HMT-West 2007: Forecast

The weak upper-level wave that the models have been bringing across the coast the past few days is just now crossing the coast a little N of the Bay Area. Earlier this mrng there were a few radar echoes in the Trinity Alps/Shasta area, and at present there are a few small showers moving through the American River Basin. I expect no accumulations of consequence.

This upper level trough is now embedded in the broader
scale difluence aloft along the West Coast, and had
120kt NW flow on its back side at 12Z this mrng, based on aircraft reports. The NAM digs this
all the way down to over Sonora state, Mexico, by 00Z
3 Jan, in the process preserving the broader-scale
difluence along the coast.

The IOP prospect for 4 Jan discussed yda is still on
track. Attm, the weather feature in question is a broad, low-amplitude trof in the
central Pacific with a 150kt, basically westerly
jet on its S side. Consistent with consensus over the
past several days, this trof is fcst to become confluent as
it apchs the Gulf of Alaska, and the subtropical high
underneath the anticyclonic shear on the south side
of the upper trough strengthens to 1040+ mb
and moves eastward out of the central Pacific to between 135 and 145W and to near
the latitude of SFO by 4 Jan.
The deep surface lows associated with the trough aloft
are moving into the Gulf of Alaska and bring a trailing front with
copious pcpn into the Pacific NW and BC by tomorrow. This
front slowly slips southward as the trough aloft comes
eastward. Rain begins from Cape Mendocino northward
by 00Z 4 Jan. However, as the front comes southward,
the surface geostrophic onshore flow ahead of it
weakens, again in agreement with earlier forecasts,
so that despite good onshore flow aloft,
by the time the front is into Nrn Ca on the
4th, onshore geostrophic flow at the sfc is weak,
and northerly flow at the surface is strengthening
behind the front. In other words, deep warm advection
south of the front is weak or nonexistent by 00Z
4 Jan.

All this plus the GFS-predicted pcpn suggests to me
that chances of Blue Canyon getting at least 1" of pcpn
out of this are ~60%. However, I think > 2"
is very unlikely, no more than 10%. Indications from the progs are
that the event in the ARB will start no earlier than about 10pm Pacific time on
the 3rd, and end by late afternoon on the 4th (say near 00Z 5 Jan). Heaviest pcpn
is anticipated to be roughly 4am to noon on the 4th.

Snow levels will drop during the event. Taking an average of my forecast of 5,500 feet initially and the CNRFC's of 7,000 feet initially puts the starting snow level at above 6,000 feet. Snow levels are xpctd to drop to near 3,500 to 4,000 feet by the storm's end.

Posted by Forecast Editor at 1:09 PM | Permalink | TrackBacks (0)

Main forecast problem today is the IOP prospect coming up in the 3-5 January range. The earlier weak southern-branch system expected to pass over or south of the IOP area late tmrw and New Years will produce little if any pcpn in the ARB. It is, however, forecast to dig southward as and after it crosses the coast, to form a cutoff low aloft over Arizona early next week. As such it contributes to preservation of the tendency for difluence in the upper-air flow in vicinity of the west coast.

As this cutoff develops and then drifts slowly eastward toward the lower Mississippi Valley, the main westerlies to the north gradually sag southward along the west coast. This is in association with a low-amplitude trough aloft currently in the western Pacific. It is this trough that is the IOP prospect for next week. It will progress eastward to reach the coast 4-5 January, to be proceeded by short-wave features in the generally WSW flow ahead of it that will bring copious pcpn to BC and NW Washington state over the next few days.

All indications are that the southern portion of this trough will begin to dig SEwd as and after it crosses the coast in response to strong ridging surface and aloft upstream and strong cold advection down the West Coast. This will serve to reinforce the low-latitude mean trough position over the SW US, but will also sever the feed of high PW from the WSW into the west coast that will contribute to the copius pcpn earlier in the week farther N. As a result, this mitigates against this storm from being the major pcpn producer over the ARB that we would like to see, but pcpn amounts in excess of 1" at Blue Canyon beginning late on 3 Jan (i.e., after 00Z on the 4th), or 4 Jan and ending on the 5th still appear to me to have a > 50% probability. This still looks like a snow producer for areas at and above 5500' elevation in the ARB, with snow levels lowering to 3500' as the event ends. A possibility (< 50%) is a rapid-deepening surface low off Nrn CA or Oregon similar to what happened on 26 December.

Dry conditions are likely for at least a few days after this event ends on the 5th.

Posted by Forecast Editor at 12:54 PM | Permalink | TrackBacks (0)

Overall trends look similar to yesterday. A very deep (944mb) low is in the Gulf of Alaska this mrng, with a frontal band with waves to its south. Over the next few days, a succession of short wave troughs and associated surface lows will move NEwd from the Central and Western Pacific, affecting mainly the coastal region from the Alaska Panhandle down to NW Washington state. A weaker upper air feature that has been noted the past couple days is still forecast to meander slowly toward central and Northern CA, to the south of the main flow, and may give a little light pcpn in the HMT, mainly in the mountains, on 31 December 06 or early on 1 Jan 07. As noted yesterday, this system has little reflection at the surface, and is likely to be contaminated by dry air at lower levels during its opportunity to affect the HMT domain, and pcpn amounts are expected to be ~ 5mm or less at Blue Canyon.

The next pcpn opportunity comes as the main jet settles slowly southward. This will come during the 3-5 January period, according to present indications. It is too early to say with any confidence that this is a good IOP prospect, but as pointed out by K. Weickmann and E. Berry in today's telecon, this might well be the best prospect during the next 2 weeks or so. I think we should be alert to the possibility of an event similar to the one that happened 26 Dec, in which Blue Canyon picked up 1.4" liquid or so. Snow levels in this event seem unlikely to be above 6000' even at the start of the event.

