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February 28, 2006

2006-02-28 WX Discussion and Forecast

Light echoes in the ARB this morning, 40 dBz tapering down to 20 over the higher terrain. There appear to be numerous echoes still on and off of the coast and these may still portend some continued precip through out the day over the ARB as they progress inland. The models indicate that the event should begin winding down with probably only lingering showers past 06h utc Wednesday.

This morning HMT run shows precipitation occurring up until 06h 3/1 utc after which it falls off rapidly, so this high resolution run also corresponds well to the NAM and GFS.

The forecast question of the day is really the onset of the next event and its forecast duration.

The NAM has the approaching precip band with the progressing closed upper level low moving inland by 06h 3/2 (Thurs). The first precipitation in the ARB from both NAM and GFS appears to begin about 09h 3/2 utc. Of the two, the NAM is slightly stronger with precipitation out to 12h 3/2 with a lull for a few time periods and picks up at the end with an additional 0.2 inches in the final 3 hours. As far as the NAM extends, it shows a total QPF of 1.58 inches between 09h 3/2 and 06h 3/3 utc.

The GFS begins the precipitation about the same time, likewise with a strong surge of moisture. Both models tend to show bands of precipitation that have been entrained in circulation around the low pressure system. As each of these bands moves over the ARB the precipitation appears to rise and fall accordingly. There is an initial strong precip event at 12h 3/2/06 utc much like the NAM followed by a tapering of precip until 9-12h 3/3 when there is another surge. This is then followed by a complete lull until 03-06h 3/4/06 utc afterwhich we see 0.84 inches in a 6 hour period. The event continues until about 09h 3/4 utc (Sunday). It is followed quickly by another system. Total QPF for the GFS for this system through Sunday is 2.82 inches with 1.14 inches falling in the same time frame as the NAM forecast termination (which is comparable).

The wind field is not as strong as this current system with winds staying mostly south at 20-30 kts and an upper level wind shift passing about 12h 3/3 utc possibly associated with the decrease (lull) in precipitation noted in the model. However, this feature has little affect at the surface being evident only above 600 hPa. After this upper wave passes it appears that precip at the surface resumes until the moisture in the system moves through. The GFS never runs far enough out to show a good surface wind shift associated with fropa.

The freezing level with this next system appears to start the episode at 850 hPa just above mountain top level and then drops to mountain top level for the duration of the event. It is definitely not as cold a system as was forecast a week ago, nor does it look as interesting to me as the event that is currently taking place.

Dan Birkenheuer - ESRL/GSD

February 27, 2006

2006-02-27 WX Discussion and Forecast

The precipitation is well underway in the ARB this morning with radar echoes in the region up to 40-45 dBz just north of the ARB as of 1545 utc.

The moisture plumes are arriving as progged in two sets. The first and heaviest is now interacting with the terrain and the second will arrive about 06h 2/28 utc. The first plume is roughly 2.5 cm TPW which increases to 3cm between 18h 2/27 and 3h 2/28. Then the second plume comes in from the NW to provide 2.4 cm of moisture for the tail end of the vigorous part of this precip event. The precip max for this event appears to be modeled between 18h 2/27 though 15h 2/28 utc. However, RH depth on model cross sections does not appear to be as deep as they did last week.

The wind field again is following prior forecasts with the NAM showing mostly southerly winds, ranging between 30-50 knots at times. At 9h 2/28 there is a fairly good wind shift from due south to more favorable SSW direction until 18h 2/28 at which time the winds turn WSW and the lighten. This probably coincides with the end of the "event" since at this same time the GFS modeled moisture plume is exiting the region moving to the south. Even so, as shown earlier last week in the GFS, the NAM today has precipitation continuing through the week with light amounts between the systems. Between 12h 2/27 and 0h 1/3 the total NAM QPF is about 4.85 inches (what I will call the "main event" here). Following this, light precipitation continues accumulating to 0.7 additional by 6h 3/2 and then another 0.88 inches by 3/3 utc. During the main event, the snow accumulates to approximately 40-53 inches in different areas of the higher terrain.

The freezing level again is following prior guidance with its mountain top presence for the major part of the heavy precip event starting there at 6h 2/27 and then falling later to 850hPa by 12h 2/28 utc. It then remains at that level for the duration of the NAM run well into Thursday afternoon local time in the ARB.

Dynamics from the NAM show the best PVA at 6h 2/28. The nature of the dynamics appears to be in waves that wax and wane throughout the event with this time period perhaps being the strongest.

The GFS model is very similar in most respects to the NAM on the first event with an almost identical QPF between 12h this morning and 0h 3/1 utc with 4.28 inches equivalent liquid. Following the first event, the GFS continues to have more of a lull, however, there now appear to be 2 moist surges coming into the ARB with the approaching closed upper level low. At this time the next event appears to begin at 9h 3/2 and continue at least to 00h 3/4 (Saturday) with the greatest precipitation maxes being at the start of the event and then a second max ending 12h 3/3 utc.

Unlike the earlier forecast runs, the temperature with the second system does not appear as extreme with the freezing level starting at about 850 hPa (where the first event left off) and then drops it to about 900 hPa at 18h 3/3 after which the level rises in altitude. Also at this time the GFS shows a SSW 30 knot wind field for the duration of the second event. The wind field never really veers to indicate Fropa between events one and two this week.

The first event is pretty well progged as far as its intensity and start and stop other than the NAM continues light precip between the two events. Both models seem to disagree slightly on when the second event begins with the GFS starting sooner and the NAM may starting the second event as late as 0h 3/3, but that time also coincides with the last panel I have for the NAM.

So looking at GFS ensembles for a maybe a better resolution of this question on the second event, it appears to be underway by 12h 3/2 utc with all panels showing precipitation, the event appears to end Saturday 12h 3/4 utc with all forecast panels dry.

Dan Birkenheuer - ESRL/GSD

February 26, 2006

2006-02-26 Wx discussion and Forecast

Major features at 12Z this morning were the upper trough, axis from near 50N/138W
to 30N/146W, and strong surface low near 37N/132W and tracking NNE.

