beyond this time.

Once the system passes the HMT area cold northerly flow ensues as the upper level ridge builds west of the West Coast, with the trough digging into the Southwestern CONUS and slowly moving to the east. Timing differences then show up between when the first in a potential series of waves that will traverse the Pacific threaten the HMT area. The Canadian would have the first one as early as late Tue/11 Dec, the GFS more like Wed, and the ECMWF delayed until Thu/13 Dec. By 240h there is quite a large difference between the 0000 UTC model runs off the Pacific in both timing and intensity of troughs. The Canadian ensemble from 0000 UTC is trending in a number of members towards a bigger trough that lingers off the West Coast in the 15-19 Dec period. The GFS ensembles from both 0000 UTC and 1200 UTC look more progressive with weaker (though rather moist in some cases) shortwave troughs and considerable spread in timing in the members for this same period. It appears overall that there would be rather low confidence in the forecast in the longer range. This shows up quite nicely for the SAC meteogram as an enormous spread in potential daily precip for the period 12-19 Dec.

ed szoke NOAA/CIRA/GSD

December 3, 2007

HMT forecast discussion for Monday 3 December 2007

Big storm currently underway in the Pacific Northwest with destructive winds and heavy rains from northern California through Washington. In California around a half inch of rain south all the way to near Santa Rosa, 3 inches up the coast at Honeydew, appears to be nothing of note in the HMT area at this point. Max winds at Blue Canyon at this point not quite 30 kts but way stronger to the north. The main shortwave trough now located near 43N/140W, with another wave back at 40N/165W, all south of an elongated closed low sitting in the Gulf of Alaska. A plume of moisture stretches al the way back to Hawaii. The easternmost shortwave is heading towards Vancouver later tonight, with the trailing one arriving a little farther south later tomorrow, by which point the plume of moisture gets detached some from the main system and sinks south, with what is left of it on Tuesday aimed pretty much at the HMT area. This should bring some echoes into the area perhaps as early as 0600 UTC tonight but mainly 1000-2200 UTC on Tuesday. Right now still looks like the most over the higher terrain areas would be in the order of 0.25", but this all depends how much is left in the plume and imagine this could be tricky for the models. The hi-res WRF NMM window has spots of over an inch, but the ARW much less. The ECMWF from 0000 UTC is also more moist, as is the Canadian run, but consensus favors lower amounts. Note though that current graphical forecasts out of the Sacramento WFO have up to 0.45" of precip max in the HMT area.

Once the plume passes, the next system still on track for later Thursday through Friday (6-7 Dec). The wave that becomes this system is just now coming into the northern Pacific, and is forecast to begin to amplify as it passes beneath the retrograding Gulf of Alaska trough later Tue into Wed. At the same time a separate upper level wave to the southwest of this system becomes detached from the main flow and closes off to the northwest of Hawaii by Wed. Tne main significance of this is that it pumps some tropical moisture north into the progressive northern system, although how much of this connection remains intact by Thu/6 Dec as it deepens and heads towards the coast is up for question. In terms of detailis so far on this next system, the 0000 UTC runs were, overall, more favorable for a decent event than the latest 1200 UTC runs. For example, the GFS had up to ~2.5" max over the HMT area, similar to the Global Canadian run, but the 12z GFS and Canadian runs are down to about an inch. The main difference is a faster moving system in the latest runs compared to the 0000 UTC runs, so less time spent with good s-ssw 700 mb flow. Too early to sort out these details but we should note that the start time in some runs is as early as 1500 UTC on Thursday. Overall the ECMWF 0000 UTC run is somewhat faster (and with less precip) than the GFS or Canadian, while the NOGAPS drops the system farthest south eventually. The 1200 UTC ECMWF is quicker to end the event than the 0000 UTC runs, with an accumulated total precip max of ~0.6" in the HMT area. The UKMET 1200 UTC run is also more progressive than the 0000 UTC run. And finally the NOGAPS is a llittle faster with the 1200 UTC run though still dives the system farthest south by Sat/Sun. Ensembles from 0000 UTC are somewhat split between faster and slower motion on the GFS, leaning more towards more progressive on the Canadian ensembles. The latest 1200 UTC GFS ensembles have an event ranging from not much precip to around 2 inches in the HMT area, with the main period from 1800 UTC Thu/6 Dec to 1800 UTC Fri/7 Dec. Bottom line at this point is a potential event still on tap, could begin in earnest (if earnest occurs!) by 1800 UTC on Thu, and trend is to end it sooner than in the earlier runs, probably mostly over by midday Friday. We'll have to see if this trend continues or if we go back to a somewhat slower solution, plus need to monitor the amount of moisture tap into the system near Hawaii.