Posted by Forecast Editor at 1:10 PM | Permalink | TrackBacks (0)

The next step in the El Nino cycle is typically a strong subtropical westerly flow, which is anticipated to dominate the Jan-Mar 2007 period. The transition to this atmospheric state normally coincides with the onset of the southern hemisphere monsoon and an eastward moving MJO-like feature. Over the North Pacific the jet stream extends to the west coast and split flow develops over the western US and Canada. However, some unusual convective patterns have been observed in the last six months with present El Nino. In particular, the Indian Ocean has been very active convectively but the dateline region shows primarily transient (< 30 day) activity. The MJO has also been episodic. An MJO in September-October 2006 helped amplify the EL NIno SST pattern by initiating an oceanic Kelvin wave. Currently there appears to be another MJO that developed over the Indian Ocean in mid-December 2006. In just the last 5-10 days convectiont shows clear signs of moving eastward. This MJO is likely to lead to the onset of persistent deep convection over the dateline, probably in weeks 2-3. In the interim as convection shifts farther east into the west Pacific and North Australia, a cold regime is anticipated for the USA Plains sometime also in weeks 2-3 (10-15 January). Some links to images in the extended entry illustrate some of these developments.

olr last two years along equator

olr last 9 months along equator

global and zonal atmospheric angular momentum within
the troposphere: a measure of the state of the circulation

150 mb vector wind anomalies dec 28 2006

wheeler mjo prediction:

gfs prediction for 00z 4 jan

gfs prediction for 00z 7 jan

gfs prediction for 00z 12 Jan

Posted by Forecast Editor at 8:58 AM | Permalink | TrackBacks (0)

Deep cutoff low has formed as predicted over the SW US. It, however, is a bit farther south than was forecast yda, probably because of the xtrmly stg Nly flow aloft down the W coast yda. Most W Coast raobs were missing winds at upper levels at 00Z tda due to these strong winds, and the initial analysis at 00Z may have suffered as a result.

This cutoff low is forecast to drift slowly E to ENE over the next 2-3 days, reinforcing the broad-scale difluence aloft along the W Coast. Specifically, there is now the beginnings of a split flow over the US. The southern branch originates in difluence along or just off the West Coast as it splits off as NW - N flow around the cutoff, whereas the northern branch is SW flow impinging on the coast N of the CONUS.

Dry weather is expected to prevail over the HMT area the next 2-3 days.

The deep central Pacific trough mentioned in yesterday's discussion is already loosing amplitude as it
rotates NEwd toward the Gulf of Alaska. In the near term this will flatten the ridge just off the W. Coast a little. The next system of potential interest to HMT follows behind the rotating trough as a s/w trough that weakens and splits off over the Ern Pacific from the main flow. This trough is forecast to
meander toward the W coast in a weak westerly flow S of the main jet, and turn Swd as it approaches the broad-scale difluence noted above. This feature can be thought of as a PV perturbation propagating on the tropopause and having too shallow a penetration depth to much perturb the surface wind and pressure field. As a result none of the model runs are forecasting either a closed surface low or an onshore surface wind component as this feature approaches California; prospects for significant pcpn (say, > 10mm water equivalent) out of this system anywhere N of the Tehachapis are nill. Greatest liklihood for any pcpn in the HMT area is on the 31st, by present reckoning. Prospects for Southern California look a bit better, on the 1st.

Beyond this, the next real opportunity looks to be 3-4 Jan 07 when some of the ensemble members from the GFS bring a short wave and weak surface low toward Nrn CA with strong onshore flow aloft. I regard this as too far out in time to say with much confidence that it is an IOP prospect. I will be considering this carefully the next 2 days.

Looked at this immediately after I entered it: found I had put 10cm, rather than 10mm as greatest conceivable upper bound on pcpn from 31 Dec system. Now corrected.

Posted by Forecast Editor at 12:04 PM | Permalink | TrackBacks (0)

Yesterday's storm in the area has ended except for a few lingering showers or flurries in the mountains. It now looks as if the next several days will be rather quiet except for some snow on the eastern slopes and along the Sierra crest later today and tomorrow as the flow at mountain-top level becomes N to NNE.

On the broader scale there appears to be downstream amplification in progress that originated with a strong low development in the western Pacific. Strong amplification of the next trough downstream from this development has occurred in the past 18-24h, and at 12Z this morning a 150+ kt jet streak was still west of the trough axis, which itself was NW of Hawaii. The downstream ridge along the west coast is also amplifying and building into Wrn Canada behind the HMT storm just ended, and surface pressures are building over the Pacific NW and BC. Strong N surface flow developed offshore northern CA overnight and KSFO had NW wind gusting to 50kt early this mrng. Farther aloft, analysed jet-level winds were analysed at NW 140 kt + at 250mb at 12Z this mrng.

Progs are unanimous in indicating a cutoff low dvlpg over NM next 24h, and moving only slowly NEwd thereafter. With the building ridge over Western Canada, the polar jet is displaced far to the N and a blocking pattern is in the offing over North America for the next several days. As a result, the HMT area will be devoid of anything but very light pcpn, mostly snow flurries and showers over the higher mountains, at least through Saturday 30 Dec.

After 00Z 31 Dec there is a broad spread in the ensemble forecasts over Western US and offshore. The most likely scenario would seem to be that the current central Pacific trough aloft will weaken and split, with a southern piece meandering slowly toward the West Coast early next week as a weak, partially cutoff feature with only a weak reflection at the surface. (The northern part of this system will affect mainly BC and se AK.) As such, as this system approaches the coast early next week, there is unlikely to be a signficant feed of > 1" PW into it unless it is considerably stronger than seems likely based on current NWP output, and it crosses the coast well to the S of the Bay Area. Rather it is likely to have to make do with dry air recycled westward from the interior western US. Accordingly, pcpn as it approaches the coast is likely to be light, well under 1". The trajectory of this feature is also uncertain, with the likilhood in my opinion that it will affect Southern CA, rather than the HMT area.

Looking beyond this, I believe predictability is very low. There is certainly nothing clearly evident that leads me to be highly optimistic for an IOP-quality event before late next week at the earliest.

Posted by Forecast Editor at 2:02 PM | Permalink | TrackBacks (0)

Current conditions feature one of the best cases
of the season so far, with a ~996mb low ofshr
southern OR and a nice batch of high PW air coming onshore
northern CA. Pcpn has set in over the Sacramento Valley
last couple hours and it looks as if rainfall amounts in the
American River basin should total into the 1.5 to 3" range
before pcpn ceases tomorrow. Heaviest pcpn will end before
dawn tmrw.

Following this, all indications from the NCEP models are
that the flat mean ridge will stay in place along or just
off the W coast, and that the large scale difluence aloft
that has been a persistent feature over the west coast
will continue, but at a latitude farther north for the next few days. This will tend to favor a feed of tropical moisture into the SW US and NW Mexico around an upper level
trough off southern CA and Baja, but is not favorable
for such a feed into northern CA.

There will be weaker systems (than the present one) coming onshore
into the Pac NW, but all indications from ensembles is that
pcpn from these will stay mainly N of 40-42N. Thus, I see
little opportunity for more than very light pcpn (< .25" total
at Blue Canyon)
between 28 Dec and 1 Jan 07. The next opportunity for an
IOP target is 2-4 Jan 07, when it appears that a stronger trough of opportunity will be approaching the west coast. It was suggested during the conference call that it would be good to consult Klaus Weickmann for his take on the
present El Nino situation, and the chances for a persistent southern branch split flow into California,
more in keeping with El Ninos of recent vintage.

Regarding Wx in Colorado, there is a very high chance of
more snow Th and F, 28-29 Dec 06. At this time, I see
6-10" for the Coop station, beginning during the day Th
and ending by late Friday. Given the large amount of snow
remaining from our last storm, plus cold ground,
this will have a bit more
impact than a typical 6-10" storm, perhaps limiting
air traffic in/out of DIA.

John B.

Posted by Forecast Editor at 12:45 PM | Permalink | TrackBacks (0)

Light rain started at Blue Cyn in the ARB at ~1430Z, pretty much on
schedule. The most recent IR satellite loop shows a well-defined SW-NE
oriented comma-cloud tail slamming into N CA, with cloud-top temps
becoming more enhanced with time. The SSM/I satellite imagery shows a
moderate atmospheric river intersecting the N CA coast, with core values
of PW ranging between 2.5 and 3.0 cm offshore. The DAX 88D shows precip
across the Central Valley, with some orographic enhancement in the ARB.
Finally, the profilers at Chico, Alta, and Cazadero indicate a snow level
ranging between 7.1-7.7kft. All in all, it is shaping up to be a wet day
in the ARB.

Yesterday’s forecast for today into tomorrow still looks on track with
respect to the timing on the best orographics, fropa, upper-level
dynamics, and snow-level behavior. Today’s models are perhaps a little
faster with cold fropa (near the 00Z Friday time period) as well as a
little weaker. Overall, QPF numbers are a bit lower than what was
advertised yesterday, although the ARB should still receive a total of at
least 1.0-1.5 inches. Expect significant precip to end between about 08
and 10Z Friday, as the shortwave trough moves SEward beyond the ARB and
the upstream ridge builds rather promptly.

Our holiday down time starts tomorrow (Friday) and extends through next
Wednesday. A couple of transient shortwave troughs may break through the
ridge during this period, resulting in some modest precip across the ARB.
However, given the generally progressive, zonal character of the flow
during this period, I do not anticipate a repeat of the wx during last
year’s Holiday Break when the ARB got hammered with copious amounts of
precip.

Paul Neiman

Posted by Forecast Editor at 11:45 AM | Permalink | TrackBacks (0)

The American River Basin will experience one more day of dry weather today before a transient shortwave trough impacts the region tomorrow (Thursday) into Friday. Thickening clouds will start streaming overheadthis evening and tonight, as moisture aloft begins to move through theflattening ridge.

Tomorrow morning at ~12Z, upslope WSW flow at 700 mb commences and thenintensifies to 30-40-kts in the pre-cold-frontal environment by 00ZFriday. Normalized 700-850 mb moisture flux anomalies (based on the 12ZWed. NAM) will approach the +1-sigma value in the ARB region tomorrowafternoon. A long, narrow PW plume is progged to intersect the N CAcoast tomorrow and then slide SEward with the advancing shortwave. Corevalues of PW should approach 3 cm just offshore. The period 18Z Thu –00Z Fri is when orogaphic forcing will be optimal. The primary dynamicforcing with the shortwave trough and cold fropa should be in the 00-06ZFriday time frame, with fropa probably occurring in the first half ofthis 6-hour window. Post-frontal precip. should linger until about12-14Z Friday, after which the atmosphere will rapidly stabilize due towarming and height rises aloft.

Light precip will likely start at ~14Z - 16Z Thu, then become steadierand heavier when the orographics become most favorable in the 18Z Thu –00Z Fri period... perhaps totalling 0.5-1.0. Expect another 0.5-1.0”between 00Z – 09Z Fri associated with dynamics tied to the fropa and tothe shortwave aloft. Lighter cold-core showers may linger until ~12-14ZFriday, perhaps totalling another tenth-inch or so of precip. Thefreezing level will start at about the 8kft level 12Z Thu before theonset of precip. Evaporative cooling in the pre-cold-frontal precip willbring the freezing level downward on Thursday to perhaps 7.0-7.5kft. Following fropa, the rain-snow line should descend to below Blue Cyn...perhaps down to ~4.5kft.

Low-amplitude ridging across the ARB over the weekend may flatten inresponse to landfalling shortwave troughs in the quasi-zonal flow aloft. Most wave energy should remain N of the region, impacting the Pacific NW,although the first half of next week may see modest transient shortwavetrough activity into the N half of CA.

Paul Neiman

Posted by Forecast Editor at 1:21 PM | Permalink | TrackBacks (0)

Cold, dry weather persisted across the ARB during the last 24 h , and it will continue through tomorrow (Wed.) under a ridge aloft. Far to the SE, the 500 mb cut-off cyclone will drift slowly Eward into NM by tomorrow morning and then intensify. This system will likely generate a potpourri of significant winter weather across CO, WY, KS, and NE tomorrow into Thursday… which could have a bearing on deployment for a possible IOP in the ARB on Thursday (see below).

The models are still on track regarding a transient shortwave trough breaking through the ridge and impacting the ARB between Thursday morning and early Friday. This Eward-moving wave is evident in the IR satellite imagery near 38N/142W. Newer model runs have increased the intensity of this system somewhat as it makes landfall on Thursday, although they have been consistent with its progressive character. The 12Z GFS shows the development of a long, narrow PW plume (i.e., atmospheric river) extending from the eastern Pacific to the northern/central CA coast on Thursday. Core values w/in the plume are forecast to exceed 3 cm just offshore of CA at that time. Meanwhile, HPC’s 850-700mb moisture flux product (based on the 12Z NAM run) portrays moderate normalized flux anomalies of ~+1.5-sigma over CA on Thursday. Finally, the 700 mb flow will become moderately strong (~40-50kts) and directed from the WSW during the wave passage… favorable for orographic precip enhancement. All in all, this system is shaping up to be a moderate event, albeit not long-lived (~12-15 h duration), with ~1.0-1.75” of precip expected across the ARB. As it looks now, a reasonably sharp cold fropa will occur in the 00-03Z Friday time period, with relatively short-lived postfrontal orographics due to quickly rebounding temperatures and heights aloft on Friday. Snow levels will ascend to near 8kft prior to fropa late Thursday, then crash to roughly half that altitude in the cold sector. This storm may be worthy of an IOP if deployment of personnel from Boulder, CO is feasible, although the storm will probably not have far-reaching hydrologic consequences.

