HMT-West 2007: Forecast

Current conditions
The 500 mb cut-off low has moved inland to a position over San Diego and appears to have opened. This has occurred in response to the shortwave trough positioned over Wyoming that is digging southward fast.

Winds aloft of ARB have increased and turned to the north/northeast. At the surface, a stagnant situation continues with fog in the Valley. Temperature in Sacramento is in the upper 30s; Blue Canyon is in the mid 30s, and upper-teens to low-20s are reported at higher elevation.

Snow depth at higher elevations ranges 6" at Blue Creek (5280') to 25.8" at Slate Creek (5700') to 48.7" at Sand Flat (6750). The snow pack at the Blue Canyon radar site is 0".

00-72 Hour forecast
The weather features of interest are the shortwave trough that will kick the cut-off low eastward and another shortwave trough barelling out of Canada into the central US around 72 hours. The GFS and NAM disagree on the westward extent of the trajectory of the second shortwave trough, with the NAM bringing the core of the jet streak into a position farther west. The location of this jet streak will have a bearing on whether the finger ridge moves inland or remains offshore. The GFS, with the more eastern position of the jet streak, brings the finger ridge inland in advance of a Pacific shortwave trough.

The bottom line for the ARB is that model guidance suggests the chances of precipitation are very slim over the next 72 hours. The only chance is for flurries at higher elevation locations, depending on the cloudiness associated with the jet streak in the 72 hour period.

3-10 days

Two members of operational GFS 00 UTC ensemble have precipitation in the ARB at 00 UTC Feb 6; Six members of Candian 00 UTC ensemble. PSD mean precipitation anomaly has precipitation in the ARB. This all hinges on a couple of factors. First, a potent shortwave trough over the central-north Pacific is forecasted to move the position of the large, broad trough eastward, while the jet stream becomes more zonal and intensifies over the central-north Pacifica. The intensification of the jet stream may be related to Tropical convection, as the Tropical ridge moves westward and forms a very nice wave train with a ridge over the Arabian Sea, a trough over the Bay of Bengal, and a ridge over the western Tropical Pacific at 144 hours into the forecast. It's this ridge over the western Tropical Pacific that would permit the Tropical convection to flare up and a northward flux of high IPW air into the subtropical and extratropical jet stream.

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The following is a discussion that was posted on the Ed Berry "Atmospheric Insights" Blog this afternoon (link below). This information may be useful for HMT concerns, particularly given today's conference call.

http://weatherclimatelink.blogspot.com/

Tropical convective forcing has consolidated around 140-160E along the equator while only sporadic convection persists across the South Pacific. In addition, tropical forcing has also been increasing across northern South America and South Africa. The latter is a response to the remnant dynamical signal of the December-early January MJO interacting with the Southern Hemisphere (SH) extratropics.

A strong Rossby wave energy dispersion (RWD) linked to the west Pacific tropical forcing is exciting the positive phase of a western Pacific wave train as I type (with SH symmetry). This pattern resembles the positive phase of the Pacific-North American teleconnection (PNA) but shifted west by about 20 degrees of longitude. As has been expected for about 10 days, bitterly cold Arctic air from central Siberia is currently being transported across the North Pole and will plunge into the CONUS by the end of this work week. The initial surge of cold air should be into Montana, with the Northern Plains, Upper Mississippi Valley and Great Lakes states being impacted the most severely.

So, what is the most probable evolution of our weather-climate situation during the next few weeks, given uncertainty and inadequate model guidance? Specific diagnostic factors to consider include transports and tendencies of relative angular momentum (AAM), the evolving SST and SST anomalies, the subseasonal tropical convective forcing and the blocking becoming established at the northern polar latitudes (with a possible warming of the stratosphere).

Poleward propagation of zonal mean zonal wind anomalies continues, with moderate westerly anomalies throughout the subtropical atmospheres, easterly anomalies around 50N and westerlies again farther north. The global signal of relative angular momentum is quite impressive with positive anomalies of roughly 2 standard deviations as of January 27th based on the reanalysis climatology. However, as discussed in past postings, this westerly flow did not evolve into a strong combined North Pacific jet typical of a warm ENSO. Strong easterly anomalies in the subtropics preceded the convection increase near the date line and may have interferred. Only during the last few days has there been a weak reversal of AAM transports with westerly flow being fluxed equatorward from the midlatitudes.

Looking farther out, a renewed active phase of the MJO may develop from South Africa into the southwest Indian Ocean during roughly weeks 2-3. Coupling with the warm west Paciific SSTs looks probable centered ~10S/160E while at least diurnally intense convection occurs across much of Brasil. This would suggest a return to a GSDM Stage 4-1 (La-Nina like) response meaning zonal mean easterly flow anomalies should re-appear across the deep tropics while the tendency of relative AAM becomes negative. For PNA sector, the large ridge currently developing west of Canada may shift northwest to Kamchatka during week 2 while the above mentioned anomalous subtropical westerly flow ”undercuts” the east Pacific ridge. Going into the middle of February there may be a situation of a cold trough extending from central Canada to just off the USA west coast interacting with a moist subtropical jet.

As mentioned above, the cold regime is on track for most of the USA for week 1. While there is likely to remain a cold air source for especially the northern states weeks 2-3, moderation of temperatures is expected. Intense convective lake effect snow is a good bet for week 1, with only light snow events across the Northern Plains on east. Portions of the Deep South to the east coast will need to be monitored for possible significant wintery precipitation. This whole pattern should shift north and west weeks 2-3, with possibly an active southwest flow storm track across the Plains by the middle of February. Much of the USA west coast should also finally get some decent precipitation weeks 2-3.

Please note: These are probabilistic statements, which we will try to quantify in future posts.

Ed Berry and Klaus Weickmann

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Current conditions
The 500 mb cut-off low continues to sink southward of the ARB, with its center now positioned west of Oxnard.

With the southward movement of the 500 mb cut-off low, the northern part of the Valley has cleared. Temperature in Sacramento is in the lo
w 40s; whereas northwest of Sacramento the temperatures are 5-10F warmer.

00-72 Hour forecast
The chances of precipitation in the ARB remain slim over the next 72 hours. It appears the cut-off low will be dislodged and slide eastward beginning around the 60-72 hour forecast period, as a another shortwave rotating around the massive Hudson Bay low may come into phase wi
th the cutoff low. A small chance of light precipitation may occur with this cold event, with precipitation amounts not exceeding a few te
nths and almost completely confined to the upper elevation locations.

3-10 days
The long-range guidance is focusing on Feb 6 as a potential IOP day. The operational GFS differs from the PSD GFS and ECMWF models in that
the operational GFS ensemble mean keeps the west coast ridge in place, whereas, the PSD GFS and ECMWF models have a reduced amplitude ridge that has shifted eastward in advance of a potent shortwave trough. IPW values are quite high, and if the ensemble mean pattern were to verify it would be the best-looking event of this year's HMT campaign.

Chris Anderson GSD/FAB

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The active tropical convective forcing has shifted to ~140-160E while weakening has occurred across the South Pacific, especially along the South Pacific Convergence Zone (SPCZ). Circulation anomalies over the PNA sector are retrograding as a response. There is a possibility of renewed MJO activity across the southwest Indian Ocean by the end of week 2 while forcing remains centered ~10S/160E. This situation would be more favorable to moist west and southwest flow for ARB.

Ed Berry and Klaus Weickmann

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Current conditions
The southwesterly flow of 500 mb cut-off low remains far south of the ARB. It is, however, moist enough to produce 0.5" rainfall amounts near Oxnard and southward.

Surface conditions are pretty much unchanged in the ARB. Fog is reported in the Valley with temperatures in the low 30s at 12 UTC. Temperature at Blue Canyon and Forrest Hill is in the mid to upper 30s. At higher elevation, where there is snowpack, the temperature is well below freezing.

00-72 Hour forecast
The chances of precipitation in the ARB remain slim over the next 72 hours. It is possible that some cloudiness and light precipitation could occur in spurts as short waves and cloud packets revolve around the cut-off low. Precipitation amounts would be at most a few tenths of an inch, but the more likely scenario is that a few high clouds will move over the ARB without producing precipitation.

3-10 days
Tropical moisture is making its way across the Pacific, within the 20N to 30N latitude band. The GFS ensemble continues to point at Feb 4 as the earliest day of potential interest, looking more like a cold event than an atmospheric river. A pool of IPW > 35mm air develops west of California about Feb 3, so that one can't rule out the possibility a small whirl could break from the larger trough and tap into that moisture pool to form a sort of mini-atmospheric river (call it an atmospheric creek? or tributary?). The more interesting period looks to be Feb 6th and beyond.

Quite a bit of discussion concerning the forecast beyond 10 days during the HMT coordination call. Ed and Klaus provided plots in another blog entry that suggested a wave train that extends across the Indian Ocean into the western Pacific may continue to connect itself with the subtropical and extratropical jet stream, causing the west coast ridge to retrograde. They believe the GFS solution probably will not be able to simulate this connection correctly, but that other ensembles, particularly those generated at PSD, have solutions consistent with the scenario of a retrograding ridge. Beyond day 10, another MJO may form as a wave packet moves off the African coast into the Indian Ocean.

Chris Anderson GSD/FAB

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Current conditions
The 500 mb cut-off low has pushed southward, producing SE 500 mb winds at Reno. The nearest location reporting SW flow is Vandenbrug. As a result, the relatively high IPW air has pushed southward. IPW Bodega Bay is 0.6"; Point Piedras Blancas is 0.6" having undergone a steady decline from 0.8" at 00 UTC. Saticoy is 0.8"

Fog is reported in the Valley with temperatures in the low 30s at 12 UTC. Temperature at Blue Canyon and Forrest Hill is in the mid to upper 30s. At higher elevation, where there is snowpack, the temperature is well below freezing.

00-72 Hour forecast
Yesterday's GSD forecasts contained some precipitation in the ARB by 00 Z Monday, Jan 29. Both wrf-schultz and wrf-thompson had about 0.2" with the wrf-thompson having a larger coverage. Today's NAM model, which provides the lateral boundary conditions for the GSD ensemble, keeps the low south of the region and the GSD ensemble correspondingly keeps the precipitation in southern CA. Precipitation amounts in the southern
Sierras are modest in the 12 UTC GSD ensemble, ranging 0.2-0.5" by 00 UTC Tuesday, Jan 30. Amounts in the ARB are spotty and do not exceed 0
.1".

The model guidance also indicates potential for precipitation in and near Reno as the low moves inland and wind direction shifts to northeasterly. GSD ensemble predicts amounts 0.01" to 0.2" along the eastern shore of coastline and in the hills to the north.

3-10 days
The long range outlook is unchange with the potential for something interesting no earlier than Feb 4. The 00 and 06 UTC GFS initialization appears to be a little dry in the western Pacific, where a belch of Tropical moisture is moving north of 20N. This may provide some relief to HMT in a week or so. Composite SSM/I 12 UTC yesterday to 00 UTC today showed > 40mm north of 20N; whereas GFS kept > 40mm south of 20N. The forecasted shift in the polar vortex is still a few days off, and, so, there is large uncertainty in how it will impact the large trough west of the Aleution Islands and finger ridge over the western US (so named because it is remarkably narrow and appears to be giving HMT the finger).

Chris Anderson GSD/FAB

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Current conditions
Fog in the Valley?! You don't say!

The 500 mb cutoff low over the southwest US has moved eastward over OK. This has allowed a bit of moisture to become wrapped into the 500 mb cutoff low that now resides along the central CA coastline. Reno is reporting a SE wind direction at 500 mb, which is an indication of how broad the cutoff low has become. IPW at Bodega Bay is pushing 0.8 inches.

Like yesterday, the Valley is foggy with temperatures in the mid 30s. Temperature at Blue Canyon and Forrest Hill is in the upper 30s. At higher elevation, where there is snowpack, the temperature is well below freezing.

00-72 Hour forecast
Day 2 is the only thing to talk about in this period. As mentioned in the Current Conditions section, the cutoff low has pulled a little bit of moisture into itself. 00 UTC GFS suggests more could be on the way with values as large as 1" in the vicinity of the low. The track of the low as it opens provides some possibility of precipitation beginning as early as Saturday evening. The likelihood of precipitation exceeding 0.5" is small whether one considers the GFS and CMC ensemble or Whitaker-Hamill analogue technique. It appears the probability of >0.5" by 00 UTC Monday is 15-20%. Of course, the GSD WRF-NMM guidance crashed on the day I was at home with my strep-infected daughter. It is unavailable right now.

3-10 days
The first signs of something potentially of interest to HMT are taking form in the Western Pacific. The 00 UTC GFS shows a plume of air with IPW > 40mm moving into the mid-latitude jet stream near 140E. By 96 hours, this moisture stream has combined with a smaller one flowing out of the Tropics just south of Hawaii to form a subtropical pool of air having IPW > 40 mm extending from the dateline to 140W.

Of course much disagreement is evident amongst the CMC and GFS ensemble members beyond 96 hours. The earliest interesting situation in either ensemble occurs Feb 3-4. A couple of members have a cold event in association with the expansion and southwest movement of the Hudson bay low. A couple of members have this low flattening the ridge and producing westerly flow over N CA. While most members continue to have a high-amplitude ridge over the area, and, worse yet, some move a large trough or cutoff into old Mexico, which would divert subtropical moisture away from the ARB.

Chris Anderson GSD/FAB

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Current conditions
Much the same weather pattern today. The 500 mb cutoff low remains in position just off the N CA coast. It has moved southward, perhaps, 50-100 miles. IPW at Bodega Bay has increased between 0.6 and 0.7 inches.

Like yesterday, the Valley is foggy with temperatures in the mid 30s. Temperature at Blue Canyon and Forrest Hill is in the low 40s. At higher elevation, where there is snowpack, the temperature is well below freezing.