However, this doesn't tell the whole story. There are lots of details. The trof offshore has at least 2 main pieces. A short-wave feature near 35N/130W at 15Z this morning was clearly associated with the main surface low off Nrn CA. Of significance is a large area of open cellular convection with cloud-top temperatures of -40C or so, between 30 and 40N, 142-149W at 15Z this morning. This has begun to turn cyclonically after heading mostly south earlier, and is currently on an ESE to E trajectory. It is also elongating and there is emerging a vorticity center estimated to be near 31N/145W at 2000Z. Beginning about 15Z there began formation of Ci to the E of this feature, and that has now expanded considerably, sgstv that there is appreciable dynamics (i.e., synoptic-scale upward motion) with this, separate from the main surface low and upper wave farther NE mentioned above. When I made a strenuous effort to do so earlier, about 16Z, I couldn't detect any separate cyclonic flow center at the surface with available data (tracking of low cloud tags, ships, buoys, etc.), but there
is clearly cyclonic shear in the sfc flow to E of the large patch of deep, open-cellular convection, probably concentrated along the surface front.

Closer to HMT, there is evidently good surface confluence just offshore Nrn and central
CA as the occluded/cold front approaches the coast. It should be noted that at the moment there is dry air over all CA, with dew points in the 40s at the sfc (35 at KACV!), and mostly offshore component to the surface flow along the coast also. However, GOES PW from RAMSDIS showed very strong precipitable water gradient just offshore and along the coast at 16Z. (This area has since become obscured by high clouds.) So, the picture emerges of a narrow wedge of high PW alg and just ahead of the cold front approaching the coast, with associated pcpn, and a wind shift behind the front into the WSW and decrease in wind speed.

At 16Z the pcpn offshore appeared to be in 2 batches: one from W of the Bay Area S
to Piedras Blancas, and the other from Pt. Arena to the CA-OR border. The southern of these has since moved NNE into the central coast and Delta areas, roughly with the 700mb flow. This morning the 12Z raobs had the freezing level near 700mb, but the wet bulb zero near 850, reflecting a very dry layer from near the surface to near 700mb. This sgsts to me that the snow level in the HMT may be closer to 5,000 ft with this IOP. There is also the likelyhood of significant evaporation initially that may delay the onset of significant surface pcpn. In the Valley, the surface dew points are highest in the rainy areas (e.g. the Delta), indicating this evaporation is already having a notable effect.

I believe the NAM may be 3-h or so slow with the onset of the pcpn over HMT, which I think should be underway by 03Z this evening. However, synoptic forcing of upward motion appears to not reach its peak till tomorrow.

Of phps more interest is what the NAM is doing with the overall sitn. It shows sfc wave formation offshore (near 30N/130W) tonight, and a new deepening surface low moving toward Nrn CA. The GFS is basically similar, but slower and slightly farther SE with the track of this wave. I am inclined to slightly favor the GFS in its handling of this feature, based on experience. This development will retard the eastward movement of the front now off the coast, setting up what looks to be a 24-30h period of pcpn over the HMT, with heavy pcpn ending before 12Z Tu as this new surface low moves rapidly ashore OR or Nrn CA and weakens. Frontal passage by GFS reckoning is anticipated to be about 08Z Tu (midnight LT), but I think this can at this point be considered only +/- 3h or more. Heaviest pcpn appears likely between 20Z Monday and frontal passage. Following the front snow levels will drop to 3500 to 4000' as low level flow continues with an upslope component and much colder air moves in aloft. However, synoptic forcing is likely to steadily diminish after frontal passage and be nil by 18Z Tuesday - 00Z Wednesday.

Then, there is a break of 36h or so before the next, colder system begins to affect the HMT. I expect this beginning will be late Wednesday, based on GFS indications.

John B.

February 25, 2006

2006-02-25 WX Discussion and Forecast

A large moisture plume is progged on the GFS that is oriented near N-S at 135W to move the upper latitude moisture max into the CONUS west coast. The plume consists of two distinct parts. A very moist potion that extends up to 35N and a uniform PW section with lower moisture that extends from that location all the way to the Gulf of AK. Both parts of this moisture system will be interacting with the ARB in some form or another with the first plume moving east and reaching the ARB 00h 2/27 utc with amounts of 2cm, lasting as long as 06h 2/28 utc with increasing moisture seen during the episode (the peak of which is about 3.0cm. After this time the plume drifts south and another remnant of the weaker moisture initially associated with the same plume passes the ARB from the north 18h 2/28 with PW levels on the order of 1.8 cm.

GFS ensembles put about a 50% chance of the event beginning before 00h 2/27 utc Monday. Ensembles also show peak precipitation prior to 00h 2/28 and ending or tapering off significantly after 00h 3/1 utc.

The UKmet has the system starting a bit later than progged yesterday with the ARB receiving light amounts between 18h 2/26 and 00h 2/27, but not really getting going until after Sunday evening local time. The Canadian model still has the system coming in a bit earlier and is more consistent with its yesterday's run. In keeping with this scenario, the Canadian model has the best dynamics coming into the area sooner beginning not too much after 06h 2/27 utc.

The high resolution HMT 3-km run is still consistent with light precipitation starting about 3h 2/27 with rates really increasing after 06h 2/27. 1.0 to 1.5 inches of precip are forecast to have fallen by 18h 2/27 utc with the highest amounts near North Fork.

Total QPF from the NAM between 03h 2/27 and 15h 2/28 utc is 5.85 inches. Total QPF from the GFS between 00h 2/27 and 06h 3/1 utc amounts to 6.31 inches. So by both accounts this appears to be a good precip producing event. The GFS total snow accumulation in the higher terrain from this one event appears to be 20-30 inches.

Winds and temperature fields remain very similar to yesterday's forecasts with the winds primarily out of the south with speeds ranging from 30 to 55 knots in a deep layer of the atmosphere. The GFS does offer a brief veering to SW directions at 12h 2/27 utc, but this persists for only about 6 hours before the winds back again to the south. The temperature fields appear to begin with a very high freezing level (650 hPa this morning) with it descending to mountain top level at 9h 2/26 utc where it remains pretty much for the duration of the event. Then prior to the next and cold system coming in at week's end, the freezing level drops to about 850 hPa 00h 3/2 utc and rapidly extends to the valley floor 6 hours later setting the stage for a very snowy second storm.