During the upcoming weekend , a low-amplitude ridge will likely be situated over CA. Some zonal shortwave energy should impact the West Coast, primary in the Pacific NW… but perhaps as far S as N CA. Another moderate, progressive shortwave is currently progged to impact the ARB on Mon/Tue of next week. Thereafter, low-amplitude ridging is expected to return for a couple of days, with more shortwave energy impacting the Pacific NW.

Paul Neiman

Posted by Forecast Editor at 12:56 PM | Permalink | TrackBacks (0)

We have had cold and dry wx across the ARB during the last 24 h, with a little precip falling in the southern Sierras and along its eastern slopes. Meanwhile, the developing 500-mb cut-off cyclone is taking shape over extreme S CA/AZ, and it will begin moving Eward starting tonight. Ridge conditions upstream of this circulation will keep the ARB dry through Wednesday.

The models are still advertising a transient shortwave trough breaking through the ridge on Thursday, resulting in some precip at that time. The 700 mb flow develops a moderate onshore Werly component during cold fropa, so some orographic enhancement should be expected. However, it will likely not be a warm system, so moisture will be somewhat limited. Depending the model, anywhere from ~1/3 – 1” of precip is expected. Although unlikely, I would not yet rule out an IOP for this storm.

Thereafter, low-amplitude ridging will likely shunt the storm track Nward into the Pacific NW for the weekend. Thereafter, the 12Z GFS solution suggests another modest storm system affecting the ARB early next week during our down time.

Paul Neiman

Posted by Forecast Editor at 12:39 PM | Permalink | TrackBacks (0)

During the last 24 hours, the ARB experienced just a skiff of light snow,with liquid equivalents on the order of 0.1 inch. Temperatures werecolder than seasonal norms in the post-wave environment. Current IRsatellite imagery shows a comma-cloud head developing east of the ARB,over the Great Basin, in response to the developing midtrop cut-offcyclone. The ARB is situated beneath the dry, subsident, westernquadrant of the circulation.All forecast models show dry post-wave northerly-component flow over theARB for the remainder of the weekend. Ridge building over the easternPacific and Pacific Northwest will also contribute to the dry weather. This pattern will persist through at least Tuesday, and probably intoWednesday.The medium-range models are still suggesting that some wave energy maybreak through the ridge during the latter part of the week... most likelyon Thursday. As of now, it does not look like a big storm, although someprecip is possible. Then comes the holiday down time.

Paul Neiman

Posted by Forecast Editor at 12:48 PM | Permalink | TrackBacks (0)

As expected, the last 24 hours yielded colder temperatures and no
significant precipitation across the ARB. The current IR satellite loop
shows the main baroclinic cloud leaf extending NEward from the California
Bight into southern NV and all of UT. A second organized band of
enhanced IR cloud tops is moving across the Nern half of CA and reflects
the trailing 500-mb cold-core vorticity spoke dropping into the
developing Great Basin trough. However, there is presently no
significant precip beneath the cold cloud tops, based on the 88D
observations. All in all, it’s a rather dull winter day across the ARB.

The 12Z family of models are quite consistent with each other and with
previous runs regarding the western U.S. weather for the next few days.
The 500-mb trough is still progged to cut off over the Great Basin late
tonight and tomorrow, then remain nearly stationary or drift slowly Eward
during the first half of next week. Perhaps a little wrap-around precip
may affect the ARB this evening and tonight, but this shouldn’t amount to
much, given that 700-mb downslope flow will develop and strengthen.
Meanwhile, midtrop ridge building will occur over the eastern Pacific the
next several days. The ridge will likely weaken by midweek with the
approach of minoring-out shortwaves. Any way you slice it, it looks like
cool, mostly dry weather across the ARB through at least the Tuesday...
and probably into our hard-down holiday period starting on Friday.

- Neiman

Posted by Forecast Editor at 12:57 PM | Permalink | TrackBacks (0)

The latest IR satellite loop shows the cold-frontal comma-cloud tail laid out WSW-ENE across the ARB and extending well out into the Pacific, with a frontal wave forming offshore of CA. The SSM/I imagery captures a continued weakening of the narrow SSM/I PW plume intersecting the central CA coast, with core values now only marginally exceeding 2 cm. Cold fropa occurred at the Bodega Bay, CA profiler rather early… in the 06-08z time period, while more subtle evidence of cold advection aloft can be found at the Chico and Sloughhouse profilers after ~13Z. A hard wind shift to Nerly has just been observed at Chico starting at 1530Z. Only moderate amounts of precip have fallen in the ARB region since ~06Z, including ~0.9” rain at Blue Cyn and ~0.5” at Alta.

The 12Z family of model solutions show moderate postfrontal onshore flow at ~700mb today, slowly weakening in intensity. Hence, continued light to moderate showery precip can be expected in the ARB today, with snow levels lowering to <5kft by this evening. After 00Z this evening, the onshore component of the flow slackens off to minimal values, at about the time the primary zone of upper-level dynamics begins propagating across the ARB. Upper level dynamics should persist through tomorrow midday… deep within the lower-tropospheric cold air. Hence, spotty light precip will fall into tomorrow, with snow level lowering to <=4kft. The ARB will be in cold-core conditions aloft tomorrow into Sunday, as a 500-mb cyclonic circulation tries to cut off to the S and SE of the area. Spotty light precip may intermittently affect the area during this period.

By Sunday afternoon through Monday, the 500-mb trough should begin exiting the area, and generally Nerly flow should become established downstream of an amplifying ridge in the eastern Pacific. If the 500-mb trough cuts off, then the ARB can actually see significant downslope NEerly flow. Regardless, the atmosphere should dry significantly during this period.

For the long range, expect midtrop ridging over CA for at least the first half of next week, with generally dry conditions. Some remnant wave energy may break through the ridge but should not yield any precip.

Paul Neiman

Posted by Forecast Editor at 12:45 PM | Permalink | TrackBacks (0)

Another 1/4 to 3/4 inches of rain fell in the vicinity of the ARB during the last 24 h, ending near local midnight. The precip likely fell in response to the passage of weak vorticity spokes and warm advection embedded in modest but moist zonal flow. The SSM/I imagery showed a narrow PW plume (~2.5 cm) sagging as far S as the ARB last night. Melting level heights could not be tagged with the nearby wind profilers or S-band radars because only shallow, nonbrightband rain fell. The Chico profiler has thusfar shown no significant signature of a Sierra barrier jet… not surprising, given that the main storm system and trailing frontal zone is only now slamming into the Pacific Northwest.

