Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 226 PM EST Tue Nov 19 2024 Valid 00Z Wed Nov 20 2024 - 00Z Sat Nov 23 2024 ...Pacific and Interior Northwest... Days 1-3... ***Impressive and long-lasting atmospheric river event to bring heavy snow to the Northwest*** An extremely amplified pattern over the Pacific characterized by an intense +4 sigma ridge near the Aleutians will help drive a downstream closed low which is progged by NAEFS to drop to as low as -6 sigma with respect to 850mb heights off the Washington coast to start the period. This intense feature will be accompanied by a strong surface low, and although this surface low will generally stall and then retrograde back to the north into Friday, the accompanying ascent and moisture will surge onshore through the period. South of this closed low, pinched mid-level flow combined with a persistent 130kt Pacific jet streak will cause strong warm and moist advection to push onshore. This will create a potent atmospheric river /AR/ which both the ECENS and GEFS probabilities suggest will exceed 750 kg/m/s IVT. As the parent low retrogrades, the associated frontal structure will stall in a NE to SW orientation, providing a favorable setup for a long duration of impressive IVT pushing onshore, focused into northern CA, but with downstream PW anomalies reaching as high as +2 sigma all the way into the Great Basin and interior Northwest. This suggests, and is agreed upon by nearly all global models and ensembles, that persistent and widespread moderate to heavy precipitation will occur through the next 48 hours, with dynamic ascent through upslope flow enhancing the intensity of precipitation rates. Snow levels initially will be low, 1500-3000 ft from north to south tonight, suggesting that the heavy precipitation will begin as snow in many areas outside of the lowlands, but including the passes. Snowfall rates will likely peak around 3"/hr in the Cascades and northern CA ranges tonight, which when combined with gusty winds will produce treacherous and at times impossible travel. During D2, the focus will shift to be more aligned SW to NE within the greatest IVT plume from the Shasta/Trinity region of CA through the Northern Rockies, but at the same time snow levels will steadily climb, reaching above most pass levels by the end of D2, around 8000 ft in CA and 5000 ft in MT. WPC probabilities D1 are high for more than 12 inches in the Shasta/Trinity region of northern CA, along the spine of the Cascades, and into the Olympics. Pass level impacts are also expected D1. By D2 the highest probabilities shift east as snow levels rise, reaching above 70% for 12+ inches in the Salmon River/Sawtooth area. In the highest terrain, especially around Mt. Shasta, snowfall of 4-6 feet s likely. During D3 snow levels continue to rise within the IVT plume, but also more broadly as a secondary surface low lifts near the WA coast to move onshore British Columbia by the end of the forecast period. This will enhance moisture and ascent once again, while at the same time a cold front drops southward from Canada into the Northern Rockies. Together, this could result in an enhanced swath of heavy snow across the Okanogan Highlands eastward to the Northern Rockies where both upslope flow and enhanced fgen will combine. WPC probabilities D3 are highest along the WA Cascades eastward through the Northern Rockies where they reach 50-70% for an additional 6+ inches. ...Northern Plains... Day 1... An impressive closed 500mb low with 500-700mb heights falling below the 2.5 percentile according to the NAEFS ensemble tables will be positioned across ND to start the forecast period. This low will drift only slowly southeast as the trough continues to amplified in response to vorticity lobes swinging cyclonically through the base of the trough, resulting in additional deepening of the trough and a pull southeast into the Upper Midwest of the primary trough axis. Beneath this, an occluded surface low will track gradually eastward across southern Manitoba, dropping into MN by the end of D1 while filling. Although this won't result in intense synoptic lift as features will be vertically stacked, impressive mesoscale ascent will persist to produce heavy snow across the Dakotas and into Minnesota. The evolution of this low will result in persistent theta-e advection, driving a pronounced TROWAL within the 600-500mb layer which will rotate cyclonically southward across the Dakotas and into Minnesota Tuesday night into Wednesday. This TROWAL will overlap efficiently with an axis of mid-level deformation and correlated 850-600mb fgen which will drive a narrow corridor of enhanced ascent into a deepening DGZ (probs for 100mb of depth from the SREF exceed 50% Wednesday morning). In general, ascent will be waning through D1, but this axis of strong mesoscale lift combined with strong winds and a cold column should result in moderate to at times heavy snow rates exceeding 1"/hr, which combined with the gusty winds will produce limited visibility and hazardous travel conditions. Total snowfall may me somewhat moderate, with locally heavier amounts likely. WPC probabilities for more than 4 inches of snow are above 70% from the Turtle Mountains along the ND/Canada border and then stretching as far south as the Sisseton Hills of northeast South Dakota. ...Ohio Valley, Appalachians, & Northeast... Days 2-3... A complex upper pattern will produce dual interacting surface low pressures across the region through Friday. Confidence in placement and evolution is lower than typical, but at least some areas are likely to receive heavy snowfall beginning Thursday. The strong closed low, characterized by widespread 500-700mb height anomalies of -2 to -3 sigma according to NAEFS, will encompass much of the region from the Great Lakes, Northeast, and Mid-Atlantic, with spokes of vorticity periodically rotating through the trough. The strongest of these vorticity maxima will shift across the Mid- Atlantic to south of New England Thursday aftn, which will drive secondary cyclogenesis (the primary low beneath the core of the closed low over Michigan). These dual low pressures will then pinwheel around each other through Friday producing periods of intense ascent within a moistening column to drive rounds of mixed precipitation. The challenge through the period is where will the heaviest snow accumulate, and, especially outside of higher terrain, how much snow can accumulate. The high confidence portion of this forecast involves the Central Appalachians where post frontal NW flow will produce impressive upslope ascent into a cold column, while at the same time the primary low from MI rotates southward to enhance ascent locally across that same area. This will result in a 2-day period of moderate to heavy snow, and WPC probabilities on D2 reach 30-50% for 4+ inches, with greater probabilities reaching above 90% for additional snowfall exceeding 4 inches on D3. Total accumulation may exceed 12 inches in the highest terrain of WV and the MD Panhandle, with locally as much as 4-6" as far south as the NC/TN border. In other places, the broadly forced ascent results in lower confidence forecast but with high risk potential for rounds of heavy snowfall rates. In the vicinity of both the primary low dropping from Michigan across the Ohio Valley and into the Central Appalachians, as well as across New England, New York, Pennsylvania near the secondary low as it retrogrades, periods of intense deformation are likely, and in fact several of the global models indicate rotating plumes of negative theta-e lapse rates which could support CSI in bands of heavy precipitation. Timing and placement of any of these more intense bands is quite uncertain, but with marginal thermal structures in place, it will likely require these heavier rates to drive dynamic cooling sufficient for significant snowfall accumulations. The most likely regions will be in higher elevations above around 1500 ft including the Catskills and Poconos, where WPC probabilities have increased to 50-70% for 6+ inches, and across portions of the Ohio Valley where they are 30-50% for 4+ inches. The probability of significant ice across the CONUS is less than 10 percent. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png