00-72 Hour forecast
There is an outside chance of precipitation exceeding a few tenths of an inch. IPW values have increased a bit, as expected in yesterday's NWP guidance, but upslope flow is nearly nill at the moment. The perfect storm scenario for this setup would include an abrupt eastward movement of a cut-off low that is now positioned over old Mexico, which could permit the cut-off low off the CA coast to tap into a bit of tropical moisture. The cutoff-low off the CA shore would then need to move onshore at just the right location to produce upslope flow. With a slight increase in IPW, perhaps to 0.9, there would be a chance for precipitation as the cut-off low moves onshore, but, again, far from the amount that is of interest to HMT, not to mention this scenario is a long-shot anyway.

3-10 days
As discussed yesterday, the polar vortex is forecasted to undergo a major change in the 5-10 day period. First, over the next 3 days, the broad north Pacific trough is forecasted to sharpen significantly west of the Aleutians. The effect is that the ridge over the West Coast is forecasted to move westward, as the vorticity dynamics intensify west of the Aleutians, and to become increasingly pinched off between day 5 and day 10. The polar vortex stretches southward during days 5 and 10 to create a series of intense mid-latitude storms that will move across the Pacific while retaining fairly high amplitude. As discussed in the HMT phone call, the vortex is being stretched in a number of directions: Siberia/China, eastern US, and Europe. This is part of the reason the response over the northern Pacific is less coherent than in early December. With the ongoing flare-up of Tropical convection, a stream of Tropical moisture can be tapped by these high amplitude storms. The pattern that is forecasted to set up would support a series, perhaps 3 to 5, of high amplitude and moist waves moving across the Pacifc. This differs from last season's iop4, the poster child, in that the flow this year would be less zonal so that a multi-day period of heavy rainfall would be unlikely, but, instead, a day of very heavy precipitation, much heavier than HMT has seen to date, is the more likely scenario to occur within this pattern.

In terms of the likelihood and timing of specific events. The GFS and CMC ensembles indicate the earliest day of interest is Feb 4, as a couple of members have the first wave making a direct hit on ARB. While this is a low-probability event given the forecast lead time, it provides a sense of how different these storms could be compared to what has been observed thus far. In the most intense precipitation event, the 12-hour precipitation total exceeds 100 mm for two consecutive 12-hour periods, before shutting off.

10-days and beyond
Ed and Klaus think Tropical convection will intensify in the Indian Ocean and west of the Dateline (~150-160E), reaching a level that could impact the flow over the central Pacific about week 2 and, especially, later. This could lead to a retrogression of the trough, perhaps permitting more vigorous cold events or westerly flow undercutting the ridge.

Chris Anderson, GSD/FAB

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An MJO excited deep convection near the date line in early January 2007 but a "combined" Pacific jet and eastward shifted storm track did not develop. A persistent pattern of subtropical easterly and mid-high latitude westerly flow anomalies may have intervened. Date line convection is now weakening; a portion is shifting southeast into the southern hemisphere and another is shifting west toward the west Pacific. Combined with eastward shifting convective activity over the Indian Ocean, a consolidation of positive convection anomalies around Indonesia has already occurred. This activity may eventually spread over the warmest SSTs currently in the west Pacific, south of the equator.

The current short term amplification and retrogression of the circulation anomalies over the Pacific Ocean will dominate the weather patterns during the next 1-2 weeks. Prospects do not look good for rain along the west coast during this time. The behavior of the circulation beyond week 2 is partially linked with the strength and location of tropical forcing. The Indian Ocean SSTs continue quite warm and we expect convection to redevelop there, possibly aided by a dynamical component from the recent MJO. Convection should also stay active over the west Pacific. If the west Pacific convection dominates, a very boring weather pattern may be in the offing for the USA west coast beyond week 2. Increased activity over the Indian Ocean would lead to a better chance for a trough along the west coast and more favorable prospects for the ARB.

Klaus Weickmann and Ed Berry

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2007-01-25 HMT Wx Forecast and Discussion

New forecaster today. No more bad dreams.

Current conditions
At 12 UTC, a 500 mb trough is positioned just off the N CA coast. Despite this normally favorable condition, the 700 mb and surface flow remains very slow and without a significant upslope component. Furthermore, IPW at Bodega Bay is between 0.3 and 0.4 inches.

Beneath the stagnant flow, fog is evident in the Valley where temperatures are in the mid 30s. Temperature at Blue Canyon and Forrest Hill are 48 and 43, respectively, where the snow pack is zero. At higher elevation, where there is snowpack, the temperature is just below freezing.

00-72 Hour forecast
There is no chance of precipitation exceeding a few tenths in this time period. The main weather feature is the low positioned offshore near N CA. This feature is forecast to wobble, but without ingesting any Tropical moisture.

3-10 days
The GFS ensemble contains some precipitation in the ARB area in days 3-5. It is entirely tied to this wobbling low. The ensemble brings IPW values between 0.5 and 0.7" inland as the low wobbles onshore. Three-hour precipitation amounts do not exceed a few hundreths of an inch, however.

Things get interesting beyond 5 days. There is a shift in the Polar jet stream that is reminiscent of a shift that occurred in early December that lead to a couple of IOPs. The long range GFS shows a shift in the location of polar air beginning Jan 29-30, such that cold air and very strong upper-level vortices are expected to move across the Siberian plains and northern China. The result is a succesion of powerful waves moving across the Pacific Jan 31 through the end of the forecast guidance on Feb 8. There are two differences between this pattern shift and the one that occurred in early December. First, the Tropical convection is kicking in gear near 120E and providing a stream of moisture into the Westhern Subtropical Pacific. Second, the pattern established across the Pacific is less zonal but consisting of more prominent storms than in early December. This leads me to think that the chances of a "big one" are improving. It seems plausible that a stream of Tropical moisture could be carried eastward with one of these large amplitude storms.

The timing of the first storm of interest appears to be Feb 2 or 3.

Chris Anderson

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The models continue to mess with my mind, this is like a bad dream. I guess handling 4-5 closed lows at 500 is just too much to expect any model to do well with. Solutions today again differ greatly from yesterday's however the bottom line remains the same. The one element that this week's forecasting has demonstrated to me is the effectiveness of ensemble forecasts especially when the weather gets into a difficult-to-model regime as it currently stands. I think in spite of the fact that the offshore low pressure system is now progged to move N today instead of S, giving the ARB better flow aloft (W to SW flow) there is not enough moisture to work with and the end result is that both the NAM and GFS keep the ARB dry.

