HMT Forecast

Present event is ending from NW to SE over the lower Sacramento basin area as upper short/wave trof comes onshore next hour or 2 and synoptic-scale dynamics begins favoring subsidence over ARB. Low-level onshore flow and lowering static stability due to cooling aloft nevertheless favors persistent but much lighter orographic pcpn over the Sierra above 2000 to 3000 ft through the day today, ending after sunset. According to reports, snow level has dropped to near 5500 ft following frontal
passage.

Next event is hard on the heels of the present one, as has been indicated by the progs for
the last few days. There will be 3 interacting aspects to this next system: a deep occluded low and associated s/w trof aloft that will track toward OR and then turn N paralleling the coast, bringing a frest batch of pcpn to the north coast of CA mainly N of Pt Arena by Sunday morning 1 Jan, the baroclinity left behind by the present exiting system, and a trailing lower latitude short-wave trough aloft presently near 35N/160W. The baroclinity, focussed in the 850-500mb layer but based at the surface, is passing into the lower San Joaquin Valley at present. It will not progress much farther south than the Transverse Ranges separating central and southern CA before the circulation with the aforementioned occluded surface low and later the trailing short-wave trof perturbs it north. Nevertheless it appears likely to remain south of the ARB, based on inspection of GFS control runs and ensembles. GFS runs have been suggesting a significant wave will initiate along this baroclinic zone (aka front) on Sunday as the trailing s/w trof approaches the coast. This
baroclinic zone will be the focus of the heaviest dynamically-induced pcpn, so I believe the heaviest pcpn from this next system, mainly midday Sunday thru late Monday, will be over the Santa Lucias along the central coast eastward into the central and southern Sierra, with Blue Canyon, being south of the strongest action, picking up a mere additional 1.5 - 2" between Sunday afternoon and midday Monday. Snow levels will remain about 5000 ft.

Looking farther ahead, the next wave and surface low in the strong zonal flow across the Pacific is forecast by guidance to take a more northerly track, affecting mainly the Northwest on Wednesday, with little or no pcpn over the ARB. As this takes place, GFS and ECMWF are both indicating massive baroclinic development in the central Pacific near 150W. Eventually this will shift eastward to
the coast, but the intensity of this development presages some ridge building along the
coast beforehand, so it still looks like midweek (Tuesday thru Thursday) will be mostly dry in the ARB.

John Brown

Posted by Forecast Editor at 06:53 PM | Permalink

Sat imagery indicates that a weak upper wave crossed the area 12-15Z this morning.
Break in pcpn indicated at some stations away from the coast is likely a response to this.

WV imagery indcts bulge in northern edge of upper moisture near 140W at 17Z today.
This and dry pocket farther W indicates upper s/w trof near 145W. At the same time, enhancement is going on offshore from Bay Area N to OR border E of 130W, indicating to me enhancement of pcpn due to interaction of ambient very strong SW-WSW flow well offshore with coastal barrier jet. These features argue for little movement farther southward of main water vapor flux, next 12-14 hours, keeping axis near or just N of ARB, consistent with the NAM fcst Paul fwdd earlier this morning.

This morning's progs are all pretty consistent in keeping the action going, with the most intense
pcpn forecast for this evening and tonite, 00 - 12Z, in the ARB, as stronger flow pushes inland, and static stability of the flow begins to diminish, allowing some shallowing of the up (Sacramento) Valley barrier flow. Expect surface winds in the valley will be gusting in excess of 35kts in some areas later today and tonight. The freezing level is exceptionally high in the ambient flow, near 10,000ft, so expect that snow will be restricted to the high ridges of the northern Sierra until 06z tonight, dropping to near 5,000 ft by noon Saturday 31 Dec. As pcpn diminishes ARB, pcpn in the central and southern Sierra will pick up, as the axis of the precipitable water flux shifts south of the ARB.

Another batch of pcpn is due in with the next surface low, centered now in the central Pacific NW of Hawaii. Thus, expect pcpn to pick up again Sunday evening 1 Jan, continuing into Monday. This event should be lighter than the present event, bringing 1.5 to 3" to Blue Canyon 00Z 2 thru 00Z 3 Jan. Snow levels will also be lower, but are not expected to drop below 4,000 ft at the lowest near the tail end of the event.

The outlook beyond Monday is for continued possibilities of rainy periods, but consensus opinion from ensembles is that the strong zonal flow will amplify, with a mean ridge near the West Coast and
mean trough offshore. This will shunt most pcpn into the Northwest and BC. Midweek (Tuesday, Wednesday, Thursday) looks like the most likely pcpn-free period over the ARB.

John Brown

Posted by Forecast Editor at 07:06 PM | Permalink

The parade of storms across the Pacific and into northern CA and the Northwest continues. The middle and high clouds from the next sytem is already evident, and precipitation will likely have started on the north coast by dusk, spreading inland and southward during the evening and night.
This promises to be a major event for the ARB in my estimation, with > 5" at Blue Canyon a good
possibility (50% chance) between 12Z Friday 30th and 12Z Saturday the 31st. There is a nice plume of high precipitable water (> 30mm) with this, by Friday morning extending from the coast far back WSW to NW of Hawaii. 700mb flow normal component to Sierra of 25-35+ kts fcst by the GFS looks reasonable.

This will be a warm event, with warm advection in the early stages of the pcpn likely raising the
snow level to above 7,000 ft MSL by Friday morning as a deep surface low turns northward offshore
of northern CA and OR. Progs are suggesting that a strong trailing wave will pass over the area later on Saturday, bringing steeper lapse rates and possibly inducing a wave on the front
offshore from the Bay area late Friday. If this happens, a heavier batch of pcpn could occur over the ARB Friday evening into early Saturday morning as dynamical forcing from the wave enhances the orographic lift.