This morning the GFS (along with the UKmet) looks to be more in line with the NAM and HMT runs as far as the onset of the event about 03ut Monday. The NAM and GFS have the event ending at about 03-06h 3/1 utc with minimal precipitation occurring after 00h 3/1.

It now appears that the next system will begin roughly at 6h 3/2 and run through 12h 3/3 utc.

Dan Birkenheuer - ESRL/GSD

February 24, 2006

2006-02-24 WX Discussion and Forecast

Today's model runs are not as exciting as yesterday's runs. The basic timing for the onset of the event is about the same with both the NAM and GFS starting things in earnest by 03h 2/27/06 utc however, the GFS has more precip by 00h (0.15 inches) starting sometime after 18h, a bit earlier. Also today the GFS appears to create a distinct gap between the two systems described yesterday with the warm event ending sometime on Tuesday and then a second system starting later in the week mainly driven by the approaching cold air. This appears to get started at about 18h 3/2/06 utc with minimal precip between the events (maybe scattered).

Another difference today, both models do not have the wind direction as favorable as yesterday with both models showing South winds at 30-40 knots instead of the preferred SW direction. The GFS does have the winds shifting to the SW near the end of the first event at about 00h 2/28/06 utc.

Today the ensembles are not as enthusiastic about the probability of precip after 15h 2/28 utc. Precipitation totals for the first event are not fully covered by the NAM model run which has a total QPF of 2.42 inches up until 00h 2/28. The GFS has a run total QPF over the ARB of 4.62 inches between 18h 2/26 and 21h 2/28. The GFS shows the greatest intensity of precipitation falling between 06h and 12h Monday 2/27 with a secondary peak between 18h to 00h 2/28. The GFS moisture plume forecasts do not show as much promise for deep moisture making its way into the ARB compared to yesterday.

The freezing level begins the run pretty high at about 750hPa at 12ut today. It drops to mountain top level 9h 2/25 utc and then remains there throughout the first precipitation episode but drops to about 850hPa at 06h 2/28 utc. The level then drops rapidly to near valley floor levels after 18h 3/2 associated with the arrival of the very cold system discussed yesterday that is forecast to arrive late next week. Even with the warmer environment, the forecast snow totals for the GFS are on the order of 30-40 inches in the high terrain areas by the end of the "first event."

Since the GFS had the event starting earlier than we had discussed yesterday, I examined some of the other models for guidance on this key issue. The new GSD HMT run (we have made some modifications as to the 3km forecast duration), shows precip beginning in the ARB at 03h 2/27 utc, which matches today's NAM. The Canadian and UKmet models also hint at an earlier start perhaps around 18h 2/26, more in line with the GFS.

Dan Birkenheuer - ESRL/GSD

February 23, 2006

2006-02-23 WX Discussion and Forecast

This morning's NAM run shows a flattened 500 hPa flow pattern initially that buckles into a ridge with the axis over the CONUS west coast at 12h 2/25/06 utc (Sat). The trof behind the ridge then progresses eastward bringing with it precipitation that finally reaches the coast of CA by 18h 2/26/06 utc. So in the immediate future, there is nothing of real consequence so the major part of this discussion will focus on the moisture patterns evolving early next week that at this time look quite promising for an interesting episode over the ARB.

The GFS PW moisture plume situation (via the U of Hawaii website) shows that the system progged for early this weekend does not have a solid connection to tropical moisture but over time this evolves. At 6h 2/28/06 utc a major tropical plume originating at about 135w moves up the coast to merge with the smaller surge of moisture that we see arriving at the coast this weekend. This surge of moisture looks significant with PW values of ~4.5cm at its core. It briefly sweeps up the coast and the northern edge of the plume reaches into the ARB on about 00h 2/28/06 utc after which it swiftly drops south and out of the area. Following this brief surge, the long range GFS moisture forecast looks pretty grim for any additional significant moisture arriving until 06h 3/8/06 utc when a more anemic band of moisture moves inland with max PW values in the 2.8cm range. A long stretch of moisture that spans the northern Pacific after traversing high latitudes finally reaches the CONUS by the last frame of the GFS moisture run on about 6h 3/11/06 utc, with again light moisture values on the order of ~2.2cm. At the end of this current model run (last night's initialization), this last plume does not make it to the ARB but looks like its trajectory will bring it in inland within about 12h of this time.

The GFS model run has a weak wave over the ARB by 12h this Sunday. Supporting the re-enforcing moisture scenario, at this same time the greatest precipitation areas remain out over the Pacific. The real action begins between 06 and 12h Monday with a good QPF area progged over the ARB. The thickness associated with this event is pretty warm at 552. This continues until about 18h Tuesday 2/28/06 utc and is followed by a short interval of decreased scattered QPF. By Tuesday the thickness falls to the 546 range. By 06h 3/3/06 utc the cold system coming in from the NW Pacific backed by a 522 thickness core reaches the ARB with additional precipitation and a cold thickness of 534. Precipitation increases with this push of cold air and the event appears to finally end sometime Friday afternoon 3/3/06.

A quick look at the wind field associated with this system and RH cross sections along with temperature has the scenario playing out as follows:
12h 2/27/06 utc south winds at 30kts very deep moisture; this continues SW winds 20-30 kts 12h 2/28/06 utc after which there is a moisture advection lull. Resumed good moisture flow at 00h 3/1/06 utc winds SW 40 with a second but weaker moisture surge; 06h 3/1/06 utc winds SW 20 continuing to 18ut 3/1/06 utc with the winds dropping to 25 kts but still out of the S to SW. Then there is another moisture lull as the cold air approaches with the colder precip event starting 12h 3/2/06 utc until 06h 3/3/06 with SW winds at about 10-15 kts. So the initial event earlier in the week has stronger winds but the wind field with both precip episodes is from a favorable direction. Temperatures begin with the freezing level at about 800 hPa, dropping to 850 at 18h 2/28/06 utc, then dropping further to 900 6h 3/5/06 utc by the event's end.