This morning's NAM analyzes a 500 hPa ridge over the NV CA border with the circulation center near Reno. The upper level low that we have been watching in old Mexico has been moving slightly to the east and now is positioned south of the CONUS between El Paso and the AZ NM border. The current analysis still does not have a closed low off in the Pacific, but the approaching wave is beginning to affect the ridge. Winds begin to pick up out of the SW 06utc Thursday as the approaching Pacific wave begins its split and the southern part begins to close off. By 00utc Friday the low has cut off in the NAM and is impinging on the OR CA coast and is in a better position to come inland for the ARB. Meanwhile at the same time the northern piece of the wave which carries more energy has closed off and is sitting atop the ridge depressing it while the Mexican closed low has slightly retrograded. This pattern looks more like the one forecast 2 days ago instead of yesterday with the exception of the better location of the offshore low. The NAM brings the 500 Pacific low a tad inland 06ut Friday into southern OR. The ARB at this time is experiencing SW winds around 25 m/s. By 00utc Saturday the low has two distinct vort centers (as we have noted might be the case in past runs) and its diameter increases and moves north off the central OR coast with a very strong vort center developing on the west side of the closed system, I would expect the low to dig after this point and sure enough 18utc Saturday has the low moving south and strengthening but still remaining offshore at about the OR CA border with a 548 height. In spite of the fact that the winds pick up more favorably over the ARB again the moisture remains right on the coast and dry conditions persist inland. It appears that the moisture plume has become wrapped up in the upper level circulation and is not in a position to be advected inland by the better winds south of the low; so the moisture goes where the low moves. By 00utc Sunday the low moves slightly north, still off shore but a rogue piece of moisture gets caught up in the circulation south of the low and appears to head for the ARB. This is certainly NOT the kind of scenario the HMT exercise is about. There appears to be no connection to a topical moisture plume with this small moist surge. By the end of this morning's run, the NAM has put down no precipitation over the ARB, even the higher terrain in spite of the late shot of moisture. This is out to Sunday 00utc (84h forecast). And as of this time the closed low is STILL off the coast beginning to interact with a new Pacific low that develops to its SW. Meanwhile the ridge axis has drastically retrograded (hats off to Klaus and Ed) now setting up over the Pacific near the AK coast. Perhaps this is the signature of the beginning of the MJO changes that Ed mentioned last week.

Today's GFS run shows no precipitation over the ARB until 06utc 31 January. So for this weekend, it is also dry. As far as the specifics in the treatment of the upper air systems, the GFS keeps the low pressure center of interest further off othe coast overall with about the same N-S excursions. However, the flow over the ARB from the GFS remains steady from the west but much weaker mainly due to the fact that the low pressure center does not come as close to the ARB.

The ensembles today are bleaker than yesterday with no precip forecast over the ARB for this system from the GFS. The Canadian ensembles are similar in that the only precip is either off shore or comes in north of the ARB (one frame).

In summary, despite poor model performance, the only reason I think that the forecast has remained dry for the ARB is the simple absence of water; certainly most of the other ingredients would have been in place had there been a good moisture plume to work with. I still see no reason for and IOP late this week.

Looking downrange, the ensembles are still indicating a possibility of something near the 2nd or 3rd of February at the earliest. Until that time the area looks dry.

Dan Birkenheuer - ESRL/GSD

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The satellite picture (below) is from the Japan Meteorological Agency (JMA) on 24 January 2007 @ 8 am MST (1500 UTC). Tropical convective forcing continues to slowly increase ~120E while weakening over the South Pacific. We are already seeing a response of the East Asian storm track that will allow for the western North American ridge to retrograde over the next week or so.

Ed Berry and Klaus Weickmann

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The following was published on my (Ed Berry) "Atmospheric Insights" Blog this afternoon (23 January). I thought this might also be useful to the audience of these postings.

Tropical convective forcing has been persistent across the South Pacific along the SPCZ for roughly a couple of weeks. This response was expected given the December 2006-early January 2007 MJO and warm SSTs (~1-2C anomalies) associated with ENSO. The global circulation has been responding with the recent large tendency in global relative atmospheric angular momentum (~30 Hadleys) and the appearance of twin upper tropospheric subtropical anticyclones around 160W. Rossby wave energy dispersion linked to these twin anticyclones (with inter-hemispheric symmetry) may be contributing to the onset of North Atlantic blocking (negative phase of the NAO). There has been poleward propagation of zonal mean westerly wind anomalies resulting in 5-10m/s anomalies at 200mb in the subtropical atmosphere. GSDM Stage 3 best describes the current global circulation pattern. However, as discussed in our January 19th posting, this El-Nino-juiced forcing has been evolving in a more persistent regime that developed just prior to December 1st. The regime has been characterized by transport of westerly momentum out of the tropics into the higher latitudes. The forcing from the South Pacific has not changed this La-Nina like (GSDM Stage 1-2) pattern.

Rossby wave energy dispersions within the regime have led to a revival of tropical convection across the East Indian Ocean into Indonesia. Fast baroclinic wave packets moving through South Asia are interacting with this forcing leading to storm development across the west and central North Pacific. A discontinuous retrogression and amplification of the North American ridge into the Arctic is expected within the next week. While a convective signal in the South Pacific is expected to persist (allowing a subtropical jet across the Deep South at times), tropical forcing may become quite robust near 120E by sometime week 2, in which case, the ridge may retrograde to 150W. The ESRL/PSD and other model ensembles lend support to this scenario. In fact, blocking may develop all across the polar latitudes as we go through February. Another MJO may also develop across the Eastern Hemisphere next month which could lead to a further demise of our warm ENSO event.

The screaming message for a good part of the country is a turn to much colder temperatures by about a week from now. Bitterly cold Arctic air that has been “bottled up” across much of Siberia is likely to plunge into the USA with a few surges, likely centered on the Northern and Central Plains/Upper Mississippi Valley. Strong winds and brutal wind chills will be probable. Best opportunities for significant wintery precipitation should be from the central-southern Rockies into the Tennessee Valley, Deep South into the mid Atlantic States. Lighter snowfall events may occur with the Arctic surges (along with intense convective lake effect snow). This whole precipitation pattern may shift northwest later weeks 2 and 3 (troughs across the Rockies/western states) while temperatures slowly moderate. By around week 3 the ridge should be far enough northwest to allow strong and moist westerly flow (including “undercutting”) to impact the west coast with significant precipitation especially California.

Please note: These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. Thus this is the time scale on which a reversal of the transports could occur

Ed Berry and Klaus Weickmann

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The moisture plume associated with the potential precipitation in the ARB late this week is detached from the tropics. By the time the plume makes its way into the ARB, amounts look to be about 1.4cm at low elevations and 0.9cm at high elevations. A meager moist surge if it could actually be called a surge since it briefly sweeps through the region from north to south in a matter of 12 hours. This is on track with yesterday's thinking that the "event" if one was to call it that would probably be less than 18h in duration. The exact timing of the "event" is still a question that remains unclear (and may never happen).

This morning the ARB is under clear skies with single digit temperatures at high elevations and some fog reported at lower elevations with temperatures around freezing. It looks like there was plenty of radiational cooling last night.

The NAM shows the closed low in N. Mex. with some signs of deformation and the tendency is to move the low center further east before it closes off symmetrically by 18utc Wednesday. It then intensifies over N. Mex. south of the boarders of AZ and NM. Meanwhile, the ridge progresses inland winding up with its axis further east than yesterday (over central MT) while the approaching closed low center looks to be too close to the ARB latitude. Winds over the ARB at 500 are from the S as the low hangs out over the coast. At 00ut Friday the closed low off shore elongates much like yesterday's GFS. The low stays offshore for the most of the run (it sort of moves back and forth, never really coming inland) putting most of the precip on the coast. The NAM's 84h forecast from this morning has the ARB receiving very little precipitation (0.03 inches) by 00utc Saturday.