Progs agree on the liklihood of another deep surface low approaching the Northwest coast late Sunday into Monday, bringing another batch of pcpn to welcome the new year. However, attm it appears this
will not have quite the PW plume with it that the earlier system has, and will therefore likely bring
lighter amounts of pcpn to the ARB. A trailing wave following close on the heels of this deep low
is indicated by the GFS to track farther south than previous systems, bringing the focus of heaviest
pcpn into the central coast and southward late Sunday into Monday.

Looking farther into the future, there is some divergence of opinion amongst the models on how much longer the current rainy period will last. The GFS suggests that a ridge will build along the
west coast the middle of next week, resulting in the focus of the heavy pcpn shifting northward toward WA and BC. However, the ECMWF and UKMO are not amplifying the ridge so much. On this basis I think there is a good possibility that the rains will continue over northern CA with only a break of a couple of days middle of next week, most likely Tuesday and Wednesday.

John Brown

Posted by Forecast Editor at 06:44 PM | Permalink

Intense zonal flow in the Pacific continues associated with a very broad trough that continues to breed short waves that are impacting the West Coast and ARB. A precipitation event is in progress today over the ARB and should be coming to an end this afternoon and evening with flow between 5000-10000 ft shifting to northwest. Things begin to dry out over the ARB in preparation for the next event setting up for late Thursday into Saturday.

The ECMWF, UKMET and GFS all look to be in better agreement today than yesterday as far as timing on the event. Yesterday the ECMWF was 6-12 hours slower than the GFS, however, the models today are in phase and are favoring a bit of a delay over yesterday's forecast. Observations show a high IPW river well formed in the Pacific with roots in the Philippines, extending toward the coast. The Univ of Hawaii GFS IPW product shows this plume moving into the coast Thursday evening and reaching the Sierras by midnight. This is well correlated with the start of the pre-wave, warm- advection period that commences Thursday afternoon (30/00Z). Light precipitation begins late evening with the heaviest rain rates late Friday morning through evening (30/18Z to 31/06Z). Winds in the 5000-10000 foot layer show a good upslope component. The ARB will be in a cold advection precip regime after 10pm (31/06Z). Showers will continue through most of Saturday, some possibly quite intense, ending sometime late (1/06Z). Totals for this event could easily go 3-5 inches in the ARB region. NWS and ESRL generally agree on intensity and timing of this event.

Another shortwave is close on the heels of the Friday event with warm advection beginning early Sunday morning and precipitation beginning in earnest around noon.
The maximum in the subtropical moisture plume stays south of the ARB and as a consequence precipitation amounts are less, possibly reaching 2-3 inches for this event. GFS indicates the peak period for rain in the warm advection period will be Monday morning (2/12-18Z), with cold advection showers continuing through afternoon and ending by evening.

The further outlook shows a change from yesterday as the wave expected Wednesday is being diverted to a more northerly track with a building ridge scenario for the western US. GFS ensembles indicate a mix of solutions, some keeping the wave and precip well offshore,; others brining it in. The GFS has been prone to over developing this ridge so a mid-week event should not be discounted.

John McGinley

Posted by Forecast Editor at 07:48 PM | Permalink

Strong flow in the western Pacific continues, helped along by a continued significant warm sea surface temperature anomaly at mid latitiudes. This is creating strong low level baroclinicity. Out of this flow, waves continued to generate and impact the West Coast and ARB area with a couple of good events during the stand down period. The next one is due to produce rain over the ARB this afternoon and evening into tomorrow with 1-2 inches possible. Since the timing places the event in the stand down period we’ll ignore this one and concentrate on the next wave upstream.

This next one is due to arrive Friday 30 Jan with rain beginning in the morning (30/12Z) and peaking out near local midnight, (31/06Z) continuing into Saturday mid day. The Univ of Hawaii integrated moisture forecasts from the GFS indicate that this is a substantial atmospheric river event. The subtropical source of this river is in the far western Pacific. The moist flow is forecast to impact the coast Friday (30/12z) in warm advection ahead of the wave. The warm advection and precipitation continues through Friday night (31/06Z) with a cold frontal passage and cold advection showers continuing through Saturday afternoon (1/00Z). Freezing level varies from 5500ft early Friday to near 9000 ft at the frontal passage and then drops again to 5500ft by the end of the event. Precip totals forecast from the GFS indicate a possible 2-3+ inch event. Flow in the 5000-10000 ft level has a good orographic upslope component to supplement the dynamic lift associated with the wave. Comparing the GFS to the ECMWF shows that the GFS is 6-12 hrs faster on this wave, so things could potentially start a bit later on Friday.

Long range outlook indicates a series of waves over the next week that will bring more heavy rain events on Monday and again late Tuesday into Wednesday, and beyond. RFC personnel indicate that this may be a series of events of epic proportions, rivaling those in the late 1980s.

John McGinley

Posted by Forecast Editor at 07:49 PM | Permalink

In the last 24 h, the coast ranges NW of SFO received more than 1.5” of rain. Blue Cyn in the ARB received over an inch, although almost all of that fell since 12Z this morning when an organized cloud band moved Eward across the region. Snow level observations from nearby wind profilers and vertically pointing S-band radars have been rather high, ranging from 8.5-9.5kft.

The satellite animation shows the next in a series of shortwaves making landfall in NWern CA. Its comma cloud tail extends SWward off the coast to a region of cooling cloud tops approaching CA. SSM/I observations in this region portray an enhanced SW-NE oriented PW filament making landfall near Pt Arena and about ready to merge with the stalled atmospheric river intersecting the Bay area. Not surprisingly, local PWs remain high, with Bodega Bay hovering in the 2.5-3cm range. The SSM/I also shows a coherent region of rain (rates exceeding 4 mm/h locally) closing in on 130W and, ultimately, on CA. This landfalling shortwave holds promise for producing significant precip in the ARB for the HMT06 radar crews before they need to wrap up later tonight or very early Thursday (see forecast below).

The 12Z NAM shows the aforementioned shortwave impacting the ARB this afternoon and overnight, with modest dynamics, enhanced 850-700 mb warm advection, enhanced upslope SWerly flow, and an impressive moisture feed that taps the subtropics. Precip totals will likely range between 1.5-2.5”. The midtrop flow veers with time from SWerly today/tonight to more zonal on Thursday. Weak vort spokes will be embedded in the zonal flow, so expect precip to persist tomorrow, and end by tomorrow night or Friday morning as the midtrop ridge pops.

The medium to long range outlook is confused at best, depending on which model, ensemble member, and initialization time is considered. There is a full suite of solutions, ranging from mostly ridge conditions from this weekend through next week (i.e., our HMT06 start-up period is next Thursday evening the 29th) to shortwaves beating down the ridge by early next week with transitory precip events possible through next week. I will address this issue during the resumption of HMT06 forecasts next Tuesday.

Paul Neiman

Posted by Forecast Editor at 07:52 PM | Permalink

Between yesterday morning and this morning, ridge building in the wake of the wet late-weekend shortwave has shunted most of the significant precip (0.5-2.5+ inches) NE of a line from Eureka (on the coast) to Bucks Lake (in the extreme Nern Sierra). Hence, only very light precip (~0.25”) has fallen in the ARB during this period. But, alas, the ridge is slowly flattening out and wetter weather shall return.

The latest satellite loop shows the next shortwave trough lifting NEward as it makes landfall in N CA. A comma-cloud tail with enhanced, but warming, cloud-top temperatures extends SSWward from N CA to near 20N, and the polar-orbiting SSM/I imagery portrays a prominent atmospheric river with core PW values of 2.5-3.5 cm associated with this landfalling shortwave. In fact, the PWs at Bodega Bay have increased from 1.5 to >2.5-3.0 cm in the last 6 h. This river originates in the tropical moisture reservoir at ~20N/140W. Meanwhile, the 88Ds at MUX, BHX, BBX, and DAX are monitoring the slow NEward progress and ultimate decay of a broad NNE-SSW oriented rainband that extended as far S as Pt Reyes earlier today. The remnants of this band will remain N of the ARB. The BBX and DAX radars actually showed an NCFR at the leading edge of this weakening band a couple of hours ago, thus suggesting that this is/was tied to a weakening cold front. One last note… the next shortwave lined up in the flow is clearly seen in the water-vapor loop, centered at ~40N between 140 and 150W, and grinding eastward with vigor. To make matters more interesting, a plume of enhanced PW (3+ cm) is becoming entrained into this circulation. Even though the vort center associated with this wave will likely pass N of the ARB (see below), I suspect this wave will be a big precip producer for the ARB (slated for tomorrow into Thursday) due to favorable orographics and plentiful deep moisture.

Now, on to the forecast. First, the easy part… snow levels will be quite high during precip episodes for the next several days (7500-8500ft). The major questions are… when and how hard will it precipitate? The main role for today’s shortwave will be to flatten out the ridge and lay out the atmospheric river across the region. Most of the dynamics will remain N of the ARB, and the associated cold front is washing out as it makes landfall, but increasingly favorable orographic upslope flow and moisture coupled with weak warm advection will likely generate some precip for the ARB later today into tonight… perhaps in the 0.25-0.5” range. The 12Z GFS shows the NE-SW oriented atmospheric river stalling out across the Bay area today and tonight, so if there are any dynamics in the SWerly flow aloft that are not presently resolved/initialized in the operational models, precip amounts could be greater… especially along the coast ranges.

Do not expect much precip across the ARB later tonight into tomorrow morning, until the next wave approaches the target area. Because the dynamics, zonal flow (with a stronger upslope component), and lower trop warm advection will be stronger with this second wave, and a developing second atmospheric river will merge with the one already in place, expect significantly more precip midmorning tomorrow into Thursday than with today/tonight’s system. Once again, a weak cold front will wash out as it approaches the coast late tomorrow. There should not be a sharp back edge to the precip, since modest zonal flow in a moist environment will be maintained well into Thursday. Rather, expect precip to slowly taper off as midtrop heights start to rebound and the ridge amplifies. Total precip with the Wednesday-Thursday system could exceed 2-3” in the ARB, and the radar folks should expect to abandon their instruments for home while it is still precipitating.

Paul Neiman

Posted by Forecast Editor at 07:53 PM | Permalink

Visible image shows broken cloudiness over the ARB region this morning. A 500 mb height ridge has developed over the area in the wake of the active trough that passed through the area this weekend. The good news is that Sunday was very rainy. Reports of 24-hr precipitation totals (Sunday 12 UTC - Monday 12 UTC) are Blue Canyon 3.72", Sugar Pine 2.64", Greek Store 2.88", Auburn 1.88", Georgetown 2.44", Chico 1.69", Alpine Meadows 3.05", and Huysink 3.68". This morning's water vapor shows an area of ehanced water vapor moving toward ARB with ETA between Monday 22UTC and Tuesday 02 UTC. Visible imagery shows some cloudiness associated with it, but it is neither an extensive cloud shield nor vigorous as suggested by the absence of deep cloud tops. Precipitation is expected to be minimal with this feature, if any occurs at all. The next large wave is positioned near 32N, 144W in the GFS 500 mb 12 UTC analysis, which coincides nicely with its location in water vapor imagery. Also, water vapor imagery indicates the wave is beginning to take a more northward trajectory. It was noted that PW at Bagoda topped out at 30mm during Sunday's event.

Weather forecasters at Sacramento largely agree with the timing of precipitation of precipitation as discussed in what follows. Model guidance suggests the next opportunity for rainfall is Tuesday evening into Wednesday morning. (Why can't you fellas get a break and have a DAYTIME event?) Although the models (GFS ensemble, Candadian, MM5, RUC) closely agree on the position of the shortwave trough passing near California during this period, large disparities exist in QPF over the ARB, as the models are not in agreement concerning the inland penetration of a low-level front trailing the short wave. A couple of models suggest 12-hr accumulation ending 12 UTC Wednesday could be as large as 1-2". A couple of model forecasts contain no precipitation in the region. The majority of QPFs lie between these two extremes. Onset of precipitation, when it is forecasted to occur, is consistently between Tuesday 18 UTC and Wednesday 00 UTC. It was noted that PWs associated with this approaching trough is as high if not higher than what was observed during Sunday's events.