Ensembles are also very favorable with the storm providing the following probabilities of precipitation:
0.75 12h 2/27, 0.83 00h 2/28, 0.91 12h 2/28, 1.0 00h 3/1, 0.91 12h 3/1, 0.41 00h 3/2, 0.91 00h 3/3, 1.0 12h 3/3, 0.75 00h 3/4, 0.25 12h 3/4, all times UTC.

It looks like a very active scenario coming up with the integrated QPF for the entire week working out to 6.7 inches of liquid with up to ~40 inches of snow accumulating in the higher terrain from this one event.

Dan Birkenheuer - ESRL/GSD

February 22, 2006

2006-02-22 (Wed) WX Discussion and Forecast

When does precipitation return to the American River Basin? The next good chance is associated with a digging trough off the west coast, with conditions favorable for moist onshore flow developing Monday at the earliest. Yesterday it looked like Monday was actually a pretty good bet, but many ensemble members don’t have the precipitation in place until Tuesday. Probability of next IOP weather before Monday is less than 1%, on Monday is 25%, on Tuesday 65%, after Tuesday 10%.

Paul Schultz

February 21, 2006

2006-02-21 (Tue) WX Discussion and Forecast

At last a nice-looking trough is appearing in the numerical guidance (COAMPS, GFS ensemble), moving close enough to the California coast to bring a good chance for an IOP on Monday, 27 February. There is an outside chance, at best, that upcoming model runs might indicate earlier landfall and the event starts Sunday, but right now Monday is the best target. Furthermore, most GSF ensemble members and COAMPS agree that much of next week will be wet in the ARB.

Paul Schultz

February 20, 2006

2006-02-20 (Mon) WX Discussion and Forecast

Northern California is in diffluent flow on the anticyclonic side of the jet through Friday, without even a whiff of precipitation anywhere nearby. There aren’t any model runs that offer any hope of precipitation in the vicinity until Sunday, 27 February.

But wait, it gets worse. If that weekend trough in digs and closes as far west as it appears in the current model runs, it could be 10 days or more before there’s any precipitation in the American River Basin.

Paul Schultz

February 16, 2006

2006-02-16 (Thu) WX Discussion and Forecast

Continental-scale trough oriented SW-NE runs the show through the weekend. Trough axis is north of the HMT area; main baroclinic zone runs through southern Oregon, across southern Idaho, northern Utah, central Colorado. Short waves rotating around the trough focus precipitation events, but they’re pretty weak and hard to identify.

Numerical guidance suggests pcpn in the ARB begins weakly Friday afternoon, as much as ¼” by sundown in some ensemble members; probability of this much is less than 25%. Overnight the snow level descends from 3500 ft to maybe 2000 ft by sunrise Saturday. The best flow for orographic enhancement appears to be in the predawn hours, probability is about 50% for 12-hr precipitation of .5” in ARB. Low-level flow is weak and poorly organized w/r/t terrain. During the day Saturday the flow remains weak and not helpful for orographic pcpn, probably light amounts off and on all day long, maybe even into the night hours. Total precipitation for the basin is probably less than 1”.

After that, a lobe of the trough breaks off toward the southwest, leaving the target area in split flow/col configuration that appears to prevent ARB pcpn for several days.

Paul Schultz

February 15, 2006

2006-02-15 forecast

Indications from this morning's data are that the event forecast yesterday for Friday and Saturday is coming both faster and drier than anticipated. The first short wave discussed yesterday and now over Northern NV and ID is clearly going to be the strongest of the sequence of waves that has or will advance from the north over or offshore of the Northwest, thence tracking southeast and east as they round the mean trough position over the West. The second one has already crossed the BC coast on its way southward offshore, with a third one close on the heels of the second. The numerical guidance (GFS from 12Z this morning in particular) is not showing sufficient cyclogenesis with this
combination to induce much shoreward-component flow across the coast of central and northern CA. Consequently, expected pcpn totals (particularly the GFS) are down from the one inch or so that appeared likely from yesterday's guidance. From what I have seen of the ensembles, there is some, but well under 50%, chance that this is too pessimistic a view, with 2 or 3 members moving this second short wave noted above more slowly southeastward than the consensus, and giving 1" in 24h in parts of northern CA (not necessarily over HMT).

The system is still anticipated to be cold, likely the coldest of the year, with pcpn mainly weakly convective due to cold temperatures aloft and the combination of weak orographic and synoptic forcing as the short wave trough approaches and moves overhead.

Beyond Saturday/Sunday, there are no clearly likely pcpn events on the horizon.

February 14, 2006

2006-02-14 forecast

Overall, things remain on track with the amplification-evolving-to-blocking
pattern currently underway. The first short wave trough to plunge SWwd out of Canada to off the
West Coast is currently over the Northwest and offshore and has a nice patch of open cellular convection with it off WA this morning, reflecting the very cold air (Quilliute -38C at 500mb 12Z this morning). I expect only weak cyclogenesis offshore with this shortwave trough aloft as it dumbbells southward down the coast. With the overall dryness of the airmass, and absence of any significant warm advection due to only weak cyclogenesis, only light, spotty pcpn can be expected out of this system as it swings inland and across the northern Great Basin over the next 24-36h.

Of more interest is the potential event for Friday and Saturday. The chief merit of this feature is it's
coldness: snow levels are likely to remain below 3000 ft, and could drop to below 2000 ft, depending
on the timing and track. This will be associated with the second of the short wave troughs to advance
southwestward from central Canada to off the West Coast. The key to both timing and amount of pcpn is how far offshore this system gets before turning cyclonically about the mean trough position over the Northwest. In general, I would say the farther it gets offshore, the later will be the onset of
significant pcpn, and the greater the pcpn totals. A greater oceanic trajectory will allow more time
for moisture accession from the sea and a better chance of a significant patch of warm advection preceeding the surface low as it comes ashore. Right now, I don't have a lot of confidence of the timing; sometime between Friday morning and evening is possible for the onset of significant pcpn.
I do believe there is a 50% chance of 1" melted or more at Blue Canyon over the duration of this event.

