Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 309 PM EST Wed Jan 29 2025 Valid 00Z Thu Jan 30 2025 - 00Z Sun Feb 02 2025 ...Northeast & Northern Mid-Atlantic... Days 2-3... Any lingering snow squalls and LES bands ongoing this afternoon will dissipate tonight as winds weaken gradually and high pressure builds in from the southeast. Thursday appears quiet across the region, but this changes Thursday night/Friday morning as a storm system approaches from the SW. By 12Z Friday, the positively tilted 500mb low located over the Middle Mississippi Valley will have directed an exceptional IVT over the eastern third of the U.S. with values topping 1,000 kg/m/s over the TN Valley. However, the air-mass over the northern Mid- Atlantic will have moderated to a point where most expected snowfall will be confined to the mountain ranges of the Northeast and throughout much of Maine. On Friday, boundary layer/surface temperatures will initially start out <32F from central PA on north through the Catskills, Southern Tier of NY, and the Lower Hudson Valley, and into central New England while a burgeoning warm nose aloft causes precipitation to fall as a sleet/freezing rain mix. Farther north, boundary layer temperatures will remain below freezing and allow for snow to be the primary precipitation type from the Tug Hill and Adirondacks on east through the Green and White Mountains. As the primary low weakens Friday evening across NY, a new coastal low will form east of the MA Capes Friday night. The low will be a fast mover with periods of snow Friday night concluding by Saturday morning. WPC probabilities show low-to-moderate chances (30-50%) for snowfall >4" across the Adirondacks, Green and White Mountains, and northern Maine with low chances (10-30%) for >6" in these same areas. WPC probabilities depict low chances (10-30%) through northern PA, the Poconos, and Catskills for ice accumulations >0.1". The Probabilistic WSSI (WSSI-P) shows low-to-moderate chances (30-50%) for Minor Impacts in all of these aforementioned areas. ...Southwest through the Four Corners... Day 1... An anomalous closed low will continue to advect slowly northeast across the Four Corners this afternoon before moving into the Central Plains on Thursday. This feature will maintain amplitude, reflected by 500mb heights fall towards the 2.5 percentile according to NAEFS, providing strong ascent downstream of the low as it moves east. The most impressive lift is likely immediately east of the upper low where height falls and divergence combined with the diffluent LFQ of a strengthening and poleward arcing jet streak, which is also where the greatest moisture is expected as a PW plume surges and lifts cyclonically into NM/CO. Heavy snow will accumulate in the higher terrain of the Southern Rockies, highlighted by WPC probabilities that show moderate-to-high chances (50-80%) for snowfall >8" of snowfall the San Juans and Sangre de Cristos, as well as the Raton Mesa. Farther east into the High Plains, including the I-25/I-70 corridors and Palmer Divide, wraparound snow aided by upslope may result in impactful accumulations across these areas. Latest CAMs guidance has keyed in on a narrow band of heavy snow from the Palmer Divide and along I-25 on east towards the I-70 corridor between 12-18Z Thursday. Latest HREF guidance shows moderate chances (40-60%) for snowfall rates >1"/hr in that span, which could lead to rapid accumulations on roadways and rapid reductions in visibilities. WPC probabilities show moderate-to-high chanceS (50-70%) for snowfall >6" along the I-25 corridor over the Palmer Divide. There are low-to-moderate chances (30-50%) for snowfall amounts >4" along I-25 in Elbert County on Thursday. It is possible, should mesoscale banding manifest itself as CAMs suggest, may result in localized amount of 8-10" from the Palmer Divide on northeast to the I-70 corridor east of the Denver metro. Travelers on these interstates and connecting thoroughfares should anticipate the potential for hazardous travel Thursday morning in these areas. Snow should conclude Thursday afternoon as the closed upper- low tracks east. ...Pacific and Interior Northwest... Days 2-3... A more active period begins in the Northwest beginning on Thursday as an atmospheric river (AR) arrives over the coasts of WA/OR before spilling inland on Friday. IVT within this AR ranges between 300-500 kg/m/s Friday morning, which is as high as the 99th climatological percentile per NAEFS. This increased moisture stream will coincide with an expanding area of precipitation, but as WAA increases as a byproduct of the AR's arrival, snow levels will climb to as high as 4000-5000 ft on Friday before plummeting to as low as 1000ft by the end of the forecast period, lowest in WA state, behind a cold front. Upslope flow into the Cascades/Olympics will continue through Saturday with heavy snow also expected farther east into the northern Rockies. Farther south, the westerly IVT will continue to pump copious amounts of Pacific moisture into the West with the northern California ranges, most notably, seeing the heaviest precipitation. While this is the case, snow levels will be rather high with elevations >7,000ft most subjected to the heavy snow in the Sierra Nevada, and >5,000ft in the Trinity/Shasta. WPC probabilities are highlighting the Cascades and Olympics above 3,500ft as having high chances (>70%) for snowfall totals >24" through Saturday afternoon. A couple other regions likely to measure snowfall in feet are the Sawtooth of central ID and the Tetons where some of the peaks of these ranges are may surpass 30" through Saturday night. Elsewhere, snowfall totals >12" are moderate-to-high chance (50-70%) according to WPC probabilities in the peaks of the Trinity/Shasta, the OR Cascades, and peaks of the northern ID panhandle. There is concern for icy conditions at lower elevations of northeast WA, northern ID, and western MT where surface temps have rarely gotten above freezing in recent days. Should snow transition over to a sleet/rain mix, ground conditions are so cold that freezing rain could ensue on these surfaces even with air temperatures above freezing. Expect wintry conditions to linger through Sunday as the IVT to the south persists and a more frigid air-mass inches its way south from southwest Canada, resulting in more mountain snow and possibly icy conditions in some valleys of the Pacific Northwest and Intermountain West. Mullinax ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png