This storm is captured well in the IR satellite imagery. The circulation center is closing in on 130W to the W of WA, and major warm-advection IR enhancement covers most of WA/OR ahead of the approaching cold front and 500-mb shortwave trough. A trailing cold-frontal comma-cloud tail extends WSWward from near OR to well out over the Pacific, and it is not yet showing signs of a Sward advance. The SSM/I imagery shows the PW plume migrating Nward toward OR as the deep-tropospheric flow backs with time ahead of the approaching storm system.

So… what is going to happen in the ARB in the next couple of days? Although the region is under the influence of weak warm advection and onshore flow today, the PW plume has migrated Nward into OR, and there is little evidence of cyclonic vorticity advection today into tonight. Hence, expect little in the way of precip (<0.1-0.2”) between now and near midnight.

Overall, today’s model solutions are similar to their predecessors regarding the storm affecting the ARB late tonight/tomorrow, if a tad slower. The cold front makes its Sward advance into N CA late tonight and across the ARB by midmorning tomorrow. Orographics will become quite favorable in the vicinity of fropa, with 50+knot WSW flow at 700 mb progged to slam into the Sierras. However, the primary band of cyclonic vorticity advection at 500 mb doesn’t impact the ARB until Friday afternoon and evening. Hence, the main frontal forcing/orographics should be decoupled from the dynamics aloft tomorrow. In addition, the primary region of 850-mb moisture flux anomaly is progged to remain north of 40N during the passage of the storm. The bottom line: this storm will likely produce moderate precip for a ~24h period starting late tonight (perhaps in the 0.75-1.25” range), with one period of enhancement during fropa tomorrow morning and a second period of enhancement as the cold air and dynamics move in aloft tomorrow afternoon and evening. Freezing levels should drop markedly from about 9+kft at the start of the storm to ~4kft at the end. One potential fly in the ointment is the possibility of the cold front stalling across the ARB tomorrow, thus allowing for phasing with the dynamics aloft…. resulting in significantly more precip than the QPF fields would indicate However, if the front stalls, it would likely be S of our domain. And none of the control solutions are showing this scenario unfolding.

The trough axis and cold air quickly moves E as an open wave across CA late Friday night and Saturday, so the postfrontal orographics should abate by ~ 12Z Sat. For the remainder of the weekend, the ARB will be under the influence of drying NW flow aloft, as ridge building commences over the eastern Pacific. Dry conditions will likely prevail through at least next Monday. Thereafter, it is unclear whether the eastern Pacific ridge will remain intact or begin to erode due to the approach of shortwave energy.

Paul Neiman

Posted by Forecast Editor at 2:09 PM | Permalink | TrackBacks (0)

Discussion for Wednesday, December 13, 2006

Another day, another half inch of precip all over the place. Good mold-growing weather.

Lots of straight flow off the ocean still causing occasional showers, some of them quite productive, over the ARB and Sacramento area. We’ve been watching a trailing cold front scheduled for Thursday evening and into Friday morning as a focus for precipitation perhaps approaching IOP criteria (1 inch for cold events, 2 for warm … although we’d settle for less). The nwp suggests that maybe ¾ inch should fall with this, probably less with the front itself than the warm/moist advection ahead of it; either way, this doesn’t look like an IOP. It should be noted that a small minority of GFS ensemble runs do have IOP weather starting after midnight Friday and lasting through Friday afternoon.

As discussed yesterday, there will be a broad trough developing along the west coast, but it doesn’t really spin up until it gets inland, and then when it does it’s too far south. Ironic for those of us who have been watching tons of precipitation fall to our north all week long. Anyway, precip associated with this development would be Saturday afternoon at the earliest, so we’ll save a decision on that for tomorrow, but right now the probability of an IOP associated with the digging trough is 20%.

Paul Schultz
ESRL/GSD/FAB

Posted by Forecast Editor at 12:43 PM | Permalink | TrackBacks (0)

Discussion for Tuesday, December 12, 2006

A low-wavenumber hemispheric pattern has positioned a broad lobe across the entire Pacific for the last several days, guiding all the (surface) cyclonic activity and pointing the firehose at Washington and Oregon. The ARB is on the anticyclonic side of all this, currently getting poorly organized but moist onshore flow, causing a few tenths of precipitation on a near-daily basis.

Finally, some ridging in the Gulf of Alaska buckles the pattern late Thursday and into Friday, setting up a hard-digging but slow-moving trough late Thursday and on through the weekend. Most numerical guidance agrees with this general scenario (ECMWF does not agree), but exactly where the baroclinic zone sets up, and where the precipitation focuses, is not clear at all right now. There are several GFS ensemble members that set up a real nice event in the ARB but others put very little precipitation in the ARB. Warm advection and onshore flow may line up for a 2-3 inch event overnight Thursday into Friday. Probability of IOP #2 starting Thursday evening is 60%.

Then, once that trough goes through, it looks like stand-down conditions for several days at least, maybe more.

Paul Schultz
ESRL/GSD/FAB

Posted by Forecast Editor at 12:52 PM | Permalink | TrackBacks (0)

Discussion for Monday, December 11, 2006

IOP 1 is in the can! Now when’s the next one?

One well-developed cyclone after another is pounding Oregon and Washington this week, offering chances of some light precipitation for the ARB in their respective warm sectors, and again with cold front passages.

The first of these is making landfall right now near Seattle; its warm sector precipitation will affect the ARB during the day Tuesday, with a total of about .75 inch over about 15 hours and snow levels above 7500 ft. This seems like a marginal IOP setup at best.

After that, a weakly organized stream of vapor meanders over the area, causing some light precipitation over the ARB every day until the next solid cold front makes landfall overnight Thursday and into Friday, total precipitation could be over an inch in a 12-hr period, snow levels getting down to 5500-6000 ft.

Some GFS ensemble members have a great cyclogenesis event in the Great Basin that would produce an excellent snow event for Reno on Saturday.

After that, a deep trough passage leaves a day or more of northerly flow and rain-free conditions for several days, this looks like stand-down weather starting Sunday.

Paul Schultz
ESRL/GSD/FAB

Posted by Forecast Editor at 12:41 PM | Permalink | TrackBacks (0)

Discussion for Sunday, December 10, 2006

IOP in progress. Precipitation estimates for yesterday and last night (24 hrs ending 1200UTC) show most of the ARB covered with .7-1 inch amounts, a small portion of the basin over an inch. Current precip seems confined to highest elevations, but there are showers in the area. Forecast models have around a half inch predicted for the area for the rest of today, none after midnight tonight.

Too bad we’re not looking farther north. The big onshore flow and baroclinic fun stuff is all aimed at western Washington and Oregon, and will be for days.