It appears that this event will end by late Saturday afternoon or Saturday evening as the
trough/upper low shifts southeastward and then eastward.

The outlook for early next week is uncertain; I cannot rule out the possibility of another pcpn event
for the HMT by Tuesday 21st or so.

John B.

2006-2-13-Forecast

Things seem pretty much on track from yesterday. Complex blocking pattern and retrogression now developing appears likely to produce a pcpn event over HMT area Friday/Saturday. This will come about as strong blocking high now building over Alaska and the Northwest Territories of Canada forces short-wave troughs on a trajectory from the Arctic south and southwestward across western Canada and out over the Pacific west of BC and the Northwest. At least 2 or 3 of these are forecast. The first of these is indicated to initiate a weak surface low offshore of the Pacific NW on Wednesday, with this low slipping southward as the short wave trough aloft swings cyclonically around the mean trough position over the Northwest and thence across northern Nevada.

With the massive buildup of the Pacific High into the Gulf of Alaska, the predominant flow offshore is
forecast to be from the north bringing arctic air offshore that has been previously modified by passage over the Canadian Rockies and coastal mountains. This air is too dry to produce much pcpn, and the cyclogenesis forecast by the models with this first short wave is so minimal that even a significant error in the guidance in the strength of this low would not favor more than a few mm over the HMT domain from this first short wave.

The subsequent wave is forecast to swing farther offshore, and shows more promise of initiating
cyclogenesis offshore in an air mass that has had another 24 - 36h to pick up moisture and heat
from the Pacific. About half the GFS 12Z ensembles visible on the Penn State Map Wall indicate appreciableoffshore cyclogenesis with this in the general vicinity of 40N/130W, the other half suggesting a more rapid southward plunge of this system with less pcpn. My confidence is fairly high that the trough will in fact swing offshore far enough to initiate cyclogenesis, with subsequent E-SE movement of the surface low and a modest (50% chance of > 1" melted to Blue Canyon) Friday and Saturday, 17-18 Feb.

This will be a fairly cold system, with the potential for good warm advection from the Pacific over the highly modified arctic air that will infiltrate southward into the Sacramento Valley behind the cold front associated with the first short wave trough Wednesday and Thursday. Anticipate snow levels initially near 2,000 to 3,000 feet, perhaps rising a little with the warm advection, but descending again as
the upper trough passes over on Saturday. Details of timing needs must await later information.

John B.

February 12, 2006

2006-2-12-Forecast

Trend toward troughing in the West and eventually off the West Coast still looking good. However, evolution is complex and appears to be favoring a delay in significant pcpn till late this week at the earliest...I see nothing significant before Friday at this point, and this is problematic.

The basic long-wave evolution scenario that Ed Szoke noted on 10 Feb is still anticipated. The bitter cold Arctic air in the Far North is approaching Hudson's Bay at the moment, but the main initial thrust of this will be into SE Canada and the Maritimes. A second thrust at midweek is anticipated to carry
the cold air westward into BC and offshore and into the Pacific Northwest, albeit with considerable modification. This is partly in response to cyclogenesis in the Western Pacific and ridge building into Alaska with the warm advection ahead of the deep cyclone as it moves into the Bering. Strong pressure rises are occurring in the western Gulf of Alaska at present. It appears that the trough
presently north of Hawaii into Alaska will split, with the southern portion cutting off N of Hawaii and the northern portion moving ashore BC tomorrow and Tuesday, then becoming part of the troughing over BC and the Northwest midweek. With widespread cold advection at low levels offshore and a trajectory from the far North, any onshore flow that does develop as result of near-shore cyclogenesis in association with these events and the overall retrogression of the western trough is going to be quite dry, making any significant pcpn in HMT (or anywhere along the West Coast) unlikely before Friday at the earliest.

However, with the the progs indicating continued retrogression over North America and Eastern Pacific, and the possibility of a major trough setting up off the WestCoast by next weekend, I believe chances are good for an event in the HMT sometime in the Sat 18th to M 20th time frame. Such a trough may well entrain the anticipated cutoff low N of Hawaii noted above (and associated moisture) into its circulation, creating a decent moisture plume into the coast and a nice pcpn event. I'll be attentive to this possibility next 2 days.

John B.

February 11, 2006

2006-2-11-Forecast

The possibilities noted by Ed Szoke yesterday for mid-late next week remain. A very cold airmass is in the Canadian Far North and headed southward, with indications in all the global forecasts that I examined (GFS, ECMWF, NOGAPS) that part of this will head SW into BC, AB and eventually into the Northwest of the US.

Preceeding this is a massive buildup of the Pacific High nwd into the Gulf of AK, to central pressure > 1040 mb. This will preclude any onshore flow along the CA coast attending the next 2 weak systems aloft drifting onshore the next 3 days, and no pcpn greater than a few mm is expected in the HMT small domain before Wednesday.

My experience over the years with the type of situation shaping up for the second half of next week is that more often than not the models tend to underdo the SW extent of the upper troughing aloft and induced surface cyclogenesis near shore with these cutting back systems. On the other hand, as resolution has improved, these errors have become less serious. The best hope for something really interesting is that the upper ridge will evolve into a high-latitude block and cutting back of the flow
to the SE of the block will be strong enough to permit bridging of a southern branch underneath and into the West Coast. (If memory serves me correctly, this is part of the evolution that led to Feb '86, a month that is ingrained in the history of many of us.) However, something of this general sort will take additional time to develop, if it occurs at all. Right now, none of the models are indicating such a development except one or 2 of the GFS ensemble members at > 10d range.

Of more immediate concern are the prospects for near shore cyclogenesis
off OR or Nrn CA W or Th, and a pcpn event as this system moves inland. I don't have a high degree of confidence pro or con regarding this at the moment.

John B.