Chris Anderson

Posted by Forecast Editor at 07:02 PM | Permalink

Rainfall is ongoing over ARB. At 1748 UTC, KDAX showed light radar echoes over ARB and a line of moderate echoes approaching Sacramento from the SW; ETA at Sacramento is 1815 UTC. San Francisco Bay radar shows scattered moderate echoes over the ocean, though banded structures are not apparent. The precipitation area is forming ahead of a 500 mb shortwave trough off the coast of NoCal. SSM/I PW values range mid to upper 20s (mm) offshore. Wind speeds above 3 km in the Chico wind profiler veered to SWerly and accelerated to 50 knots between 08 and 14 UTC. SWerly winds in excess of 40 knots now are observed as low as 1.5km. The onset of persistent precipitation was coincident with the change in wind speed and direction. Reported precipitation amounts between 06 and 12 UTC were 0.53" Blue Canyon, 0.40" Auburn, 0.72" Sacramention International Airport, 0.48" Folsom, 0.48" Georgetown, 0.36" Sugar Pine, .36" Greek Store, and 0.59" Chico. Between 12 and 18 UTC, precipitation totals were 1.28" Blue Canyon, 0,84" Auburn, 0.60" Sacramento International Airport, 0.84" Folsom, 1.08" Georgetown, 0.80" Sugar Pine, 0.80" Greek Store, and 0.74" Chico. At 1833 UTC, Blue Canyon ASOS reports 4-dot rain. The surface met station at Blue Canyon reports 2.03" in the 00 to 18 UTC period. Sacramento notes convective elements in the central valley.

Rainfall will continue through the next 12-18 hours. RUC 15 UTC analysis/forecast cycle brings the shortwave over SW Oregon and NoCal between 00 and 03 UTC Monday, as a result 700 mb height contours suggest WSW flow remains persistent through 03 UTC, producing orographically induced vertical velocity >10 microbars/sec through 03 UTC. RUC 3-hr precipitation totals range 0.10" to 0.25", giving a range in 12-hr total amounts of 1-2" near and within the ARB. Short-range ensemble forecasts contain 3-hr precipitation totals >10mm through 12 UTC Monday in 7/10 members and >25mm through 09 UTC Monday in 3/10 members. The end of the heavy rain period is dependent upon on slackening gradients of 850 and 700 mb geopotential height, thereby reducing orographic lift. The majority of models forecast this to occur during the 06 to 12 UTC Monday period. An extrapolation of the current track of the shortwave in water vapor imagery is consistent with the majority opinion of the model forecasts.

Unlike previous model forecasts, today's 12 UTC cycle (GFS, MM5, Short-range ensemble) does not contain a single short wave trough traversing the ARB region in the Wed-Thurs period. Instead, ridging is more prevalent with a series of weak short wave troughs producing intermittent wet periods between 00 UTC Wednesday and 12 UTC Thursday.

Chris Anderson

Posted by Forecast Editor at 07:10 PM | Permalink

Observations at 17:45 UTC show fog in the valley and a band of light rain along a Sonora-Audubon-Chico line. Radar (WSR-88D) estimated hourly rainfall is less than 0.1". This band of precipitation is associated with a shortwave at 500 mb that is well resolved in the 16 UTC 13-km RUC analysis. Of importance for subsequent periods is a serious dent made in the HMT-death ridge. It is unlikely that the ridge will reform thanks to another shortwave trough that is extending westward a deep low over the northern tier of the United States. However, low-level wind in Chowcilla and Bagoda Bay profilers remains S-SEerly below 1.5 km, even after the passage of this shortwave. One of the key forecast questions heading into the 12-36 hour period is whether the low-level flow will turn SWerly prior to the arrival of the next shortwave. SSM/I water vapor imagery indicates a swath of mid-20s PW ahead of the next trough.

NWP model guidance (GFS, MM5, ETA, Short Range Ensemble, Canadian model) is in agreement in that precipitation totals are in the 1-10 mm range prior to Sunday 12 UTC. The models that produce ~10 mm prior to Sunday 12 UTC, which is a minority of the models, contain a weak short wave that is absent in the other models. The timing of much heavier precipitation after Sunday 12 UTC depends on the forecasted speed of a potent short wave trough. All model forecasts contain a potent short wave. The model solutions of the short-wave trajectory are consistent with a 24-hr extrapolation of the short wave as seen in water vapor imagery. Probability of precipitation in this period is very high. The relevant forecast questions are: when will heavy rain commence? and how much rain will fall?
A minority of models produces 10-25mm in the Sunday 06-12 UTC period.
About half of models produce 25-50mm in the Sunday 12-18 UTC period; the other half produces 10-25mm.
All but two models produce 25-50mm in the Sunday 18 UTC to 00 UTC period.
Total precipitation in the Sunday 12 UTC to Monday 00 UTC period exceeds 50mm in 21/27 models.
During the forecast discussion, weather forecasters indicated that widespread 1-2" totals between Sunday 12 UTC and Monday 00 UTC is a reasonable expectation. HPC analogue forecasts suggest amounts as high as 4" in the ARB headwaters. The snow level was expected to be about 3000-4000 ft at the onset of the wet period and ~6000 ft during the period of heaviest rain. Storm duration is expected to be midnight Sunday to early morning Monday.

Forecast models suggest a break between Monday midday and sometime on Wednesday. On Wednesday, the GFS ensemble has a river stretching into central CA with PW 30-32mm. Although the wave tracks north of CA, a trailing front pushes into central CA in the Wednesday 06 UTC to Thursday 00 UTC period, producing 30mm of precipitation. This shortwave creates a large ridge that deflects subsequent waves northward.

