Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 309 PM EST Mon Feb 13 2023 Valid 00Z Tue Feb 14 2023 - 00Z Fri Feb 17 2023 ...Pacific Northwest and Northern Rockies... Days 1-2... A shortwave dropping into the Pacific Northwest will amplify as it crests the Cascades this aftn and shift eastward while closing off across the Northern Great Basin Tuesday morning. During this evolution, a secondary vorticity lobe will stretch out of Canada and rotate almost due southward through the base of this amplifying trough, driving rapid and impressive height falls and PVA along the Pacific Northwest coast through Tuesday. Aloft, a Pacific jet streak amplifying towards 150kts will arc southward as well, aligning the favorable LFQ diffluent portion along the coast upstream of the sharpening trough axis, with the overlap of these synoptic features likely resulting in surface low development moving into the OR. Impressive forced ascent through the deep layer lift aided by the wave of low pressure and low-level fgen moving onshore will likely result in rounds of heavy snowfall, especially into the Cascades of OR, where upslope enhancement will contribute additional lift. Regional soundings during Tuesday are quite impressive with steep lapse rates beneath the cooling column combined with strong omega below and within the DGZ. As the column cools, snow levels will crash rapidly behind the front, with additional lowering likely provided as intense vertical ascent leads to heavy snow rates which will drag down the cooler air as well. This will likely result in heavy snow rates, which are progged by the WPC snowband tool to reach 1"/hr, reaching even into the lowlands around Portland, OR, resulting in light snow accumulations Monday night into Tuesday, with moderate accumulations even in the coastal ranges of OR and northern CA. The heaviest snowfall is still expected to be along the Cascades and into the Shasta/Trinity/Siskiyou region where WPC probabilities are high for more than 6 inches, with 1-2 ft likely in the higher terrain of the OR Cascades. Snowfall across the coastal ranges may exceed 4 inches, with an inch or two possible in the lowlands. With the lowering snow levels, heavy accumulations are expected at many of the area passes, especially in OR, where WPC probabilities for more than 6 inches are above 50% at Santiam and Willamette Passes, with several inches of snow also likely at Snoqualmie and Siskiyou Passes. At the same time, the amplifying trough will drive a series of cold fronts southward through the Northern Rockies and into the Northern High Plains, resulting in low level convergence and frontogenesis, aided by post-frontal upslope enhancement to produce rounds of heavy snow into the terrain. The guidance is suggesting that two distinct cold fronts will settle southward during this period, with the more impressive fgen accompanying the latter front which will move out of Canada Tuesday. Heavy snowfall is likely as the behind the first front from central ID through the Absarokas and into the Northern High Plains as modest upslope flow combines with upper level synoptic ascent within the tail of a departing jet streak, but more intense snowfall rates exceeding 1"/hr are progged to develop Tuesday aftn as the second front sags southward leading to more intense low-level fgen aided by upslope ascent. WPC probabilities on D1 are high for more than 6 inches from the Bitterroots eastward across the Absarokas and into the Big Horns, where locally 12-18" is possible. Later D1 into D2, heavy snow will spread into the High Plains of southern MT with additional enhancement surging into the Black Hills of SD where WPC probabilities indicate a high risk for an additional 6+ inches of snowfall. ...Four Corners, Central Plains, through Upper Midwest... Days 1-3... An extremely active period of winter weather is expected as two significant low pressure systems emerge from the Rockies and lift northeast through Thursday. Each of these will likely result in swaths of heavy snow, with strong winds also developing leading to blowing and drifting of snow, with possible blizzard conditions. The first of these will develop beneath an anomalously strong closed low characterized by 700mb heights that approach -4 standard deviations according to the NAEFS ensemble tables by Tuesday aftn, producing increasingly strong deep layer ascent through height falls and downstream divergence. This upper low will pivot from the Four Corners into the Central Plains Tuesday aftn, when it will likely reach its maximum amplitude, becoming negatively tilted before gradually filling as it surges towards the western Great Lakes Wednesday morning. At the same time, a poleward arcing jet streak will intensify downstream of the primary longwave trough axis and surge towards the Great Lakes, efficiently placing its diffluent LFQ atop the greatest mid level height falls, which should result in a deepening area of surface low pressure from the lee of the Rockies through the western Great Lakes. This amplified trough will drive impressive PW being drawn northward from the Gulf of Mexico Tuesday, with the resultant theta-e ridge lifting isentropically into a robust TROWAL rotating into the Upper Midwest late Tuesday into Wednesday. Mixing ratios within the most intense isentropic ascent