February 10, 2006

2006-02-10 WX Discussion and Forecast

Sharp high amplitude ridge continues to be found right along the West Coast extending all the way to Alaska keeping the HMT area dry. The longwave pattern around the hemisphere is dominated by an expansive upper level low over eastern Asia into the Pacific and another over eastern North America. A strong jet extends across the Pacific and the latest wave to break off of that flow is now near 135W but is weakening and lifting north as it hits the West Coast ridge and as was noted yesterday is not expected to bring any precipitation of note this weekend...although by Monday what is left of this system forms a weak small-scale upper low that drifts southeast near the HMT region perhaps with a few showers around but without moisture of note present.

Another wave in the strong Pacific flow follows the one noted above and splits well off the coast with a piece of the wave heading south and closing off near Hawaii. This may have significance for later next week as it will allow for moisture to build near Hawaii and start advecting northward. Whether this becomes a potential factor later next week and beyond will depend on the evolution of the overall upper level pattern which is forecast to undergo a significant change with the retrogression of the trough position in North America that should push the upper level ridge off the West Coast by midweek (Tue/Feb 14). As is explained below, this gives a threat beginning next week, so this discussion is far less pessimistic than yesterday's that dry conditions will persist through the next 15 days. That is not to say that an event will occur, just to make note that changes are expected that could set up a potential event (or events) as early as about next Wed/Thu or perhaps waiting for something by the weekend. Latest trends in some 12z/Friday model runs are now suggesting a potential cold event for Wed/Thu that we will have to carefully watch.

Confidence is high in the retrogression of the North American trough that will push the ridge off the West Coast and bring colder Arctic air into much of the Northwest eastward across the Rockies by early to midweek of next week. Generally the issue has been whether the retrogression might eventually lead to a piece of the trough extending off the West Coast and opening up the area to an intrusion of moist systems beginning perhaps next weekend. The 00z GFS and a good number of its ensemble members show such a scenario. In fact, if the 00z GFS were to be a perfect prog, a strong piece of the trough would retrograde off the West Coast by next weekend and close off, setting up a significant event beginning late in the weekend continuing well into the future. The latest 12z GFS run, however, tends to be farther east with the deep trough moving south across the HMT next weekend rather than heading off the coast, so still a likely event but for next weekend and then not the prolonged situation into the following week.

Now for the really confusing part! While it earlier appeared that the really interesting possibilities might wait until next weekend, some of the 12z models are forecasting a far bigger system intensifying off the coast as early as Tue/Feb 14, apparently as a wave in the fast Pacific flow phases with another coming south around the retrograding North American trough. The 12z ECMWF just in is most notable with this solution, but it is also seen with the 00z/Canadian at 144 h and also strongly with the 12z NOGAPS run. The latest 12z UKMET run also is similar to the ECMWF, maybe just a tad faster. All would have a significant trough moving across the HMT area perhaps as early as Wednesday (more Wed night in the EC) into Thursday. This would be a cold system, producing low snow levels, with a big question as to how much moisture it will have (the NOGAPS, for instance, doesn't produce much in California but more in Nevada). What about the GFS? The GFS appears to split the energy so that this first system is more of a minor trough and then the next piece of energy drives the main system off the coast, as opposed to a bigger earlier trough. JUST got a look at the set of ensembles from the 12z GFS and a number of the members are supporting a stronger system for Wed/Thu like, but not quite as deep as, the ECMWF. The main issue with the GFS solutions that are favorable appears to be limited moisture as the system passes the HMT area.

Before looking at all the 12z models I must admit to being more excited about the longer term potential for a prolonged event. Now one cannot ignore the possibility of an HMT event midweek. There definately appears to be some oscillation between somewhat deeper vs. not as deep solutions depending on 00z vs. 12z initiallization times, so it will be interesting to see if that happens tonight or instead if we start to converge on a solution for next week. Ed Szoke NOAA/GSD

February 09, 2006

2006-02-09 WX Discussion and Forecast

A 500 hPa ridge still dominates the west Pacific CONUS coast with its axis almost parallel to the coastal longitude but displaced to its west. This sharp ridge of high pressure is forecast to progress eastward to be over the coast 18h Friday. This morning's NAM appears to be well initialized with the main feature being the closed low circulation under the ridge (part of a dissipating Rex block) that has much cloudiness and moisture over Mexico south of CA. The NAM 84 h forecast has a short wave making its way over the ridge and the ARB (valid 21h 2/12/06 utc) but no precipitation is indicated with this passage in this model run.

Looking at the GFS, I think it shows slightly better initialization than the NAM with respect to the closed system and cloudiness south of CA. But it is not all that different than the NAM. The GFS brings in a wave of precipitation off of the coast over the weekend but it dissipates to zero over land. Similar to the NAM, it has a wave entering the state over the weekend and the latest run has it remaining as an open wave as it passes the ARB, with some minor precipitation in the 00ut initialized run by 18h 2/13/06 utc, but in the later initialization (12ut) this morning there is no precipitation forecast. An arctic cold front appears to move over the region mid-week but the model generates most of the precipitation with this system south of the ARB keeping the ARB dry for the forecast duration.

Examining moisture plumes in the GFS, there are no substantial tropical pipelines for moisture for any of the above mentioned events. It appears that the cold air surge that is progged for mid to early next week will bring in drier arctic air and also act as a barrier for penetration of moisture from the Pacific, weak as it may be, at that time. Looking out to 384 hours, the forecast looks pretty grim for any sign of copious moisture making its way anywhere near the ARB area.

Looking at the long term GFS ensembles, they also give the same dismal probabilities for measurable precipitation in the region. A slight increase in probability shows up at the end of the run with a less than 50% chance (still very slim) at 384 hours Friday 12h 2/24/06 utc.

The bottom line here is continued dry for an extended period.

Dan Birkenheuer - ESRL/GSD

February 08, 2006

2006-02-08 (Wed) Daily Wx Forecast Discussion (1945Z)

As mentioned in yesterday’s forecast discussion… we are essentially dead in the water through at least early next week, with ridge conditions and only an occasional weak, dry vorticity spoke propagating through.