Chris Anderson

Posted by Forecast Editor at 07:22 PM | Permalink

The HMT death-ridge remained in place overnight, creating conditions favorable for intense fog this morning but unfavorable for precipitation over the next 12-24 hours.

The 36-72 hour period appears very likely to be wet. The magnitude of precipitation and onset in the forecast models is dependent on the eastward propagation of a shortwave trough and inland penetration of the surface front associated with the trough. Forecasts from the GFS ensemble, Canadian model, and MM5 model suggest the period likely to be wettest is Sunday 18 UTC through Monday 06 UTC. The majority of the models produce largest 6-hr precipitation amounts in the Sunday 18 UTC through Monday 00 UTC period with the majority of models producing 25-50 mm. A minority of models have light precipitation (6-hr totals < 10 mm) beginning as early as the Saturday 18 UTC to Sunday 00 UTC period.

The ESRL/GSD experimental 3-km NMM nested within the GFS suggests the precipitation may not be as widespread as the GFS and Canadian models predict. The particular GFS run that is downscaled fails to push the surface front inland. As a result, low-level flow remains SSE over and south of the ARB. The downscaled model positions the heaviest rainfall northward of ARB during the Sunday 18 UTC to Monday 06 UTC period with only 10-20 mm accumulating over the ARB but >50 mm accumulating north of the ARB and along the coastline near the surface front.

Given this model guidance, it appears to be almost a certainty that rainfall exceeding 25 mm in 6 hours will occur near or in the ARB Sunday afternoon. The onset of lighter precipitation may occur as early as Saturday afternoon and may linger into midday Monday. However, discussion during the weather briefing phone call focused on whether the shortwave troughs would remain vigorous as they approached California. Forecasters were less optimistic than the models that the ridge would crash and felt that upslope flow would be weak in the Central Sierras, indicating that 0.5 inches on Sunday afternoon was a more realistic expectation than 1.0 inch or greater.

Long-range forecasts contain two more shortwaves near CA next week. Disparity of storm track among GFS ensemble members and the Canadian model is large, creating a large range of forecasted amounts in two periods: Tuesday 12 UTC through Wednesday 00 UTC and Thursday 00 - 12 UTC. In the majority of model forecasts, 700 mb wind is oriented with at least a slight upslope component and PW values remain steady in excess of 28 mm throughout the period. During the weather briefing, the weather forecasters were more impressed with the Wednesday evening-Thursday event but cautioned the events of this weekend could significantly alter the forecast.

Chris Anderson

Posted by Forecast Editor at 07:21 PM | Permalink

Synoptic pattern forecasted to remain unfavorable for precipitation over next 24 hours. Closed low over SoCal expected to drift SSE.

Sacramento WFO, ESRL, and HPC in general agreement concerning increasing chances of precipitation for the weekend, especially Sunday.

72-84 hour period is forecasted by global models to be a minor wet period. Global models (GFS, ECMWF, Canadian model) indicate a weak shortwave trough will impact NoCal, embedded within larger-scale warm advection, and will create a period of precipitation between 00 Z Saturday and 00 Z Monday. Heaviest rainfall period is forecasted to be 12Z Sunday to 00Z Monday. Models are in agreement that dynamics of the trough are expected to be weak but orientation of moist 850-700 mb flow is forecasted to impinge upon west face of Sierras. Disparity among model forecasted precipitation arises from orientation of flow and humidity of air mass. Accumulations in that period range 10 mm to 25 mm. Not a classic atmospheric river case, however, precipitation appears likely.

5-7 days ECMWF and GFS show westerlies extending across Pacific with a major trough impenging on CA coast 21-22 December. Atmospheric river is well developed in GFS forecast at points northward of SF Bay.

Posted by Forecast Editor at 07:43 PM | Permalink

And the beat goes on… midtrop ridging and dry weather will remain locked over the ARB through the end of the week.

A weak shortwave is still progged to penetrate the ridge during the weekend, but it will shred apart in the process. Hence, significant precip is not likely. Interestingly, the 12Z Canadian spectral solution shows a more robust wave penetrating the ridge and impacting the ARB next Monday. Given, however, that this model tends to be overly progressive with its handling of transient shortwaves, and the fact that it is an outlier solution, I would not put too much faith in this scenario. Having said that, the 12Z EC model is not as strong with the West Coast ridge for early next week than the previous model runs. Is this a significant trend that supports the new Canadian solution? Unknown just yet.

The GFS keeps the West Coast ridge generally intact through the middle of next week, only to begin breaking down later in the week…. with Pacific westerlies encroaching on CA. This scenario would suggest a change to wetter weather… just in time for the holiday stand-down period. Go figure…

Paul Neiman

Posted by Forecast Editor at 07:56 PM | Permalink

Midtrop ridge conditions and low-level Nerly to offshore flow will keep the ARB dry for the remainder of the week.

For this weekend into early next week, the GFS continues to show some shortwave energy breaking through the ridge, giving the ARB only modest precip at best. In contrast, the ECMWF and Canadian models maintain the West Coast ridge throughout the weekend, with shortwaves deflected Nward from the eastern Pacific into BC and AK. None of the solutions are very encouraging.

Paul Neiman

Posted by Forecast Editor at 07:38 PM | Permalink

The persistent upper low will finally exit CA today and tonight, with only scattered light precip… mainly in the southern Sierra. In its wake, dry Nerly flow will cover the ARB through midweek, with midtrop ridging situated offshore.

Shortwave energy will chip away at the ridge the latter half of the week, with minoring-out vort spokes riding over the ridge into the Pacific NW and BC, and additional energy undercutting the ridge and establishing a Sern branch stream into Baja and extreme SoCal. Throughout, the ARB will remain at the epicenter of the split flow, with little in the way of wet weather.

The 12Z GFS shows a transitory shortwave breaking through the ridge during the weekend, with only a modest precip. event (at best) across the ARB. Ridging re-builds thereafter. In the 12Z ECMWF solution, the ridge remains locked across CA throughout the weekend. All in all, the outlook is considerably less optimistic for significant precip in the ARB than it was just a couple of days ago…. bummer!

