Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Mar 22 2023 07:32:38 FOUS11 KWBC 220732 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 332 AM EDT Wed Mar 22 2023 Valid 12Z Wed Mar 22 2023 - 12Z Sat Mar 25 2023 ....The West... Days 1-3... A broad closed low over the Intermountain West will slowly fill and advect east today and tonight. While the core of this trough is progged to shift into the Southern Plains by Thursday, residual lowered heights are likely to persist across much of the West as weak impulses embedded within the flow continue to pivot onshore and through the general height weaknesses. During D2, a more pronounced shortwave will dive out of the Gulf of Alaska and into the Pacific Northwest bringing more significant height falls and PVA across WA/OR and into the Northern Rockies. For D1, the subtropical jet streak downstream of the large scale trough across the West will gradually pivot northeastward leaving favorable LFQ diffluence across the region to enhance ascent which will already be impressive due to the mid-level troughing. This will result in widespread precipitation across the terrain, especially from CA eastward through the San Juans and including the Great Basin terrain and as far south as the Mogollon Rim. As this exits to the east D2, an equally potent jet streak upstream of this trough axis will approach the Pacific Northwest and then dive into the Great Basin during Saturday, placing renewed LFQ diffluence for ascent across the terrain farther north than what is expected D1. This will result in heavy snowfall spreading from the Olympics and WA/OR Cascades D2 into the Central and Northern Rockies by D3, but with continued significant snow in the Cascades likely through the weekend as mid-level flow orthogonal to the terrain drives pronounced upslope enhancement with lowering snow levels. WPC probabilities on D1 are high for 6 inches or more from the Sierra and San Gabriels eastward across the higher terrain of the Wasatch, San Juans, CO Rockies, and as far north as the Wind Rivers. Locally, up to 2 feet of snow is possible in the San Juans and northern Wasatch/Wasatch Front of Utah. During D2 and D3, the heaviest snow shifts northward in response to the secondary and impressive diving shortwave and accompanying moisture plume. By the end of D3, heavy snowfall will likely encompass much of the Pacific Northwest terrain above 2000 ft, extending as far east as the Big Horns of WY, with WPC probabilities indicating a high risk for more than 6 inches from the Big Horns, Absarokas, Sawtooth, and Blue Mountains of OR. However, the heaviest snowfall is almost certainly going to be in the Olympics and WA/OR Cascades both D2 and D3 due to higher column moisture and more intense ascent in the favorable upslope regime. Two-day snowfall across the Cascade crest could exceed 4 feet. Additionally, as snow levels lower to just a few hundred feet, significant accumulations are likely at the important passes, with WPC probabilities indicating a high chance for at least 6 inches as Snoqualmie, White, Santiam, and Willamette passes. With snow levels becoming quite low D3, even some light snow is possible into the lowlands and valleys around Portland, OR. ....Northern Plains through the Upper Midwest... Days 1 & 3... A lead shortwave and associated surface low will be departing into Ontario early Wednesday morning, but a subsequent impulse and secondary low will track closely in its wake. This secondary low will be driven rapidly eastward by a sheared shortwave advecting northeast from the Central Rockies, as a lobe of strung out vorticity emerges from the broad trough enveloping the Intermountain West. This vorticity lobe will remain sheared and modest in amplitude as it moves into more confluent flow to the east, but impressive LFQ diffluence atop it owing to a strengthening subtropical jet streak should still result in robust synoptic ascent across the Plains and into the Upper Midwest. Additionally, the placement of the upper jet overlapped with a modest baroclinic boundary between a surface high to the north and the wave of low pressure moving across KS/MO will yield a band of frontogenesis to further enhance lift from west to east. Moisture will be generally modest as the best advection pivots more E/NE vs N, but an area of precipitation is still likely to overspread the region Wednesday aftn and night. The guidance has backed off on the intensity of this precipitation tonight, but a stripe of moderate snowfall is still expected, and WPC probabilities indicate a 10-30% chance for more than 2 inches of snow across northern NE, with isolated higher totals still possible. During D3, southern stream energy reflected by an amplifying shortwave ejecting out of the Southern Plains will attempt to amplify and take on a negative tilt as it lifts northeast towards the Great Lakes by Saturday morning. The guidance is not well clustered with this evolution so confidence in timing, intensity, and track is low, but with this feature strengthening in good proximity to impressive upper level diffluence within the RRQ of a jet streak arcing over New England, it is likely a surface low will develop and strengthen across the Upper Midwest or Ohio Valley. The ECMWF is well NW of the consensus, while the NAM is well southeast, leaving a GFS/CMC preference at this time. The initial column will likely be too warm for wintry precip, but as the low deepens and dynamic cooling can occur, especially NW of the surface low, an axis of heavy snow may develop early Saturday morning. At this time, WPC probabilities are modest, noted by less than 5% chance for 4 inches or more in southern WI, but this will need to be monitored with future forecast cycles for potentially more significant snowfall. ....Northern Maine... Day 2... A potent closed mid-level low moving south of Hudson Bay will leave downstream divergence across New England on Thursday. This will combine with increasing upper diffluence within the LFQ of a potent jet streak to drive potent deep layer ascent into the Northeast. At the same time, a warm front extending from a triple point near the Great Lakes will lift northeastward, driving enhanced warm and moist advection to fuel PW anomalies that may reach as high as +2 standard deviations by late Thursday. The overlap of this lift and moisture will yield an expanding area of precipitation from southwest to northeast, but a marginal thermal structure and a northward advancing warm nose will keep most of this as rain. However, across far northern New England and especially northern Maine, the column will remain cold enough for periods of moderate to heavy snow, and WPC probabilities for more than 4 inches of snowfall are as high as 50% in the far northern part of the state. The probability of receiving at least 0.1" accumulation of ice is less than 10%. Weiss $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .