Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 02 2024 20:07:52 FOUS11 KWBC 022007 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 307 PM EST Sat Mar 02 2024 Valid 00Z Sun Mar 03 2024 - 00Z Wed Mar 06 2024 ....West Coast & Northern/Central Rockies... Days 1-3... While the period of more prolific snowfall rates across the Sierra Nevada will gradually decrease this evening, the synoptic-scale pattern will continue to favor periods of heavy snow for the remainder of the weekend and into the first half of next week. The robust 500mb shortwave trough rounding the base of the mean 500-250mb longwave trough over the western U.S. will pass over the northern and central Rockies this evening while the left-exit region of a 150kt 250mb jet streak remains ideally located over northern UT, much of WY, and northern CO. NAEFS focused at 06Z tonight continues to show 200mb heights over the Pacific Northwest that fall outside the observed CFSR climatology (1979-2009), while the winds at 500mb from along the central CA coast to the CO Rockies are cranking to levels above the 99.5 climatological percentile, giving credence to the ongoing anomalous setup for strong synoptically-forced dynamics aloft into early Sunday. A persistent influx of 700mb moisture combined with added lift via topographically-favored upslope flow through tonight and into Sunday morning will keep periods of heavy snow ongoing from the Sierra Nevada, the northern CA Coastal Range, and Wasatch to the CO Rockies. Latest WPC PWPF through Sunday evening continues to show high chances (>70%) for additional snowfall amount >18" along the spine of the Sierra Nevada above 5,000ft, with similar high probabilities in the Wasatch >7,000ft and CO Rockies >9,000ft for snowfall totals >12". By Sunday morning, a stationary front will become draped across the mountain ranges from west to east with additional 700mb moisture flux being funneled over the front throughout the day. The dearth of moisture and strong synoptically-forced ascent will back off some, but prolonged upslope flow will keep periods of moderate-to-heavy snow in the forecast through Sunday. Finally, snowfall rates will back off Sunday night and into early Monday morning from the Sierra Nevada to the CO Rockies. By late Monday morning, however, any break will short lived in northern CA, southern OR, and into the interior Northwest as another 500mb shortwave on the backside of the longwave upper trough positioned over the the northwestern U.S. and southwestern Canada pivots towards the Pacific Northwest. While not nearly as vigorous as the shortwave trough late Friday into Saturday morning, added 500mb PVA and another surge in 850-700mb moisture is set to track through these aforementioned regions late Monday morning and into Monday afternoon. As a weak frontal boundary forms Monday night and becomes positioned from the OR/CA border on east to the Tetons, it will act to give additional lift at low levels to foster heavy snow from the northern CA mountains (Salmon, Shasta), the northern Sierra Nevada, and on north along the Cascade Range to as far inland as the Boise/Sawtooth mountains and the Tetons. WPC 48-hr PWPF (00Z Mon - 00Z Wed) depicts high chances (>70%) for >18" along the northern Sierra Nevada and neighboring northern CA ranges on north along the Oregon Cascades. With the event picking back up in the Boise/Sawtooth mountains Monday night into Tuesday, WPC 24-hr PWPF shows moderate chances (40-60%) for additional snowfall totals >12" there, as well as low chances (10-30%) around Jackson, WY. Impacts-wise, this afternoon and into Sunday morning the WSSI continues to depict Extreme Impacts along the Sierra Nevada as the seemingly relentless heavy snow and blizzard conditions continue to make for impossible travel conditions, extensive and widespread closures, and the potential for life-saving actions in the most extreme cases. From the central Great Basin and Wasatch to the Bear River Range, Tetons, and both the CO/WY Rockies, anywhere from Moderate to Major Impacts are expected through Sunday largely due to the combination of heavy snowfall accumulations and blowing snow. Major Impacts (considerable disruptions to daily life) are most likely along the Wasatch and the Bear River Range. From Monday into Tuesday, Major Impacts return to the Coastal Range of northern CA and southwest OR, as well as the southern OR Cascades. The Probabilistic WSSI (WSSI-P) shows low chances (20-40%) for Moderate Impacts late Monday into Tuesday in the WA Cascades and the Boise/Sawtooth mountains of Idaho. ....Northern High Plains... Days 1-2... Strong 500mb PVA and efficient divergence at upper levels thanks to the left-exit region of a 150kt jet streak atop the Sierra Nevada is fostering fantastic upper level ascent over the Northern Plains this afternoon. This is translating at the surface as well, spawning an anomalous sub 990mb low over northeast Wyoming that will track north and east along a stalled 850mb front that will flex north from northern SD this afternoon into central ND later tonight. It cannot be understated how strong this