Paul Neiman

Posted by Forecast Editor at 07:46 PM | Permalink

The pesky upper-level cut-off cyclone that has spun offshore of SoCal the
last several days is now starting to kick out to the NE. Some minor
cloud/vapor enhancement in the deformation zone on the N side of the
circulation is presently streaked across portions of the Sierras. As the
upper low exits tonight/tomorrow, there is a slight chance of scattered
light precip within this zone, mostly S of the ARB. Ridging in its wake
will keep the ARB high and dry through at least Wednesday-Thursday
(possibly until the end of the week), as discussed by Steve Koch
yesterday.

Thereafter... it’s a mess! The various medium-to-long range models have
shown very little consistency from day to day. For example, yesterday’s
solutions were suggesting a prominent omega block setting up in the
eastern Pacific toward the end of next week, with undercutting waves in
the zonal current drawing in subtropical moisture and slamming into the
ARB for an extended period thereafter. Today’s GFS still tries to pinch
off the upper-level ridge over the NEearn Pacific later next week, but
very little wave energy reaches CA until later in the weekend. The waves
that are ultimately progged to impinge upon the ARB are more modest in
scope than yesterday’s wet GFS solutions. Meanwhile, the ECMWF maintains
a full-latitude ridge near or just offshore of the West Coast through the
end of next weekend. What is certain is that the confidence level of the
model solutions is very low. With this in mind, we need to remain
mindful of Klaus and Ed’s recent medium-range discussion suggesting
significant wave energy may yet break through the ridge and affect CA
later next week. Hopefully, we will have a better handle on things
tomorrow, but don’t count on it.

Paul Neiman

Posted by Forecast Editor at 07:55 PM | Permalink

Eta model shows the cutoff low finally lifting out right across central CA and then into NV by Monday morning, so… kiss good-bye to the last several days of offshore flow around the northern side of this system. A weak short wave riding over the top of the persistent upper-level ridge approaches British Columbia by Monday, then merges with the old cutoff system as it digs into the mean long-wave trough position over the eastern U.S., effectively causing a discontinuous, but temporary, retrogression of the trough axis. The GFS tells us that split flow should continue through most of next week, with the polar jet stream from the Gulf of Alaska merging with the essentially zonal subtropical jet stream coming from the Pacific. This pattern of strong confluence over the Rocky Mountain region promotes the continuance of dry-dry-dry conditions over the ARB region through mid-week, but it looks like a major change will commence thereafter.

A tantalizing scenario is beginning to be forecast in the GFS solutions by next Thursday, as a very strong jet streak rips across the top of a wave in the Gulf of Alaska, the ridge builds northward into the polar regions in its wake, and a high-latitude omega block becomes established. The GFS ensemble is consistent with this split flow-omega block scenario. This pronounced ridge brings down very cold air into southwestern Canada and a quasi-stationary frontal system sets up across the northwestern U.S. from Thursday through Saturday. At the same time, the Gulf of Alaska PV max drops straight southward into the strong Pacific subtropical jet and then shoots eastward as it undercuts the ridge. Having broken through the ridge, this wave then sets up the situation for a train of disturbances within the strong jet over the Pacific to head towards the California coast beginning on Thursday. An extended interval of heavy rainfall results over the ARB region through Saturday, as a very strong onshore component develops. Should this scenario play out, ESRL/NSSL crews would need to depart for the region on Tuesday.

Steve Koch

Posted by Forecast Editor at 06:56 PM | Permalink

Between now and the middle of next week…. boring, boring, boring! During the weekend, the cut-off cyclonic circulation will spin offshore of SoCal, with deep-layer offshore flow across the ARB. By Sunday night and Monday, a transient shortwave will break through the ridge, but most of its energy and associated moisture will impact OR/WA. This wave will help dislodge the cut-off circulation… which should make landfall near the CA/MX border. Midtrop heights and flow will remain high and weak, respectively, in a rex-block pattern across the eastern Pacific and ARB through the middle of next week. Hence, it will remain dry for at least the next 5 days.

Thereafter, some (but certainly not all) model solutions are now hinting at significant wave energy grinding eastward to the CA coast near the end of next week… in line with yesterday’s medium-range forecast discussion issued by Klaus Weickmann and Ed Berry. We’ll all monitor the situation during the next few days.

Paul Neiman

Posted by Forecast Editor at 07:42 PM | Permalink

Only light precip was observed in and near the ARB last night into today, with Blue Cyn reporting ~0.3” rain. Meanwhile, there are no significant radar echoes in the SAC Valley and Sierras. The latest satellite imagery shows the developing midtrop cutoff slowly drifting Sward just offshore of CA, with the bulk of the modest colder cloud tops already beyond the ARB into S CA. In a nutshell, there is simply not much wet weather to be had across the ARB.

The cutoff is still progged to drift SSWward during the next 2 days, with mid-trop height rises, offshore flow, and generally drying conditions on tap for Friday and the weekend. A shortwave trough should still cut underneath the ridge early next week, but this system will likely be weak… not producing much precip in the ARB area.

The models are backing away from sending a shot of more significant wave energy into the ARB the middle of next week. Instead, some wave energy heads up into OR/WA, and a modest zonal current is now progged to develop well S of the ARB. All in all, not a favorable pattern for HMT06. Having said that, enclosed below is a note of optimism offered by Klaus Weickmann and Ed Berry in their current Medium/Long-Range outlook. Let’s hope it verifies…

Paul Neiman

Medium Range Discussion by Edward Berry and Klaus Weickmann
A NW-SE band of persistent positive convection anomalies across Indonesia have been contributing to La Nina-like impacts during the last several months. These include low atmospheric angular momentum represented by easterly wind anomalies in tropical regions and a weak Hadley circulation. Occasional transient convective events over the west Pacific Ocean are also been impacting the circulation. The current strong westerly flow in the subtropics and easterly flow near 55N appear related to an extratropical feedback to a west Pacific convection episode in early November 2005.