A bit of warm advection-related precipitation brings about a half inch to the ARB on Tuesday afternoon, but it’s a quick shot, less than 10% chance of IOP weather. After that, some GSD ensemble members suggest maybe Saturday…

Paul Schultz
ESRL/GSD/FAB

Posted by Forecast Editor at 11:36 AM | Permalink | TrackBacks (0)

Discussion for Saturday, December 09, 2006

IOP in progress. Radar-based rain estimate for the period ending 1200UTC today shows about a half inch fell before sunrise, pretty well in line with expectations.

Today’s precipitation in the ARB looks continuous but light, around .25 inch in the morning, another .25 inch in the afternoon. It starts to pick up after sundown, another .5 inch before midnight. It’s the 6-hr period before dawn tomorrow where the heaviest rates are expected to occur, probably over an inch. This coincides with the coldest air of the event, a 12-18 hr stretch where the 1000-500 mb thickness appears to hover around 540 dkm, resulting in a snow line around 4500-5000 ft.

Rain rates back off a little Sunday morning, and then there’s one more shot of about .75 inch Sunday afternoon, tapering off before midnight. Thus endeth IOP #1.

Onshore flow continues to the north of the area of interest through Monday and even Tuesday but there’s little chance that action slips far enough south to impact any HMT-related decisions. That band of moisture could end up making a couple 6-hr periods late Tuesday with .25-.5 inch, but big deal.

We’ve mentioned the prospects for an IOP late next week, but those chances have diminished in recent model runs.

Paul Schultz
ESRL/GSD/FAB

Posted by Forecast Editor at 8:40 AM | Permalink | TrackBacks (0)

Discussion for Friday, December 08, 2006

Yesterday’s estimate for the onset of precipitation in the ARB was early – it appears that will be in the predawn hours, maybe 0900UTC Saturday. That suggests a later sonde launch time, probably midnight.

The first of three short waves is just offshore, and should produce .5-1 inch of precipitation during the day Saturday, heaviest around noon. The rain-snow line would be 5500-6000 ft. Precipitation may not stop completely before the next short wave.

The second short wave is about 155W at 1200UTC today, and should produce another 1-1.5 inches overnight into Sunday, heaviest around midnight. This is the period of minimum 1000-500 mb thickness, bottoming out below 540 dkm, the rain/snow line down to 4000-4500 ft

The third short wave, now at about the dateline, should produce another .75 inch before sunrise Monday, heaviest in the early evening hours Sunday. The snow level goes up a little in response to warm advection with this wave, up to 6000 ft or so. Again, there may not be a break in the precipitation between these waves.

Most models agree that precipitation could continue into Tuesday farther north (Oregon), but right now it looks like things settle down in the ARB by Monday afternoon at the latest.

Still looks pretty good for another IOP is Thursday or Friday, with the potential for a nice atmospheric river event.

Paul Schultz
ESRL/GSD/FAB

Posted by Forecast Editor at 12:51 PM | Permalink | TrackBacks (0)

Posted by Forecast Editor at 1:35 PM | Permalink | TrackBacks (0)

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-12-06 12 UTC Synopsis
-------------------------
The apex of an upper-level ridge has moved overhead of ARB. A potent and fast moving short-wave trough is about 2 days west of the CA coast. IPW values are less than 0.5" along and west of the CA coast.

Current conditions in the ARB region
------------------------------------
Clear skies with fog in the valley. Temperatures in the valley are in the mid-30s. As one walks up the hill, temperatures ris into the low 50s. Blue Canyon is reported 53 at 16 UTC. ETL instruments at Blue Canyon measure 3" of snow. Bodega Bay reports IPW of 0.3", with winds directed offshore below 1-km. Upper-level winds are southerly at Oakland, and the 0C level is 650 mb, about 12000 ft.

0-72 hour forecast
------------------
A nearly ideal scenario has been predicted by the 00 UTC Dec 6 Canadian model forecast. You can view the forecast at http://www.meteo.psu.edu/~gadomski/CMC_0z/cmc60.html. Follow the forecast from 60-hours through 120-hours. It is, unfortunately, the only model solution produces a near ideal scenario. Much of yesterda's forecast for Friday is valid today. The first wave looks as though it will hit the coastline as a coherent, potent wave. There will be no precipitation prior to its arrival, as the IPW values are below 0.5" ahead of the storm. Most likely period of precipitation onset is 21 UTC Friday through 03 UTC Saturday. The most likely range of precipitation accumulation during the 18 UTC Friday to 18 UTC Saturday period is 1"-1.5". Even under a nearly ideal scenario, the Canadian model predicts no more the 2". Freezing levels will drop significantly to as low as 1500 m around 18 UTC Saturday.

The second wave has a much less certain evolution. As anticipated by the Sacramento forecast office, the models have shifted to an evolution in which two vorticity centers are active. The forecast question then becomes which center will become more dominant. The models are all over the map on that issue, providing a range of scenarios from bust to nearly ideal conditions. Most are somewhere in the middle. Observations are, of course, sparse. The latest water vapor imagery could be interpreted to mean the northern vort max is dominant, or it could not be. Going by the middle portion of the model guidance, t
he most likely precipitation range is 1.0"-2.0". Precipitation onset is most likely to occur between 03-12 UTC Sunday.

Discussion notes:

00 UTC Model projections

All GFS members have wave coming onshore between 12 UTC Friday and 00 UTC Saturday. No precipitation prior to landfall, during this period most GFS members produce 1" of precipitation. The control run produces 0.25" between 18 UTC Friday and 00 UTC Saturday, and accumulation between 06-12 UTC Saturday is 0.5".

NOGAPS produces 0.5" by 12 UTC Sat and another 0.5" by 00 UTC Sunday.

CMC brings first wave onshore just before 00 UTC Saturday with nearly perfect orientation of 850 mb height field. 12-hour precipitation accumulation at 00 UTC Saturday is 0.25" and 12 UTC Saturday is 1-2". It ends precipitation by 00 UTC Saturday.

Next wave comes in as a splitting wave with the northern wave progged to hit mainland between WA and BC and the southern wave hits at San Diego. First contact is between 00 and 12 UTC Sunday.

NOGAPS brings it ashore 12 UTC Sunday, with 0.5" in the previous and subsequent 120hour periods.

CMC brings it ashore earlier than 12 UTC, and keeps the northern wave much stronger. It produces 1" in the 12-hour period ending 12 UTC Sunday, during which nearly perfect upslope occurs.