along the 290-295K surface will reach 4g/kg, suggesting anomalously moist air being wrung out as heavy snow in the presence of the robust ascent. On the NW side of this low, the overlap of a strengthening deformation axis with increasing fgen and at least modest ageostrophic flow out of the Canadian high pressure into the deepening low should result in rapid cooling of the column, and the guidance has again increased in its potential snowfall in a band from SD through eastern ND and into much of northern/central MN. Despite the lack of a clear signal to CSI, snowfall rates of 1"/hr seem likely as this band pivots northward, and WPC probabilities for more than 4 inches are 40-60% from along I-90 in SD northeast through the Coteau des Prairies and towards International Falls, MN. WSE plumes indicate still quite a bit of spread across this area, so locally more than double that amount is possible. Lighter snows are likely as the cold air collapses back into the low as it pulls away, with a few inches of snowfall possible as far east as Minneapolis and Duluth. Behind this first shortwave, another, almost equally impressive and strong closed low, will drop across the Great Basin beginning Tuesday before amplifying further into the Four Corners with NAEFS ensemble tables indicating 500-700mb heights dropping to -3 standard deviations below the climo mean Wednesday. This closed low will then dig through the base of the longwave trough and begin to shift northeast into the Plains Thursday but then rapidly shear out as it becomes entrenched in downstream confluent flow between the Southeast ridge and a secondary northern stream shortwave dropping into ND/MN. The result of this evolution will be an impressive winter event across the Four Corners, especially in the terrain, but with lowering snow levels to bring moderate snowfall to even the lower terrain/valleys, before developing into another deepening surface low in the lee of the Rockies which will spread snowfall across the High Plains and into the Upper Midwest by the end of the forecast period. At jet level, the subtropical jet streak will spread northward from the Four Corners through the Ohio Valley which will bring Pacific moisture into the region but also place favorable RRQ and LFQ diffluence aloft to enhance deep layer ascent. Where this overlaps with the most intense height falls and PVA, heavy snow will occur, with rates enhanced beneath steep mid-level lapse rates and along a sharpening band of fgen which will spread northeast from the High Plains into the Upper Midwest with associated negative theta-e lapse rates to drive the potential for CSI. While heavy snow (rates and accumulations) are nearly certain in the Four Corners and High Plains, there is some more uncertainty farther east into the Central Plains and Upper Midwest due to a separation of the theta-e ridge to the cold air as moisture is initially slow to return northward behind the initial storm. The guidance has become a bit more aggressive with this band, but uncertainty is high with the evolution beyond D2. For D1 into D2, WPC probabilities for more than 6 inches are high within the terrain from the southern Wasatch through the Kaibab Plateau, along the Mogollon Rim and into the White Mountains of AZ, and through the San Juans/Sangre de Cristos, then continuing across these areas while spread into the Front Range and Raton Mesa/Palmer Divide through Wednesday. Impressive snow rates and snow amounts drive high probabilities for even major impacts within the pWSSI, especially along the Mogollon Rim D1, and moderate probabilities along the Raton Mesa where upslope flow as the low develops to the southeast will enhance snowfall and impacts. Total snowfall during D1-2 could reach 2-3 feet in the higher terrain of some of this region, especially in the San Juans and Sangre de Cristos. Although maximum snowfall amounts will be heavily influenced by elevation, the I-25 urban corridor of eastern CO will also likely receive impactful snowfall of more than 4 inches. During D3 confidence lowers due to the reasons mentioned above, but there is likely to be at least a narrow stripe of heavy snow from western KS through southern IA, with a secondary maxima possible as far east as the western Great Lakes depending on how this low and shearing mid-level impulse evolve. At this time, WPC probabilities indicate a 30-50% chance for more than 6 inches across this area. Weiss ...Key Messages for Feb 14-17 Winter Storm... --A winter storm will develop near the Four Corners on Tuesday and spread a swath of heavy snow northeastward across the Plains on Wednesday. This system may continue into the Midwest and Great Lakes Thursday. --The combination of heavy snow and strong winds will produce blowing and drifting of snow, resulting in hazardous travel conditions and possible impacts to infrastructure. --A corridor of mixed precipitation, including sleet and freezing rain, is also possible along the storm track over the Plains. --Uncertainty remains with the timing and location of the storm track, which determines where the axis of heaviest precipitation and most significant impacts will occur. --Much colder temperatures, falling to well below normal values and perhaps near record lows over the Southwest, will follow in the wake of this system.