In the last 2-3 days convection has increased again over the northwest Pacific as a strong cold air outbreak impacts southeast China and the west Pacific Ocean. The strongest convection is still centered at 120E and we expect this to hold steady or shift west in the next 1-2 weeks. A strong jet is expected to move across the Pacific Ocean and impact the west coast during the Monday-Wednesday time frame next week, including the HMT operations region although the synoptic details are unclear. The ridge is then expected to retrograde back to around 140W (all models are struggling with this evolution), which should allow a trough to deepen across the Rocky Mountain states, and possibly extend southwestward off the California coast. Subtropical disturbances re-enforced by tropical convection in the warm regions of the SPCZ will continue to support additional subtropical jets interacting with the western USA trough as we go into week 2. Monitoring will be critical to determine if any closed lows interacting with subtropical jets will have an impact on the operations region during week 2.

Posted by Forecast Editor at 07:54 PM | Permalink

Based on the IR and water vapor satellite loops, the shortwave trough is presently impacting N CA as it migrates slowly SEward… confirmed by reports of R- in the Sac Valley and even some R along the N coast region. In addition, a weak comma cloud head is developing in the cloud/vapor field, suggestive of the beginning stages of the shortwave cutting off from the northern-branch flow. The 88D at KDAX shows scattered mostly light precip approaching the lower Sierra foothills from the Sac Valley. SSM/I imagery reveals a PW plume of 2-3 cm over the eastern Pacific approaching CA’s N coast, although the wind profilers at Bodega Bay and Chico show only weak westerly-component 850-700mb flow of <5-10 m/s. Hence, even as the atmosphere moistens up in the ARB, there should not be a significant orographic mechanism to wring out the moisture.

Mostly light precip is still progged to fall across the ARB this afternoon and Thursday as the developing cut-off circulation slowly drifts Sward, but don’t expect amounts to exceed ~0.5”, given the weak orographics and dynamics. Snow levels should range between 5-6kft during this modest event.

As the cyclonic cutoff circulation drifts Sward on Friday, downsloping offshore flow will develop. This, in combination with rising midtrop heights, will make for a dry and pleasant weekend across the ARB. However, none of the model solutions develop a full-latitude ridge along the West Coast during the weekend or beyond… unlike several days ago. Rather, we have a ridge-over-trough scenario developing, whereby wave energy over the Pacific should be able to undercut the ridge next week. The big question is: what will be the strength of the undercutting shortwaves? The latest GFS and ECMWF solutions suggest only weak shortwaves reaching the CA coast next week, with a general breakdown of the midtrop flow over the eastern Pacific. It is too early to determine if next week will bring significant precip to the ARB, although it is becoming increasingly likely that full-latitude ridge conditions along the West Coast will not materialize.

Paul Neiman

Posted by Forecast Editor at 07:56 PM | Permalink

The shortwave trough approaching the Pacific NW is seen clearly as a quasi-organized cloud band in the IR satellite imagery. The coldest cloud tops are presently over coastal BC, WA, and OR, while shallower cloud tops extend SWward from OR to the eastern Pacific. SSM/I imagery shows enhanced integrated water vapor values of 2-3 cm in the offshore-portion of the cloud band…. which is slowly approaching northern CA. The shortwave is presently shearing apart as it penetrates the ridge offshore of CA. Mostly clear skies across the ARB will give way to increasing clouds late tonight and tomorrow morning, as the wave approaches.

On Wednesday afternoon and Thursday, the shortwave will slowly drop SEward along the CA coast, putting the ARB in a region of enhanced moisture coupled with modest large-scale ascent and weak orographic ascent (upslope component at 700mb should be less than ~15kts). Consequently, expect light precip to develop Wednesday afternoon and persist into Thursday. Both the NAM and GFS produce a weak cutoff circulation just offshore off the SF Bay area on Thursday afternoon, and only slowly drops the circulation Sward into Friday. Hence, there is a chance light precip may linger into Friday am. The CNRFC actually puts out ~3/4” of precip in the ARB area from Wednesday afternoon into early Friday, although only in piece-meal fashion. Snow levels should range from 4.5-5.5kft with this event. The heaviest precip will likely fall closer to the upper low center near the coast.

As the shortwave drifts Sward on Friday, downsloping offshore flow will develop. This, in combination with rising midtrop heights, will make for a dry and pleasant weekend across the ARB.

The medium/long range model solutions are showing the possible resumption of shortwave energy beating down the West Coast ridge next week, first with a minoring our shortwave on Monday, and then perhaps a somewhat more substantial system for the middle of next week. Time will tell.

Paul Neiman

Posted by Forecast Editor at 08:26 PM | Permalink

Ridge conditions and dry weather will remain over the ARB today through early Wednesday.

The models are still showing a shortwave trough breaking through the West Coast ridge Wednesday and Thursday, although this forecasted disturbance is a bit weaker in the 12Z cycle simulations. As the shortwave rounds the ridge and then drops SEward along the CA coast Wednesday afternoon and Thursday, it may produce some light precip across the ARB with snow levels in the 5.5-6.5kft range. However, because the associated dynamics and orographics will both be modest at best, don’t expect heavy precip. The shortwave may cut off from the northern branch later Thursday/Friday. Interestingly, the 12Z GFS is the first model to suggest that the cuttoff circulation may stall W or NW of the ARB rather than to its S. If this is the case, light precip may persist into Friday. If, however, the cutoff drops S of the area (as has been progged by all other model solutions), the ARB would see drying downslope easterly-component flow at mid levels by Thursday night and Friday.

Midtrop heights should rise behind the departing shortwave by the end of the week, with the prospect of dry wx through the weekend. The long range models do still hint at the possibility of more modest shortwave energy breaking through the West Coast ridge after this weekend, but as of yet, there is no indication that a major zonal race-track of embedded frontal waves will develop.