The GFS tends to keep the low as a single unit but with 2 distinct vort centers, moving it slightly to the same latitude of the ARB and supplying S winds to the ARB (not favorable). This morning's GFS looks far different from prior runs with the low staying off the coast for an extended period and gradually dropping south putting the ARB in unfavorable wind flow (easterlies) with time. Precipitation amounts from this morning's GFS are zero over the ARB during the episode timeframe. This is now beginning to look more like the ensemble means we have been observing over the past few days, and it now appears that both the NAM and GFS are beginning to converge to a similar solution.

Today's ensembles show less probability with 2 frames of 12 showing precip over the ARB with one of these showing a fairly good amount of precipitation by 12utc Saturday. All in all, the ensembles are in more agreement that the region will stay dry. The Canadian ensembles show one frame with precip over the ARB by 00utc Saturday. Again all other runs look doubtful. One of the Canadian frames plays out very similar to the GFS forecast, bringing more moisture in at the coast with little or nothing in the ARB area.

In summary, the system at week's end still looks too weak for an IOP and might be falling apart. Furthermore, the models are beginning to come into agreement, but to a solution that is not favorable for precipitation. The system still bears watching since the models are radically changing the movement of the closed low systems run to run (and there are several low centers interacting, for example one going over the ridge), but there have been no major changes in the forecasts to bring this to potential IOP status.

Dan Birkenheuer - ESRL/GSD

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This morning it looks like a weak front is poised to move over the ARB mainly distinguished by the drier dewpoints behind it and northerly surface winds. South of the area the air remains mostly near saturation. The interesting picture proceeds at 500 hPa and the evolution of the approaching wave that we mentioned yesterday as a possible player for something at week's end.

This morning the NAM has the 500 cutoff over N. Mex. and directly south of AZ with the persistent ridge still over the pacific coast. This closed low intensifies through 12utc Wednesday and moves slightly southward but deepens with time. The ridge is just about centered over the ARB with very light winds aloft up until Wednesday. Then the approaching sharp Pacific wave splits just as we discussed yesterday and the southern part comes in under the ridge. Just as this Pacific closed low comes into the scene, the NAM shows the ridge centered over central WA with its axis aligned with the closed low in Mex. The Pacific low is moving into the ridge center with time and is off shore at about the latitude of the ARB increasing the winds at 500 from the SSE though Thursday at 00ut, after which they begin to pick up strength. On the other hand the GFS has this as an open wave until Thursday 12ut and it is rather elongated north to south with the center of the low slightly further north in a better position. By Thursday morning winds pick up from the SE at 500. By Friday at 12ut the low in Mex. begins to lift out and the GFS has the second low coming on shore just to the north of the ARB; winds become favorable out of the SW to W and there is a vort max just north of the area. Then by 00ut Saturday things change rapidly. The first low begins to leave the scene moving over SE Colorado, the low north of the ARB elongates E-W and the winds drop in speed very quickly. Meanwhile, another very sharp wave approaches rapidly from the Pacific. This causes the high pressure ridge to retrograde almost directly over the coast and it begins to amplify. Winds over the ARB go NW and remain fairly strong. The sharp wave closes off well offshore, the ARB low weakens and falls apart and the off shore low heads way south of the CONUS. This leaves the ARB under the influence of a very strong 500 ridge with an axis off shore. By Thursday 00utc 1 Feb, the weather pattern over the CONUS is dominated by a strong high pressure centered north of Manitoba that in combination with the strong ridge off shore of the Pacific coast brings arctic air to most of the CONUS. Winds over the ARB are strong out of the N.

The winds appear favorable for a short time near week's end; the next question is what kind of moisture situation is progged to occur and whether the models show any precip action over the ARB. First the GFS Pacific TPW forecast shows a weak moist plume associated with this possible surge that barely comes inland by 12utc Thursday 25 Jan. with a moisture depth of about 1.4 cm TPW (pretty weak) and much lower over higher terrain. It is quickly swept south and dry continental air quickly replaces any moisture, probably due to the deforming low center and a possible cold air outbreak moving into the ARB from the NE. For precip probability and timing, I looked at the ensembles for guidance. This appears from all upper air activity to be a short-lived event if it occurs. The exact timing of the event might be in question, but if this does occur, it will not be a protracted event.

Friday at 12utc, 3 of the 12 GFS ensemble runs have precip north of the ARB, none of which are in an ideal location. Saturday 00ut 27 Jan, again 3 runs show precipitation over the ARB in a more favorable position than the prior time. Sat at 12utc we have 4 frames showing precip with one of those marginal. By Sunday 00ut, one frame shows minor precip over the ARB, after that time the area remains free of precipitation until something looks fairly substantial by Wed 00utc Feb 7th. The best probability for precip looks to be about 0.25 at the end of this week, and the amounts look light.

Looking at the Canadian ensemble, there are 3 runs that show some precip by 00ut the 27th. Two runs for 12 utc the 29th and one panel 00utc the 29th. (Note there is a gap in the runs between the 27th and 29th.) This result is in fair agreement with the GFS, showing about the same probability of 0.25. It should be noted that the ensemble forecasts for this "event" have remained fairly consistent with this probability since late last week.

It should be noted that the GFS total snow accumulations with this "event" show between 0.5 and 1.0 inches at higher elevations with total liquid equivalent of 0.08 to 0.1 inches over the ARB area.

In summary, I see no IOP opportunities until perhaps the end of this week, followed by a long dry spell possibly breaking in about 384 hours. The system at the end of this week appears to be short lived, low on moisture, and with a consistent low probability of occurrence (I would prefer 50% or greater from the ensembles, we only have 25%).

Dan Birkenheuer - ESRL/GSD

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There are slight differences in today's weather from yesterday's forecast, but nothing that will make a real difference in the overall message, we still remain dry in the short term and watch the high pressure ridge dominating the Pacific coast. Yesterday in the telecomm, we discussed the possibility of something happening late this week, possibly Thursday into Friday and I will go into more detail on that at the end of this discussion. But first I will review the current weather situation in the near term and discuss some of the very strong dynamics aloft.

At 500 hPa this morning we have a closed low pulling out of the western US, centered just north of Omaha. On the east side of the Pacific ridge we have a vigorous vorticity center and digging wave in southern UT with the major vort max over Las Vegas. This intensifies the developing low but in contrast to yesterday's progs, it now digs further to the west, eventually setting up in just about the same location as the closed system that has departed the scene. The high velocity N winds at upper levels over the ARB are now in place as progged yesterday and the day prior. The ARB area is just to the west of the strong vort max over NV. N winds are unfavorable for precipitation; furthermore the moisture plume we watched crest the ridge remains well away from our forecast area in CA. By 00utc Monday we see a new cutoff form over west central AZ. This intensifies and digs further south finally ending up over Baja and N. Mex just south of the US by 18utc Tuesday. The strong N winds over the ARB give way to NE weaker flow by 12utc Monday after which, the winds lighten in force to about zero through 00utc Wednesday. A fairly intense moisture plume impinges on the Pacific coast 12utc Tuesday but remains too far north of the ARB to play any role in precip. No precipitation is forecast for the ARB through the entire NAM run.