12 UTC Model projection

NAM has a slightly earlier arrival of the first wave, but also pushes it northward. 12-hour accumulation ending 00 UTC Saturday, 12 UTC Saturday, and 00 UTC Sunday is 0.01, 0.3, and 0.05. for a grand total of almost 0.4".

>72 hour forecast
----------------

00 UTC 6 Dec (Wednesday) GFS ensemble continues to show a zonal pattern emerging after the shortwave on Sunday. This is good news in that it will permit higher IPW values into the area for a more extended period, enhancing the likelihood of upslope flow ahead of short wave troughs that would create heavy precipitation periods. The ensemble indicates IPW values exceeding 1.0" continuously from Mon through Friday. The timing of short wave troughs is tenuous in the model guidance at this forecast lead. It isn't unusual in this pattern to have short wave troughs affect the US West Coast every 2-3 days. Aside from the nonlinear dynamics of the polar jet stream, it does appear interaction between the westerlies and Subtropical convection is possible, as convection has been persistent over the past couple of weeks as far north as Hawaii. The first wave in the period 5-days and beyond is forecasted to affect the West Coast on Tuesday. A long-wave trough is progged to develop Thursday through Sunday. Of course, it is possible that shorter-wave troughs could be embedded within that long-wave pattern.

Posted by Forecast Editor at 10:45 AM | Permalink | TrackBacks (0)

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-12-05 12 UTC Synopsis
-------------------------
An upper-level ridge remains overhead of the ARB, but a potent shortwave is only a couple days away offshore.

Current conditions in the ARB region
------------------------------------
Severe Clear. Fog is reported in the Sacramento Valley. Temperatures at 15 UTC range low 30s in the Valley to low 50s in the Blue Canyon vicinity. Snow depth measured by the ETL instruments at Blue Canyon is about 3.5". Bodega Bay IPW is 0.35" with winds directed offshore at low-levels and from the northwest aloft. Level of 0C is 750MB in the Oakland 12 UTC sounding.

0-72 hour forecast
------------------
We finally have something to talk about. Bottom line is that ARB is very likely to receive >1.0" Friday night into mid-day Saturday and not quite as likely but very possible on Sunday.

Regarding the first system, the most likely time for precipitation onset is between 21 UTC Friday and 03 UTC Saturday, though an 18 UTC onset can't be ruled out. Heaviest precipitation rate should occur overnight, with precipitation almost certainly tailing off to a trickly by 21 UTC Saturday. Storm accumulation could exceed 1.5", though 1.0"-1.5" appears to be the most likely range. Freezing level might drop down to 1500m late in the storm.

The second system has a larger range of possible tracks compared to the first. There is a remote possibility the system will dive too far south to effect the ARB, but the most li
kely scenario begins precipitation no later than 12 UTC Sunday, with heaviest precipitation occurring 12 UTC Sunday through 00 UTC Monday. Precipitation accumulation could exceed 2.0", with much of that falling as snow above 1000m. The most likely range of precipitation amount is 1.0"-2.0".

Discussion concerning the guidance for these storms follows. The cannonical soaker in the ARB has an upslope stream with IPW >2.0" offshore that sets up 12 hours or more in advan
ce of a serious short wave trough. In the present case, IPW values ahead of the short wave trough are 0.5" at best. Don't expect a pre-game show, so to speak, with this storm.
Most, if not all, of the precipitation will occur as the short wave hits mainland US. IPW values will exceed 1.0", but are not the juicy values of a formal atmospheric river. The 12 UTC GFS forecasts of 6-hour precipitation rate are 0.35", 0.45", 0.25", and 0.05" beginning 00 UTC Saturday and ending 18 UTC Saturday. The 00 UTC GFS ensemble has a range o
f precipitation amounts for the 12 UTC Friday through 00 UTC Saturday period of 0.25" to 1.0" and 0.5"-2.0" in the 12-hour period 00 UTC Saturday through 12 UTC Saturday. The Ham
ill-Whitaker analogue method for the periods 00 UTC Friday - 00 UTC Saturday and 00 UTC Saturday - 00 UTC Sunday have pops for measurable, >0.5", and >1.0" of 55%, 35%, 15% and 80
%, 55%, 20%, respectively.

The precipitation forecast for the second storm is somewhate dependent on the amount of ridging that occurs ahead of the short wave trough. If the ridge is low-amplitude, upslope
flow will continue in the period between storms, so that precipitation never really ends. At this point, most of the guidance suggests a fairly high-amplitude ridge ahead of the
short-wave trough, suggesting a period when precipitation might be non-existent. The remaining uncertainty in the forecast of this second wave lies in the trajectory it will tak
e. Some NWP forecasts have it tracking south of LA. That would be too far south to really impact ARB. At this point, I would go with the mean forecast, which has it on a track
more favorable for ARB precipitation. Hamill-Whitaker pops for measurable, >0.5", and >1.0" on 00 UTC Sunday - 00 UTC Monday are 75%, 57%, and 35%.

>72 hour forecast
----------------
Beyond Sunday, it isn't clear when the next storm will affect the area. The GFS ensemble initialized 00 UTC Dec 5 (Tuesday) does have a significant wave in many members in the ARB area at 12 UTC Dec 12 (next Tuesday). However, this feature could track farther north and leave the area unaffected. If it stays on a course aimed at the ARB, it could set up a killer upslope pattern with IPW values >1.0", resulting in a heavier precipitation event than might occur this weekend. Generally, a zonal pattern impenging somewhere along northern CA, OR, WA is expected through the end of next week, meaning more than one such event is possible between Tuesday and Saturday of next week. Basically, the events this weekend "prime the pump" for what could be more intense events next week, but, of course, the evolution of storms in forecasts for next week is much less certain than this weekend.

Posted by Forecast Editor at 11:50 AM | Permalink | TrackBacks (0)

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-12-04 Synopsis
-------------------------
Upper-level ridge resides over ARB. The amplitude has been reduced a bit by a shortwave crashing into WA, British Columbia. The next significant upper-level trough is passing south of the Aleutian Islands.

Current conditions in the ARB region
------------------------------------
Clear skies reported at 21 UTC. Tempreatures range from low 60s in the Valley to low 40s at higher elevation. Snow pack at Blue Canyon ETL site has been reduced to 4.5". Front is moving through southern OR.

0-72 hour forecast
------------------
Weather will remain uninteresting until Friday. Dry frontal passage is expected tonight through tomorrow, resulting in a slight uptick in IPW values at Bedoga Bay.