surface low is this evening as NAEFS shows barometric pressure readings in northeast WY that are less than all 00Z values in the CFSR climatology for a 3-week period centered on 00Z March 2. As strong 850-700mb warm air advection and strong 290K isentropic glide ensues over ND and northern MN, snow will fall to the north of the 850mb front along the International border. To the south of the front, the >0C warm nose will bulge enough to where an icy wintry mix will ensue over central ND and points east to the Red River of the north. This is due to the fact that while the 850mb front is north of the area, the surface front is actually as far south as central SD and north-central MN. This is helping to keep surface temps sub-freezing longer, and allowing for a fully saturated sfc-850mb layer to support periods of freezing rain and freezing drizzle. This will likely continue into Sunday morning as the surface low in northeast WY this afternoon eventually makes its way to northeast SD around 12Z Sunday. Latest WSSI-P does depict low chances for Minor Impacts (10-30%) from Bismarck and the I-94 corridor in central ND on north and east up to Devils Lake. This aligns well with WPC PWPF which shows similar low-chance probabilities (10-30%) for >0.1" of ice accumulations through Sunday. Slick roads are possible in some area and could make for hazardous travel the late morning hours, but the ever strengthening March sun angle should help to mitigate icy conditions on more heavily traveled roads during the midday and afternoon hours. On the western and northern flanks of the emerging surface low, rich 850mb moisture being wrapped around the storm in the form of a warm conveyor belt will allow for a TROWAL to form and become to focus for heavy snowfall through Sunday. Snow will become increasingly heavy this afternoon and evening over north-central MT as southeasterly 850mb winds increase to the north of the low, then snow will fall heavily in northeast MT for the predawn hours Sunday morning. By 12Z Sunday, the TROWAL axis will be oriented NW-SE from southern Saskatchewan to northwest ND with a compact surface low tracking into central ND. Not only will snow fall heavily at times in northeast MT and northern MN, but gusty winds on the northern and western flanks of the low will cause blowing/drifting snow through midday Sunday and into the late afternoon hours. Snow will begin to wind down by Sunday evening as the low tracks northeast into southern Manitoba, although lingering wind gusts over northeast MT and northern ND may still cause additional blowing snow through the overnight hours. Latest WPC PWPF shows moderate-to-high chances (50-70%) for >6" of snow in far northeast MT and northwest ND through 00Z Monday. Depending upon the positioning of the TROWAL axis and its duration, there are enough members in the WPC super ensemble that suggest a low chance (10-30%) for >12" in far northern ND. There are still some members, however, that suggest a more progressive TROWAL or the axis of heavier snowfall occurring more into Canada, thus still making this event a tricky one to forecast for. Residents in northeast MT and northern ND should expect some moderate-to-heavy snowfall, and even should lesser amounts take shape, the elevated wind gusts will still make for hazardous travel conditions. This is captured well in the WSSI-P which shows a large area of high chances (>70%) for Minor impacts throughout northeast MT and northwest ND with Snow Amount and Blowing Snow the primary hazards of note through Sunday evening. ***Key Messages for the Major Western Winter Storm*** --Heavy Mountain Snow Continues Extremely heavy snow rates of 2-6 inches per hour will continue across the Sierra through tonight, with 1-3 inches per hour likely across the other Intermountain West terrain. Additional snow accumulations of 2-4 ft in the Sierra, and 1-3 ft in other terrain above 5000 ft, are forecast through late Sunday. --Blizzard Conditions in the Sierra Nevada Strong winds gusting 60-80 mph, locally over 100 mph at the highest peaks, will cause whiteout conditions, making travel impossible in the Sierra Nevada, and many passes are closed. Substantial, long-lasting disruptions to daily life are expected in the higher elevations through Sunday, and avalanches are increasingly likely. --Widespread Damaging Wind in the Western U.S. Wind gusts of 55+ mph are forecast across much of the West, particularly across higher elevations and the Intermountain West, where 75+ mph gusts are possible. These winds will cause difficult travel, and likely down trees and power lines, resulting in widespread power outages. --Colder Temperatures and Lower Snow Levels Colder temperatures and lowering snow levels behind this system to bring accumulating snow down into many valleys. Temperatures will drop to 10-20 degrees below normal. --Another round of snow early next week Another system may bring additional heavy snow to parts of Northern CA, the Sierra, and the Cascades Mon-Wed. Mullinax $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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