Looking ahead as discussed in yesterday's telecomm to the scenario late this week we see an approaching strong wave west of the Pacific ridge begin to split into two very strong systems (at 500). One advances over the ridge and flattens it to the point of obscuring it completely for a time, the other part closes off and dives south and is off shore of CA by Friday 00utc. This is the teaser we observed yesterday. It should be noted that it is not associated with a good moisture plume, but rather a cutoff system that might bring weak precipitation to the ARB if it moves to a favorable location. However, the GFS has this system remaining off shore and dropping south finally putting the ARB area in easterly flow at 500 by Sun 00utc (about a week from now). This low then ejects leaving behind a very positively tilted ridge with an axis now offshore (retrograde) of the Pacific coast putting the flow over the ARB again in the N direction with probably some high velocity winds. By 31 Jan Wed 00utc, there appears to be an approaching short wave coming down this side of the ridge that might bring some interesting weather to the ARB later in that week.

Looking at both the GFS and Canadian ensemble forecasts for the near time of interest (late this week). Both are favoring a dry scenario over the ARB for the Thursday timeframe that we discussed yesterday. The Canadian ensemble has one and a close second run putting some precip in the ARB early Friday this week while the this morning's GFS ensemble set shows nothing at 00utc Friday over the area precip wise, but does have a few 500 hPa runs with a closed low that might be favorable for flow. My thinking is that this system at that point might be too dry. Again I don't see a favorable moisture plume associated with this event.

To summarize, the system late in the week bears close tracking; however, again in the near term there is no reason to call an IOP.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 12:42 PM | Permalink | TrackBacks (0)

This morning's analysis and model runs clearly show the moisture plume discussed 2 days ago reaching the apex of the high pressure ridge and moving inland over Canada. This morning there are some radar echoes over Portland and surface forecasts show there will be additional precipitation farther north (Seattle) likely associated with this moist surge. However, the current forecasts agree with those of Thursday showing that the moisture is caught up in anti-cyclonic flow and basically driven off shore, or advected east before any significant part of the moisture can reach the ARB, leaving it high and dry.

The NAM shows that the closed low we have had over S CA is lifting out, maybe a bit earlier than prior forecasts since this morning's analysis has the 500 hPa low center over Mex. AZ and NM boarders. This low opens up by 00ut Sunday with a negative tilt over the TX panhandle. At the same time, another vigorous short wave travels down the east side of the Pacific ridge reaching N UT at about this time and putting the ARB under strong N flow at 500. A second stronger vorticity center follows this wave and by 12ut on Sunday begins to close off a low over the 4-corners and is well established by 18ut Sunday. With a good jet on the west of this cutoff it intensifies and moves SSW (slight dig to west). This puts it over the AZ Mex. boarder by 12 utc Monday. During this evolution, the high pressure ridge over the ARB becomes more positively tilted and deformed on the east side reducing the flow over the ARB with winds from the N persisting, not favorable for orographics. The upper level cutoff moves further south and then heads toward El Paso by 00utc Tuesday. During the model run, there is only one time frame that shows possible light precipitation in northern CA (north of the ARB) and that is ending 00utc Sunday.

The short range forecast would not warrant an IOP prior to Tuesday of next week.

GFS model ensembles out to Feb. 5 00utc still show dismal prospects for any IOP for the extended range; however, more of the ensembles do place some minor sporadic pricip activity over the ARB. This is in stark contrast to yesterday when there was absolutely no action shown in ANY run. Even so, this is a minor change but in the right direction.

Dan Birkenheuer - ESRL/GSD

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Examining last evening's GFS Pacific TPW forecast runs, several moisture plumes (including the one mentioned yesterday) tease the coastal areas north of the ARB but never make it far inland. The earliest that a plume looks like it might impinge on the ARB is maybe a week from today, Friday 1/26 at 06utc, or around midnight early Friday morning. Other than that, the progs have not changed much since yesterday. The west coast is still dominated by a high pressure ridge and a few radar echoes this morning were seen advancing westward as the surface winds were gusting out of the east in central CA.

This morning's NAM has the 500 hPa closed low pulling out of the CA region and tracking east and north making it to the TX panhandle by this Sunday at 3ut. Meanwhile, an intense short wave advances down the east side of the coastal ridge passing Salt Lake City around Sunday 12utc. This feature continues due south and finally cuts off over El Paso by Monday. The effect of this action on the ridge is to amplify it and give it a slight positive tilt. This continues to provide weak upper air velocities over the ARB and strong northern flow over UT and AZ, a bit farther east than yesterday's forecast. No major precipitation occurs over central CA for the NAM run, however there is some minor precip forecast during the last few frames of the NAM early next week, but nothing that would warrant and IOP.

Last night's GFS ensembles still paint a very dry picture for the ARB out through Sunday February 4th.

Dan Birkenheuer - ESRL/GSD

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

The strong MJO that came out of the Indian Ocean in early January has weakened considerably. As expected. it excited convection near the dateline and attempted to generate a strong combined Pacific Ocean jet stream. This MJO also helped force the cold regime over the USA that we discussed on 29 Dec 2006. However, two factors indicate our scenario of a strong combined jet stream over the Pacific for this winter is now less likely.

First, since about 1 December the atmospheric momentum transports have been moving momentum out of the subtropics and into mid-higher latitudes. This pattern has been so strong that the forcing produced by the MJO over the warm El Nino waters appears only as a small perturbation in a persistent flow regime. Regionally this zonal mean regime is characterized by split flow patterns, especially over the oceans. Convection is currently increasing over the Indian Ocean and Indonesia while a portion of the convection at the dateline is moving westward at ~4 m/s as an equatorial Rossby wave. This combination suggests convection will become centered somewhat to the west of its current position during the next 2-3 weeks, more in line with the current anomalous circulation regime. We expect this perhaps by the end of week 2 and its implications for the east Pacific would be a retrogression of the east Pacific-North American ridge and a better shot at the undercut scenario people have been talking about.

The second factor is more relevant for the atmospheric circulation beyond week 3. The onset of the southern hemisphere monsoon (also associated with the strong MJO) has produced strong anomalous northerly flow across the equator. This appears to be linked with an amplification and deepening of the cold water (negative anomalies) below the surface in the equatorial Pacific Ocean. This cold signal may continue to deepen and eventually reach the surface putting an end to the basinwide aspects of this El Nino.

(These are probabilistic statements, which we will try to quantify in future posts. The decay time scale for momentum transport anomalies is on the order of 1-2 days. This then is the time scale on which a reversal of the transports could occur.)

Posted by Forecast Editor at 11:37 AM | Permalink | TrackBacks (0)

Snow in Malibu, but not much going on in the ARB. The 00ut GFS12 hour forecast run shows a good water plume connected to the tropics at roughly 180W bringing moisture up along the western side of the synoptic ridge that sits atop the forecast area. The future model runs have this plume detach from the tropics and begin an inland circulation in northern latitudes (Canada) only to follow an anti-cyclonic circulation under that ridge and become redirected out to the Pacific by the time the plume makes its way south into the CONUS latitudes. With time, the ridge continues to dominate the circulation pattern bringing in drier continental air east of the forecast area further west which probably obliterates any chances of a good precipitation event in the near term and possibly through the remainder of the upcoming week.

This morning's NAM has a 500 hPa cutoff just off shore of Southern CA and with time this ejects which modifies the flow over the ARB area from weak westerly flow at 12ut this morning to NNW in 84 hours (Monday 00ut) as the ridge increases in amplitude with an axis that is parallel and slightly offshore of the western coast.

The GFS 90h forecast from 06ut shows about the same upper level pattern as the NAM with the presence of a closing off low over western Colorado. This feature briefly cuts off and drops south in to New Mexico. Meanwhile the ridge over the ARB area stays in place with possibly a relocation directly over the ARB. In spite of the passage of numerous short waves all the way out until Thursday of next week when another off shore cut-off forms under the ridge threatening a split flow kind of pattern, the ridge dominates. Regardless of whether a split flow pattern forms, the ridging remains, directing Pacific flow well to the north of the forecast area and puts the ARB region in a weak flow regime at 500 hPa. Both model solutions are grim in terms of potential events, I spent the remaining part of this morning examining the possibility of the next potential system that will probably occur beyond this week using GFS ensembles.

Last night's ensembles have the ARB dry with the exception of 2 model runs, one 00h Sat 27 UTC and a different run (not the same forecast that produced the evening precip) 12ut Sat 27 -- both very weak. Other than that, the ensembles look dry for the ARB to the end of the ensemble forecast run (384 hours). Given this result, I would say that the distant future looks poor as far as additional precipitation opportunities.

Dan Birkenheuer - ESRL/GSD

Posted by Forecast Editor at 12:55 PM | Permalink | TrackBacks (0)

500mb low passed pretty much on track over San Francisco Bay at 09Z in LAPS
analyses with accumulating precip staying near the coast. Trace amounts of
snow were indicated by LAPS analyses in the ARB vicinity.

In today's 12Z GFS the next upstream wave passes north over WA/BC on Saturday,
then by Monday cuts off near GJT and progresses south all the way to NRN Baja
CA by Wed 24 Jan. Some GFS ensemble members have this cutoff farther east with
all members showing a ridge persisting off of or near the CA coast through
early next week. The ECMWF is slightly more progressive with the cutoff
while still maintaining a ridge over CA early next week.

The following trough shows some divergence in successive GFS runs for
the 25-27 Jan timeframe as to whether it rides north or cuts off well to the
WSW of LA. The most recent GFS run favors a more northerly position of the
still open wave near 43N 130W about to do battle with an upper level high
over ERN OR on 26 Jan at 00Z.

Overall it looks dry for the ARB through the remainder of this week and next
week. Just an outside chance of the trough late next week taking a favorable
trajectory based on model uncertainty. If we want to look at the 240hr GFS
and beyond there is a suggestion the west coast ridge may weaken by 27 Jan
and the next trough in the series could have a shot at NRN CA around
28-29 Jan. The 06Z GFS however still has a stronger ridge illustrating the
model uncertainty.

Steve Albers

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Latest GFS run has the upper low passing SWD over Oakland about 10Z Wednesday
so this is slightly east of yesterday's solution yet still only a few hundredths
of an inch of precip indicated over the ARB region. What precipitation does
fall should be snow down to fairly low elevations. GSD's WRF-NMM run also
keeps precipitation mainly along the coast. WRF-NMM cross-section shows
easterly downslope developing with the passage of the upper low helping to inhibit
precipitation over the ARB.

The next wave passes way north in the GFS/ECMWF over British Columbia on
Saturday, then dives down near the 4 corners on Monday in the GFS.

The following wave later next week around Jan 24-25 has some model divergence
as to whether it stays in the northern branch or tries to break through the
west coast ridge. This shows up both between successive GFS model runs and
in the GFS ensemble for 12Z. If the ridge holds over the west coast and the low
cuts off to the west we could see an omega block pattern setting up as the deterministic
GFS run suggests.

Looks like light precip possible over the ARB below IOP criteria Wednesday
followed by dry conditions. Next potential precip (slight chance) is second
half of next week.

Steve Albers

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ARB is in a clear and cold regime today behind the secondary trough
now starting to cut off in ERN AZ.

Wave of interest for the mid-week period initializes this morning at 150W.
GFS/UKMET/ECMWF models agree on placing a fairly strong cutoff low just W of
Santa Barbara by 00Z Thursday. GFS determinstic run keeps most of the precip
along the NRN CA coast, then progressing to be off the SRN CA coast as the
low drops south. The low eventually comes ashore in SRN CA on Saturday with
most of the precip still remaining S of the ARB.

The dprog/dt for the GFS shows the cutoff located various distances W of SFO
in the past several runs valid at 12Z Wednesday, we have to go back to the
run initialized 36 hours ago to find a solution with the cutoff over the
ARB itself.

GFS ensemble members mainly support the above scenario with the earliest
member having the 500mb cutoff reaching LA by 00Z Friday 19 Jan and the
other members more to the WSW at that time.

In the longer term, the GFS has the next two upstream waves splitting off
more to the south to try and develop more of a southern branch of the jet.
The ridge that has been parked off the west coast may then get pinched a
bit and more inland over Idaho. This may open some potential for these
southern waves to come into SRN or possibly Central CA around 25 Jan.

So it looks quiet this week with little or no potential for IOP criteria
to be met.

Steve Albers

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The ARB should be mostly dry today, except for a few light snow showers
possible with the secondary push of cold air, mainly along and east of the
crest of the Sierra. The GFS ensemble is uncertain even with this system
regarding how much of the energy will cut off in the SW US between now and
Wednesday.

Our wave of interest for the week at the eastern end of the PACJET initializes
12Z today at 163W longitude. Dprog/dt for the GFS valid at 12Z Thursday
continues to oscillate between a cutoff W of Santa Barbara and a more
progressive trough inland along the NV/UT border so we have a similar
uncertain forecast for mid-week. We either get a light/moderate precip event
over the ARB by Wednesday, or the system hangs off the coast until next
weekend. The latest GFS at 12Z favors the cutoff solution. The 12Z GFS ensemble
shows the possible location of a cutoff low by 06Z Friday anywhere on an
E-W line from SE CA to offshore at 128W. So a reasonable scenario would be
to see the ARB in the region of a pinching cutoff developing sometime on
Wednesday. The GFS ensemble mean has only trace amounts of precip over the
ARB on Wednesday with just a few hundredths along the coast and somewhat
more offshore.