By the middle of next week, there seems to be two preferred operational numerical solutions (depending on the model and initialization time), both hinging on large-scale trough retrogression: (1) cold air spilling across the Intermountain West, with the ridge remaining locked just offshore of the West Coast, and (2) much more vigorous trough retrogression, with the cold air spilling out over the eastern Pacific, and a suggestion of a more zonal fetch developing across CA. The first option would mean the ARB remains dry and becomes a bit colder, whereas the second option could begin to bring at least some precip back into our focus area. It is simply impossible to say what the answer is right now, although for the last decade or so the midtrop atmospheric flow has generally tended toward low-amplitude configurations. If this qualitatively-perceived long-term persistence is maintained, then the second option would be more likely. We shall see. In any event, this situation needs to be followed closely in the coming days.

Paul Neiman

February 07, 2006

2006-02-07 WX Discussion and Forecast (1940 UTC)

A horrendous HMT weather pattern is in place for the foreseeable future. Now is the time to work on that backlog of ASAP projects, wash your clothes, and sleep. A large amplitude ridge is in place over the West Coast. Off the coast, a shear vortex has developed. However, it is dry, and it's impact on the ridge either will be nil or significant, depending on whether it quietly passes to the south or causes the ridge to intensify. The GFS ensemble favors a track across Mexico. Either way, nothing good for HMT will come of this swirl.

We have to look out to 120 hours in the forecast cycle (12 UTC Sunday) to see any hope of precipitation for HMT. Unfortunately, the GFS ensemble suggests only an outside chance of a glancing blow to northern CA; all precipitation is well north of the HMT domain.

I hate to dignify forecasts beyond 240-hours by discussing them. But... 10-days is a reasonable spin down period, and the GFS forecasts buy into that property of the atmosphere. The ridge is broken down by a potent trough 228 hours into the forecast. From that time on, a series of waves is forecasted to pass near or over the HMT domain.

Chris Anderson

February 06, 2006

2006-02-06 WX Discussion and Forecast

Both NAM and GFS models show a tendency for a Rex block (High over Low) with the central axis aligned over the Pacific CONUS coast well established by 18h 2/8/06 utc. Leading up to this setup and following its slow breakdown, there is nothing but short episodes of weak, transient precipitation forecast by either model for the next 240 hours. The 240 GFS has a hint of precipitation in the area but again very weak.

Examining the GFS moisture plume tracks from the Pacific tropics it appears that the next potential system in the near term might be 06h 2/11/06 utc (Saturday) but the plume barely manages to make the coast in this forecast run before it is diverted west out to sea, this is in line with the GFS forecast of no precipitation with this potential surge of moisture. This has a very anemic appearance, however, if this were to change, it could turn into a decent system, but as we will see, the odds of this are slim. Following this, there appears to be a second moisture surge mid-week 18h 2/15/06 utc but this one does not have a good connection to the tropics.

The remaining focus is the far future and the likely time scenario of the next system.

Examining the GFS ensembles out to 384 hours it appears that the event this weekend is given a probability of about 0.50 12h 2/12/06 utc. Following this short lived and weak looking event there is an extended dry period until a 0.58 probability of precip on 00h 2/17/06 utc.

It is probably worthwhile to keep an eye on the system for this coming weekend, however, since it is the only game in town for the near term. There might be something coming for the end of next week (near Friday the 17th), but at this time it is too early to tell for sure.

Dan Birkenheuer - ESRL/GSD

February 05, 2006

2006-02-05 (Sun) Daily WX Forecast Discussion

HMT Forecast 5 Feb 2006: With the wave from the last IOP exiting the area, dry ridging will be dominating the ARB over the next few days. Models are generally in agreement with a sound consensus scenario over the next 5 days. Recent runs were available from the GFS, ECMWF, NOGAPS, and CMC.

The general forecast series of events: On the 6th a weak wave off the coast closes off and drops south. This, coupled with major-wave ridging over the entire west coast, sets up a stable high-over-low block by the 8th. Easterly mid-level flow will dominate the ARB through the 11th when the flow becomes southwesterly again. Even at this time conditions are not favorable. The nearest system forcing any precipitation is a short-wave trough that will move into WA and OR on the 11th and drop a cold front through the ARB. All signs indicate no significant precipitation for the ARB. GFS ensembles sustain this block in one form or another to the 11th.

For the long range it looks like a major longwave trough retrogression into the Rocky Mountains (15-16 Feb) will make things interesting. Some solutions have the trough over or west of the coast with a closed-off low possible, with very cold air and precipitation likely. Others keep the ARB in northerly upper level flow with the trough to the east that would eliminate any significant precipitation opportunities. The U Hawaii precipitable water product from the GFS doesn’t indicate a moisture plume of any significance in the ARB region until Feb 19 / 00GMT, and this event looks like it favors S. California.

….So, it looks like everyone will get at least a week’s rest and maybe more.

John McGinley

February 04, 2006

2006-02-04 (Sat) Daily Wx Forecast Discussion (1940UTC)

Hi Everyone.

What a dud! Unlike the last couple of storms that impacted the ARB, this was a big under-producer of precip. As of 19Z, Blue Cyn reported ~0.7" of rain, while Alta had ~0.8”. Precip is tapering off at these sites and across the Sierras, as is also revealed by the DAX radar. A well-defined cold fropa was observed along the coast at the Bodega Bay profiler just shy of 12Z, but this feature became much more diffuse as it crossed the windward slopes of the Sierras (e.g., see the Sloughhouse profiler). Not surprisingly, the 88D radars recorded a modest narrow cold-frontal rainband that died as it impinged upon the Sierras. Oh well...

The storm is winding down, and midtrop post-wave subsidence aloft will efficiently kill any remnant precip well before 00Z this evening. West Coast ridging will ensue for tomorrow and during the upcoming week, although minor vorticity spokes approaching California from both the east and west will cut through the ridge from time to time. These features are weak and dry, so don’t expect any precip. Week-2 looks lackluster as well right now.