Looking farther ahead, most models have a break from ~9-12 Dec, but then the threat of systems out of the wnw with the upper level ridge far enough to the west for the period 13-18 Dec. Consensus is towards progressive shortwave troughs digging into the Intermountain West with the threat of enough digging before they hit California to produce an event.

ed szoke NOAA/CIRA/GSD

December 2, 2007

HMT forecast discussion for Sunday 2 December 2007

The main changes for this forecast cycle include increased optimism for a potential event late Thursday through Friday and perhaps into Saturday (6-8 Dec), and then a pattern that could keep the area vulnerable to systems out of the northwest in the longer range. Right now the changes that we've been talking about have occurred, with the upper level ridge now pinched off over northern Alaska, while a strong wave sits below it near 150W/40N. To the east of this strong system is a shortwave trough that will hit the Oregon coast shortly with increasing wind and precipitation. The heavy precip extends from Washington south to near Eureka at this time, and only a slight southward shift is expected through most of the event through Monday, with many inches of rainfall up the coast through Washington. There could be a little snow shower activity through the higher HMT elevations this afternoon through Monday. It looks like a better chance of some rain/snow would prevail on Tuesday as the remains of the plume sag south. At this time it still looks like the plume will be much diminished by this point but would not be surprised to see something in the order of 0.25" or so in some areas on Tuesday, or at least some more substantial echoes in the area. Until then high winds will be the main threat for the higher HMT areas. The 0600 UTC high-res NCEP WRF windows both have very little precip for the HMT area, but from 7 to 9 inches total through 1200 UTC Tue from Eureka northwards.

The upper level flow is forecast to shift to west-northwest by Wed/5 Dec with drier conditions moving into the area. However not far away is another wave traveling across the Pacific that showed signs of deepening while still approaching the California coast yesterday, and this trend continues with todays 0000 and 1200 UTC model runs. For instance, today the GFS 1200 UTC run now has a pretty strong upper low about to hit north-central California on 1200 UTC Fri/7 Dec. A new twist also is that this system is forecast by both the GFS and the 0000 and now 1200 UTC ECMWF to slow down and head more to the south towards Southern California with time, keeping the HMT area potentially in precipitation with southwest flow for a longer period, perhaps from late Thursday into Saturday. There is some risk now that the heaviest precip could actually be too far south, but other models (Canadian, for example, and to a lesser extent the UKMET) are quite a bit farther north and do not close the system off over Southern California. GFS 0000 UTC ensembles favor something eventually cutting off but only slightly over a more progressive but deep trough. The Canadian ensemble set from 0000 UTC favors a more progressive trough with the main precip threat for HMT on a Friday time frame. One interesting note is that there could be a tap into some moisture associated with an upper low that is hanging out nnw of Hawaii, shown best in the GFS PW loops at the U of Hawaii site.

Beyond this period both ensembles keep the ridge far enough west off the coast in enough members to have some potential for the following week (9-14 Dec). A similar message from the latest GFS 1200 UTC ensembles.

ed szoke NOAA/CIRA/GSD

December 1, 2007

HMT forecast discussion for Saturday 1 December 2007

The very narrow and high-amplitude ridge remains in place from the North Pole south across central Alaska and then further south near 140W this morning, but is about to be undercut by an eastward moving shortwave near 40N/150W, as well as a retrograding trough now in British Columbia. This will set up the strong west-southwest flow and moisture plume aligned at the Pacific Northwest that was discussed yesterday. The aforementioned shortwave in the Pacific will hit the Oregon coast near 1800 UTC on Sunday and bring strong winds and rains south into Northern California as well. This may be the closest tease for the onslaught into the Pacific Northwest, and one would think there could be at least a little precip perhaps into the higher elevations of the HMT area, but it looks like a sharp cutoff will be in place with the heavier rains/snow staying too far north. The hi-resolution runs of the WRF (ARW and NMM NCEP windows) run at ~5 km resolution both have accumulated precip up to about 0.30" in the higher elevations of the HMT area, mainly falling between 0600-1500 UTC 2 Dec, then another close call later on Sunday. The strong onshore flow will continue aimed at the Pacific Northwest through Monday and gradually weaken by Tuesday, but not before some areas from extreme Northern California to Washington receive 5 to maybe 10 inches of rain, so quite an event. Cold enough for snow in some areas and all kinds of warnings are in place, but more wind warnings than precip by the time you head south to near the latitude of the HMT area. The 1200 UTC NAM is a little farther south with the main precip than the latest GFS run. Overall though pretty good agreement with a close call but the heavy rains staying too far north. Same with the short range ensembles, though again some members do bring a little precip into the higher terrain of the HMT area on Sunday.

A couple of possibilities are present after the big event weakens on Tuesday. One remains whether there may be something left to the plume as it sinks a bit south resulting in precipitation possible in the HMT area in the late Tue through Wed time frame. Right now this looks marginal but certainly we'll keep an eye on it. A better chance might be with a shortwave later this week that amplifies as the upper level ridge gets reestablished off the West Coast. This was a minor feature yesterday that amplified farther inland but today there is more spread with this feature, with some models developing it more before it reaches the California coast. This would be a threat of a relatively quick moving and cold event in the Friday/7 December time frame. We have time to watch this one.

Looking further ahead the ensembles generally continue to predict the upper level ridge to build off the West Coast with most systems dropping farther inland, so a dry pattern for the HMT. However, a little more uncertainty now in the GFS ensembles, (and some remaining in the Canadian ensembles) with the chance the systems could amplify more as they approach from the northwest and before passing the coast. This would not likely lead to any sustained atmospheric river into the area, but a potential event out of the northwest with reasonable precip possible. The uncertainty is seen by comparing just the ECMWF 240h from last night's run with the latest GFS, with the EC having a decent wave by early next week in the northwest flow.

ed szoke NOAA/CIRA/GSD