By 12Z Wednesday the GFS/UKMET/ECMWF all agree on a cutoff low just SW
of SFO, while the NCEP WRF appears a bit more progressive in its run.
The 00Z ECMWF run favors a cutoff low W of San Diego around 00Z Friday like
the GFS.

There is still just a trickle of a PW river coming in over the ridge with the
wave for this week. By next weekend, the system entrains some high PW air and
shunts it toward SRN AZ with a subtropical jet evident coming off the Pacific
into NRWN Mexico and TX.

So far an IOP is conceivable this week around Wednesday, though it would not
appear to be likely.

Next possible system could drop in once again from the north about Jan 22.

Steve Albers

Posted by Forecast Editor at 12:47 PM | Permalink | TrackBacks (0)

The ARB should be dry today. For the short term forecast we have a secondary
push of cold air riding down the east side of the ridge that could give some
light snow showers over the Sierra on Sunday afternoon and evening. WRF-NMM
cross-sections show predominatly NE winds so the ARB will be on the downslope
side of things, similar to the past system.

In the medium term The GFS dprog/dt shows the 12z run has changed in the
position of the system for Thursday 18 Jan actually bringing a more respectable
trough onshore over CA. This is the system currently located near the eastern
end of the PACJET at about 170W longitude. The ECMWF concurs, so it now looks
like yesterday's GFS ensemble outliers may have a greater likelihood of
verifying. The deterministic GFS has a closed low sliding SEWD over SFO about
15Z Jan 17. The wave then opens up a bit into a positively tilted trough.
Heaviest precip should be near the coast with light/moderate amounts over
the ARB. We'll want to watch this in case it could reach minimum IOP criteria.
The possibility of some downslope wind component could be an inhibiting factor.
The GFS ensemble spread shows this event could happen anytime Wednesday or
Thursday.

A weak to moderate PW plume is associated with the mid-week system in the
GFS although it is connected to the tropics via a rather narrow feed.

Longer term around 23 Jan there is some model divergence with the ECMWF having
a ridge just off the west coast and the GFS developing a cutoff low WSW of
San Diego with the ridge farther west. Subtropical jet action now looks a bit
less pronounced and farther east over northern and central Mexico at this time.

Steve Albers

Posted by Forecast Editor at 12:49 PM | Permalink | TrackBacks (0)

LAPS sfc analyses show continued NE upslope flow near Reno over the past
24 hours. The organized radar echoes near Reno kept going yesterday
afternoon, then activity diminished to snow showers overnight having
a secondary peak in intensity around 10Z, about the time of the 500mb
trough passage. The upslope on the eastern slopes has deepened as discussed
yesterday, and most of the remaining clouds continue to the east of the ARB,
just including the crest of the Sierra.

The system of interest for the second half of next week is currently located
at about 170E longitude. GFS dprog/dt valid 12Z Thu Jan 18 continues to show
a Rex block with the cutoff low oscillating off the CA coast. The latest run at
12Z shows the low a bit closer to the coast at about 35N by 128W. From Jan 18-20
it wanders slowly and approaches the coast ending up just W of SFO with some
precip making it onshore. There is still limited potential for precip over
the ARB with this solution. The ECMWF starts out with a sharper trough
as of Tuesday near 140W, and eventually drives the 500mb low farther SW
compared with the GFS as of 00Z Jan 18. The ECMWF however shows a trough
axis extending to the ENE across central CA and NRN Nevada.

The subtropical jet does reach a moderate intensity south of the cutoff low
and over Baja CA in the Jan 19-21 time frame in the GFS.

So probably no IOP would be indicated for the next week though some limited
moisture could make it onshore by the weekend of Jan 20-21. This could
theoretically come in earlier as some GFS ensemble outliers have the weak
cutoff low over California as early as Jan 18. In any case only moderate PW
values are suggested to be entrained in this low by the GFS.

Steve Albers

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Tropical convective forcing remains stationary ~0/140E with development of a new region ~160W within 15 degrees of the equator (see figures below, click on them to enlarge). The latter is associated with downstream twin subtropical cyclones that were part of the response with the recent Madden-Julian Oscillation (MJO). Most diagnostic tools have shown a rapid weakening of the MJO signal during the last week. The remnants of the MJO combined with the pattern of tropical SST anomalies, including the warm ENSO signal, suggest organized tropical forcing during the next couple of weeks is most probable around 10S/160E while extending from the South Pacific back into Indonesia. Some shift slightly to the west may occur afterwards.

Zonal mean westerly flow aloft has been increasing throughout the tropical and subtropical atmospheres (within 15 degrees of the equator) during the past week. In fact, some of this westerly flow has already worked its way around through southern Asia and is enhancing the East Asian jet. This supports a GSDM Stage 1 weather-climate pattern more typical of La Nina conditions than El Nino. The current negative phase of the Pacific-North American teleconnection (PNA) is a charcateristic of this stage. Westerly flow should continue to increase throughout the subtropics. As shown by most models, the current trough-ridge-trough wave train across the PNA region should progress eastward leading to a ridge position just off the North American west coast for week-two (GSDM Stage 2). A strong subtropical jet is expected to “undercut” the ridge. During weeks 3-4 it is probable this pattern will shift westward and allow the subtropical jet to impact southern California. This scenario differs from an El Nino composite where a combined, extended jet across the North Pacific would be expected (GSDM Stage 3). However, this possibility cannot yet be ruled out for this winter/spring.

After the major winter storm of the next few days, much of the USA is in for a cold and dry regime, particularly the northern and central states during week 1. Locations in the Deep South will have freezing temperatures while Santa Ana conditions may develop across California. As the subtropical jet begins to undercut the ridge, another cold/wet storm system may impact locations from the Desert Southwest/Deep South into the Mississippi and Ohio Valleys and on to the northeast during week 2 (starting roughly next weekend). Afterwards, this storm track may shift northwest and locations such as the California coast may finally get some much needed precipitation going into February.

Ed Berry and Klaus Weickmann

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At present some radar echoes are showing up near Reno and Truckee and light
snow reported in Reno. As mentioned in the telecon, Truckee profiler is showing
upslope to 2.3km MSL for the eastern slope of the Sierra helping to support
the precip, and this may deepen somewhat as the upper low approaches.
Some additional echoes are present moving southward off the NRN CA coast.
In the near term the NCEP WRF run keeps about 0.1" of forecast liquid
equivalent (of the cold powdery snow) over the ARB through about 18z
Friday as the current trough and associated 500mb low propagates SEWD
directly over the ARB about 12Z Friday. The GSD WRF ensemble is showing
the possibility of somewhat higher amounts in the 0.2" to 0.5" range
over the higher terrain.

Medium term about 5 days out there is a system coming out of the Pacific Jet
that has some energy splitting off into the developing Rex block to the south.
The GFS has more energy going south compared with the ECMWF so it will be
interesting to see which solution verifies around next Tuesday.

A dprog/dt animation of the longer term GFS deterministic runs, valid at
12Z Thu 18 Jan, shows some oscillation between a Rex block off the west
coast and a positively tilted trough making it onshore. In either case
California is dry at this