Cheers,
Paul

February 03, 2006

2006-02-03 (Fri) Daily Wx Forecast Discussion 1945UTC

Hi All,

Animated GOES satellite imagery shows the next Pacific storm system already approaching the West Coast. The remnant warm-advection comma-cloud head is making landfall in OR/WA, with the 88Ds recording some light precip beneath these clouds. Meanwhile, a well-defined cold-frontal comma-cloud tail is rapidly closing in on 130W. At the moment, there is not much cold cloud-top enhancement associated with the cold front, and PW values have decreased along the cold-frontal band since yesterday (now in the 2.25-2.75 cm range), but the cloud band itself is quite sharp… thus suggesting it may be associated with tight baroclinic structure. In addition, the trailing cold-core convection has become more enhanced in the last few hours, thus suggesting the possibility of frontal-wave development in the near future. More locally, skies are generally fair in the ARB, with residual fog and stratus in the Central Valley. Expect a good deal of sun today across the ARB in the shortwave ridge environment preceding the approaching shortwave tough. Clouds will likely start streaming in and thickening some time this evening or early tonight as the cold front approaches the NW tip of CA.

The forecast is still on track for precip to begin in the ARB between ~07-10Z Saturday as the storm’s cold-frontal cloud band begins impacting the region. Snow levels will start out relatively high… in the 7-8kft range. Although the largest midtrop height falls with the approaching shortwave trough will occur in extreme N CA and in the Pacific NW, strong tightly-confined frontal dynamics and favorable orographics should extend southward across the ARB tomorrow morning, so once it starts precipitating it should pick up in intensity fairly quickly. Expect a period of strong winds straddling the front, as the pressure gradient across our domain becomes rather intense. We should see cold fropa and perhaps a narrow cold-frontal rainband about 12-16Z Saturday, followed by rapidly descending snow levels (down to ~4kft). The precip will likely quit fairly abruptly at or before 00 Sunday, as strong postwave subsidence aloft begins to spread across the region. I still think total precip in the ARB can exceed 1 inch.

The medium range is still looking rather bleak. West Coast ridging aloft builds in earnest starting Sunday, and it will likely persist for most or all of next week. The GFS ensembles and latest week-2 GFS forecast suggests the possibility of southern-branch wave energy impacting CA in mid-February, but that is a looooong way off.

Paul

February 02, 2006

2006-02-02 (Thu) Daily Wx Forecast Discussion 2000UTC

Hi All,

The storm that impacted the ARB is presently winding down, with only remnant post-wave popcorn convection persisting over the higher terrain. This precip is ending, as the atmosphere stabilizes due to significant mid-trop warming. Our storm was a relatively warm one, with the brightband at Alta, Colfax, Bodega Bay, and Chico hovering generally in the 7.5-8.5kft range. This is not surprising, given that the SSM/I imagery showed that this storm was associated with a respectable landfalling atmospheric river. This river is still intersecting our region, but the core values of PW are slowly decreasing with time offshore of CA. Precip totals in the northern Sierra were not huge but respectable, generally in the 1.5-2.75” range.

Transient shortwave ridging aloft tonight and tomorrow (Fri) will keep our focus area dry, but the next storm is already taking shape in the eastern Pacific between 150-160W and 30-40N, as seen in the animated GOES satellite imagery. Cloud tops are cooling rapidly in the baroclinic cloud-leaf region of the developing cyclone, and the cold-core convection is becoming more pronounced with time. This storm will move rapidly ENEward, spreading high, then middle, clouds across CA by later tomorrow. The storm is already beginning to entrain large PW values (>4cm) associated with the remnant atmospheric river that spans from the tropical moisture reservoir SW of Hawaii to California.

Precip will likely commence in the ARB between ~06-10Z Saturday as the storm rapidly approaches the West Coast. Although the strongest midtrop height falls and associated dynamics will impact far N CA and the Pacific NW, a relatively tight zone of strong polar cold-frontal dynamics and favorable orographic flow will likely affect the ARB during the morning Saturday. Prior to fropa, the melting level will be fairly high… in the 7-8kft range. However, it will descend rapidly to ~4kft following fropa (perhaps between 14-18Z Sat). The postfrontal flow will turn to an orographically unfavorable NWerly direction on Saturday afternoon in the cold sector. This, coupled with midtrop height rises by 00Z Sunday following the passage of the shortwave trough aloft, will likely mean a quick end to the precip by later Saturday afternoon. QPF amounts vary significantly from model to model, but judging from the stout cloud-field evolution of this storm over the central Pacific, I think we could pick up a quick 1”+ of precip in the ARB, although probably not 2”.

West Coast ridging aloft builds in earnest starting Sunday, and it will likely persist for most or all of next week. The GFS suggests the possibility of some minor shortwave energy cutting through the ridge late next week or later, but don’t hold your breath.

Paul

February 01, 2006

2006-02-01 (Wed) WX Discussion and Forecast

Light spotty precip has been indicated by radar over the ARB this morning.
It looks like a slight delay in getting the moisture plume to move south,
apparently due to a wave (visible in satellite) forming along the front.
Both the GFS and NAM do have heavier precip starting up around 00Z Thursday,
then lasting until about 00Z Friday. The AFWA MM5 is going for nearly 2"
over the ARB, with the CMC Regional having about 1.5". The GFS this time
comes through with 1.1" with the NAM slightly behind at 0.8". The GFS
ensemble mean is 0.5" and the UKMO holds with just about 1/4". So if the IOP
can be extended a bit it looks like a good shot of precip tonight and Thursday.
In the short-range the 3km LAPS WRF-ARW model run indicates about .35" over
the ARB for the initial phases of the event in the 12 hours ending 03Z
Thursday. These hot-start runs are re-initialized every 3 hours.

The Saturday storm begins about 18Z in the GFS with good dynamics in the
OR/WA coasts. This progressive shortwave brings in a 500mb low to the BC coast.
The ARB is on the southern edge of a short wave that breaks off from the
main low to the north, receiving about .2" precip. This could be somewhat
more if the wave digs slightly to the south.

In the long range the GFS continues to build a west coast ridge with
some indication of a Rex block, with a weak low riding over the ridge
and locating itself near the ARB mid-week. This should be a mostly dry
system in a mostly dry week.


Steve Albers