Paul Neiman

Posted by Forecast Editor at 07:47 PM | Permalink

Weather in the northern Sierras will remain cool and dry today through
Tuesday, with tropospheric ridging aloft and Nerly flow at low levels.

By midweek, a shortwave trough is progged to break through the ridge and
impact the Sierras. Although this system will likely be relatively
modest in strength and possibly associated with low-level frontolysis,
the various models are now all showing this feature. Once rounding the
ridge, the shortwave will dive SEward along the CA coast on Wednesday
(probably starting in the afternoon) and Thursday... and will likely cut
off from the main northern branch in the process (well S of the ARB).
Upslope Werly flow will likely precede the wave passage, but it should be
relatively weak (<10 m/s) and in a stably stratified environment... thus
don’t expect major orographic precip enhancement during this phase of the
storm. However, as the wave drops SEward past the ARB it will drag in
cold air aloft, so some convective activity may be expected toward the
tail end of the storm... especially if the unstable conditions coincide
briefly with the remnant upslope flow. All in all, total precip with the
storm should not be impressive. I will stick with John’s assessment from
yesterday.... >50% chance of less than 0.5”.

Ridging should build in quickly behind the departing shortwave toward the
end of next week, with the prospect of dry wx through at least next
weekend. The long range does hint at the possibility of more modest
shortwave energy breaking through the ridge late next weekend or beyond.
We shall see.

One last comment: if this Wednesday/Thursday scenario were to have
evolved during an active weather pattern, it would have been worth
considering executing a mini-IOP. However, because there are no other
storms on the horizon, it will probably not be worth sending folks out to
CA.

Paul Neiman

Posted by Forecast Editor at 08:01 PM | Permalink

Ridge is building surface and aloft along and off the West Coast.

All indications point to an absence of significant pcpn over the ARB for at least the next 60h, and possibly much longer. Upper trof is maintained in the Central Pacific by GFS and ECMWF next few days and the main storm track will be from north of Hawaii into the Gulf of Alaska. The only possibility I see for any pcpn over the ARB within the next 120 hours is Tuesday-Wednesday of next week. A moisture band (probably mostly above 850mb by the time it reaches the northern Sierra) ahead of a short-wave trough moving through the mean-ridge position maintains sufficiently in the model forecasts (GFS, ECMWF) to allow some pcpn over parts of Northern CA ahead and along the weakening surface front. I put the chances of Blue Canyon getting a total of more than 0.5" out of this event at under 50%.

Ensembles suggest that this Tuesday-Wednesday event will be at best a minor intrusion in an otherwise dry pattern for the ARB. Northerly-component flow likely at mid levels later next week as the short-wave trough aloft evolves to a weak cutoff low southeast of the area.

John Brown

Posted by Forecast Editor at 06:15 PM | Permalink

The storm is gone, and the clearing skies shall commence…

Rainfall totals with yesterday’s storm included a very respectable 6 2/3” at Blue Cyn and almost 10” a little farther N at Bucks Lake. Given the high snow levels (>7-8kft), only the highest terrain received significant snowfall. Meanwhile, remnant midtrop troughiness will migrate Eward across the ARB tonight, but the ARB will not receive significant precip due to the deep-layer drying that has occurred.

Midtrop height rises will occur over CA during the weekend and into next week in response to offshore ridge building. Hence, the ARB should remain high and dry through at least early next week. Most medium range model solutions (including a mojority of ensemble members) keep the ridge in place for all of next week (and perhaps beyond), although some solutions suggest some weak shortwave energy breaking through the ridge… with just a slight chance of a little light precip during the middle of next week. The main message here is that there is a good chance the ARB will remain mainly dry all of next week.

Paul Neiman

Posted by Forecast Editor at 07:44 PM | Permalink

Well… it’s show time! The latest satellite water vapor loop shows the N-branch vortex dropping SEward offshore of the Pacific Northwest, while a series of mesoscale waves in the S-branch vapor band extend WSWward from the ARB to at least 140W. The SSM/I imagery reveals that this mesoscale wave train is embedded within a potent atmospheric river, with core IWV values in the 3-4 cm range. Land-based GPS measurements from sites in the N half of CA indicate that IWV values have surpassed 3cm… and rising. The wind profiler at BBY showing stout SWerly flow of 30-40 kts between 950-700mb. Surface winds at Blue Cyn are now gusting past 40kts, accompanied by R+ at times. The 88D at SAC shows 25-40 dBZ echoes (with some 45+ echoes… perhaps brightband induced) anchored to the Sierras. Rainfall totals have already surpassed 4” at places like La Porte, Brush Creek, and Strawberry Valley… and Bucks Lake has ~6” and counting. Blue Lake has received 3” of rain, with plenty more on the way. Snow levels are presently quite high. I just had a discussion with Dave Kingsmill, who is operating the X-band radar at Colfax…. he said the radar is seeing brightband conditions above 9kft. Finally, thunder has been reported at Santa Rosa. Bottom line: it’s a helluva day in the northern Sierra for our official debut of the HMT06 winter season!

Now, on to the short-term forecast. All the ingredients are in still in place (see previous two discussions) for significant precip in the ARB through tonight. The presence of the southern-branch mesoscale waves embedded within the ideally situated atmospheric river would suggest the possibility of periods of rather heavy precip, although trying to time the passage of these waves will prove challenging. Cold fropa is slated for ~05-08Z tonight, likely resulting in another episode of enhanced precip. The precip will quickly become more showery after fropa, then end fairly quickly early Friday a.m. Snow levels will descend quickly after fropa, dropping to the 4-5kft level by 12Z Friday.

The longer-range outlook for the weekend and onward through at least the middle of next week (based on ensemble GFS output and the NOAA/CDC experimental 6-10 day outlook) calls for rising midtrop heights associated with ridge building offshore. It is unlikely that significant precip will fall in the ARB during this period.

Paul Neiman

Posted by Forecast Editor at 07:48 PM | Permalink