>72 hour forecast
----------------
Potent short wave forecasted to affect the ARB region on Friday, Dec 8. A wide swath of IPW >1.0" has moved into place ahead of this trough that is now situated south of the Aleutian Islands. UK and CMC move the wave westward at a slightly slower speed than GFS; whereas, the GFS ensemble is in near unison that precipitation will begin Friday, CMC and UK push it back to Friday night into Saturday.

From Hamill-Whitaker, probability of measurable, >0.5%, and >1.0" rain 00 UTC Dec 8 - 00 UTC Dec 9 (Friday) are >40%, ~25%, and ~15%. All but one GFS ensemble members have precipitation in the range of 0.5"-1.0" during that period. In the next 24-hour period, 00 UTC Dec 9 to 00 UTC Dec 10, pops for the same thresholds are 25%, 20%, and 10%. Pops during 00 UTC Dec 10 through 00 UTC Dec 11 are 35%, 30%, and 15%.

It appears more than one wave will affect the ARB Friday through Monday. The IPW values may not exceed 1.5", and, so, these are not expected to be extremely heavy rain events with 24-hour amounts more likely to be in the 0.5"-1.0" range and amounts >1.0" to be isolated. Most likely period of precipitation onset is Friday afternoon.

Posted by Forecast Editor at 2:58 PM | Permalink | TrackBacks (0)

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-12-03 12 UTC Synopsis
-------------------------
An upper-level ridge has moved over head. Winds are light northwesterly from near the surface upwards. A shortwave trough is evident in the Gulf of Alaska.

Current conditions in the ARB region
------------------------------------
Severe Clear. Snow depth measured by the ETL instruments at Blue Canyon is steady at 5.2". The 1"-6" is the most common report of snow depth in the ARB by SNOTEL measurements. fog is reported in the Sacramento Valley. Temperatures are in the low 40s from the Valley up the slope of the slopes of the Sierra.

0-72 hour forecast
------------------
Expect a dry frontal passage Monday night into Tuesday with the most noticeable impact being an increase of IPW values reported by Bedoga Bay.

>72 hour forecast
----------------

The northern Pacific regime shift is underway. A broad trough now extends across much of the north Pacific from Russia to the Bering Sea in the latest GFS analysis. A potent shortwave trough is progged to travel south of the Bering Sea on Tuesday, establishing a persistent flow of cold air and troughing in that region for a week or so.

Though the first system affecting the US West Coast after the regime shift is expected on Thursday, the odds are that it won't appreciably affect ARB. Thursday is now the Day 5 period in the Hamill-Whitaker anologue forecast technique. Their technique provides calibrated probability of 24-hour precipitation exceeding selectable thresholds. Probability of measurable rain in the 24-hour period 00 UTC Thursday through 00 UTC Friday is about 10%. On Day 6, that is 00 UTC Friday through 00 UTC Saturday, pops for measurable, >0.5", and >1.0" are 30%, 20%, and 10% with the actual frequency of occurrence given these pops being 30%, 15%, and 5%. On Day 7 (00 UTC Saturday through 00 UTC Sunday), pops of measurable, >0.5", and >1.0" are nearly 40%, 25%, and nearly 20% with actual frequency of occurrence being 37%, 22%, and 10%.

The climatological robability of >1.0" is about 15%. Since the probabilities of >1.0" on Day 6 and Day 7
are slightly less than climo, these storms are not expected to produce unusually heavy precipitation amounts, although they are the most potent thusfar of the HMT campaign.

Posted by Forecast Editor at 8:34 AM | Permalink | TrackBacks (0)

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-12-02 12 UTC Synopsis
-------------------------
An upper-level ridge is entering the ARB region, producing northerly flow from near the surface upwards.

Current conditions in the ARB region
------------------------------------
Dry. At 16 UTC clear skies were reported across the ARB with temperatures in the low 40s. A few fog reports were seen along the coast and in the Sacramento Valley.

0-72 hour forecast
------------------
Dry.

A shortwave trough is expected to move over the NW Monday into Tuesday of next week. The weather conditions in the ARB resulting from that trough are likely to be dry. A dry frontal passage is expected on Tuesday.

>72 hour forecast
----------------
The long-range guidance continues to forecast a large-scale pattern change over the northern Pacific, createing a more zonal flow that would guide upper-level storms on a more direct path to California and tap into Tropical moisture. This shift in the polar jet stream is already under way. Satellite imagery corresponds well with the 12 UTC GFS analysis in showing a massive trough moving off the Asian continent, which will pull the the main center of long-wave troughing eastward over the northern Pacific.

Details for ARB operations are still very uncertain: 4 days, 4 different forecasts of where the first storm will hit. Today's GFS forecast targets the OR, WA and northward. Thursday remains the earliest possible date for a storm to hit ARB, and it is likely that more than one will affect ARB in the week following Thursday. Probability of measurable rain valid 00 UTC Fri - 00 UTC Sat next week is about 35%, using Hamill-Whitaker's analogue forecasts, and pops >0.5", >1.0" is 35%, 15%, respectively. The actual frequency of measurable, >0.5", >1.0" for forecasts of 35%, 35%, and 15% are 35%, 30%, and 10%. Positive skill exists in this forecast method with this forecast lead in this region; in other words, it's likely these forecasts are better than one based on climatology.

Posted by Forecast Editor at 10:51 AM | Permalink | TrackBacks (0)

Forecast prepared by Chris Anderson, ESRL/GSD/FAB

2006-12-01 12 UTC Synopsis
-------------------------
A surface front is approaching the Sacramento Valley carrying with IPW values at Bedoga Bay exceeding 0.6". Westward a large ridge is knocking on the door of ARB.

Current conditions in the ARB region
------------------------------------
Dry. Clear skies reported across ARB despite the approaching front and elevated IPW values as 700 mb flow remains nearly parallel to the coastline and Sierra Mountain range. Temperatures in the low 50s in the Valley to low 40s around Blue Canyon.

0-72 hour forecast
------------------
Dry.

A shortwave trough is expected to move over the NW Monday into Tuesday of next week. The weather conditions in the ARB resulting from that trough are likely to be very similar to today's.

>72 hour forecast
----------------
Again, the long-range guidance is showing consistency with previous runs in bringing about a large-scale pattern change over the northern Pacific such that a more zonal flow would stretch from Asia to nearly the US coastline. Ensemble spread in the northern Pacific is relatively small, lending more confidence to the timing of this pattern shift. Details for ARB operations are still very fuzzy, but it appears next weekend is a likely period for heavy precipitation in the ARB. There is an outside chance operations may begin Thursday. That would seem to be the earliest possible storm.

Posted by Forecast Editor at 1:16 PM | Permalink | TrackBacks (0)