Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 404 PM EST Wed Mar 01 2023 Valid 00Z Thu Mar 02 2023 - 00Z Sun Mar 05 2023 ...Four Corners... Day 1... Tonight and Thursday will finally bring an end to the long duration and impressive snowfall event that has been plaguing the West much of the week. The bowling-ball upper low moving across the Four Corners will gradually push eastward into the Southern Plains by Friday morning, but through D1 height anomalies at 500-700mb will remain below -3 standard deviations according to the NAEFS ensemble tables. This potent upper low will drive steep lapse rates aloft, supporting greater instability while also lowering snow levels to just 500-1000 ft beneath its core. As this low pivots eastward, the downstream subtropical jet streak arcing from near Baja California into the Ohio Valley will strengthen to above 170 kts, placing favorable RRQ diffluence over the Desert Southwest, while a more modest secondary jet streak digs down upstream of the primary trough axis to result in at least modest coupling. The overlap of upper diffluence, height falls, and mid-level divergence in the presence of steep lapse rates to drive elevated instability will result in ample deep layer ascent on D1. This impressive omega will act upon an environment with above normal moisture as IVT of 250-500 kg/m/s according to the CW3E probabilities shifts inland into AZ, supporting widespread and heavy precipitation. PW anomalies within the core of the IVT are progged to reach as high as +.5 standard deviations above the climo mean, and it is likely that snow rates at times will reach 2-3"/hr, both due to additional ascent through convergence/fgen along an eastward moving cold front, and through upslope flow into the terrain. While the heaviest snow still appears to focus in the terrain where SLRs will be at least a bit fluffier, the intense ascent and convective snow rates should drag down sufficient cold air for at least modest snowfall accumulations in the valleys and foothills. This is likely to be a very impactful event, especially for southern UT and AZ where pWSSI for moderate impacts reaches above 90%, and is even well above 50% for major impacts, especially along the Mogollon Rim. WPC probabilities on D1 are high for more than 6 inches along the Mogollon Rim and into the White Mountains, with additional high probabilities extending through the Kaibab Plateau eastward through the Chuska Mountains, the San Juan and Jemez mountains, southern Sangre de Cristos, and across some of the higher terrain east of Tucson. Locally as much as 2 feet of snow is possible across the Rim and White Mountains. ...New England... Days 1-2... Low pressure moving out of the Northern Plains tonight will race eastward along a wavering front through the Mid-Atlantic to off the New England coast Thursday night. This wave will be driven eastward by a shortwave traversing near the Canada/CONUS border, but should remain modest in amplitude due to downstream confluent flow across New England. While the confluent flow at 500mb will limit wave amplification, it will also help to keep cold air locked in across New England even as the Canadian high pressure begins to retreat. This will be important to p-type Thursday as WAA along and ahead of a warm front extending northeast from the surface low tracks into the region tonight. Ascent through this WAA will be overlapped by modest upper diffluence as dual, nearly zonal jet streaks couple overhead during D1. This will likely result in the surface low subtly deepening as it moves offshore and encounters at least subtle baroclinicity offshore, which will also help lock in the cold air across primarily northern New England. As this low intensifies despite rapidly pulling into the Canadian Maritimes, it will throw moisture back into Maine while also dragging cold air down as CAA commences in its wake. This should result in an area of moderate to heavy snow Thursday, with significant accumulations possible, especially where the theta-e ridge maximizes into northern Maine. WPC probabilities D1 into D1.5 peak above 50% for 6 inches across generally the higher terrain of north-central Maine. South of this heaviest snow, some freezing rain accretion exceeding 0.1" is possible for parts of the Berkshires, Catskills, Adirondacks, and into the NH Monadnocks. ...Pacific Northwest through Northern California... Days 1-3... After a brief respite in the busy winter pattern to start D1, snow will return to a large portion of the area Thursday night through Saturday. The primary driver of this return to active winter weather will be dual mid-level troughs and accompanying confluent/moist flow surging onshore from the Pacific. The first of these troughs will be a modest shortwave progged to rotate onshore near the British Columbia/Washington border Thursday evening, which is then followed by persistent confluent and zonal 700-500mb flow on Friday, with a more impressive shortwave amplifying into a closed low off the OR coast by the end of the forecast period. This latter closed feature will drive more intense ascent through divergence and height falls into WA/OR, with confluent flow downstream providing intense moist advection into CA. As the mid-level forcing persists and intensifies into the weekend, a continuous stream of Pacific moisture will begin to surge onshore within a zonally oriented and slowly southward sinking jet streak, providing a modified Pacific air stream into the region. PW anomalies are progged to be normal to slightly below normal according to the NAEFS ensemble tables, but the persistence of the moisture advection combined with continues ascent, especially including upslope enhancement where mid-level flow is orthogonal tot he terrain, will provide a long duration moderate to heavy snow event. Snow levels will be quite low through the period, only rising to 1500-3000 ft during the period of WAA late D1 into D2, but then crash quickly during D2 as a cold front crosses onshore, falling to the surface east of the Cascades, and to as low as 500-1000 ft in OR/WA, and then remaining generally steady within the zonal flow into Saturday. Snow levels will waver D3 but should generally be 1000-2000 ft across the region, and these persistently low snow levels will allow for notable and impactful snowfall accumulations even at the area mountain passes. WPC probabilities D1-2 for more than 6 inches are high, but generally confined to the Olympics, Cascades, and Northern Rockies, with some spread into the NW WY ranges on D2. However, with the lower snow levels, this will result in notable snowfall impacts at the Cascade Passes each day, with 3+ feet possible in the higher terrain. During D3 as the secondary low approaches and moisture funnels more impressively southeast, WPC probabilities for more than 6 inches reach 50-80% from the Olympics southward through the OR Cascades, into the northern CA Shasta/Trinity/Siskiyou region and into the Sierra. Snowfall may reach 1-2 feet in the higher terrain of northern CA D3, and with low snow levels continuing, even the coastal ranges could receive significant snowfall, with light accumulating snow sinking even into the lowlands around Portland, OR. ...Central Plains, Upper Midwest into the Great Lakes and Northeast... Days 2-3... A potent closed mid-level low emerging from the Four Corners Thursday night will eject northeastward across the lower MS VLY Friday morning before racing northeast to be over New England Saturday morning. By the end of the forecast period, the primary vorticity structure associated with this low will be sheared out into the northern stream westerlies placed over Canada, reducing the amplitude of the wave as it pivots northeast during D3. However, before that time, impressive height anomalies at 500-700-850mb of -3 to -4 standard deviations are progged by the NAEFS ensemble tables to lift across the Southern Plains and into the Ohio Valley before weakening. This intense feature will be accompanied by downstream divergence, and increasingly impressive upper diffluence as the downstream zonally oriented jet streak intensifies to 190 kts over New England in conjunction with a jet streak rotating through the base of the primary trough and arcing poleward to produce an intensely coupled jet structure. This overlap of robust synoptic ascent will result in a rapidly deepening surface low moving from OK to the Great Lakes Thursday night and Friday, before energy begins to transfer to off the coast of New England Saturday leaving an eastward moving coastal low by the end of the forecast period. In addition to the deep layer ascent which will spread northward, moisture will become quite anomalous downstream of the parent trough as meridional flow surges out of the Gulf of Mexico. Isentropic ascent becomes impressive along the 285-290K surfaces, with PW anomalies surging to +2 to +3 standard deviations above the climo mean in a narrow ribbon collocated with strongest low-level winds out of the south. This will concurrently push a strong theta-e ridge axis northward which will rotate cyclonically through the WCB into a TROWAL, and it is likely that by Friday aftn an expansive shield of precipitation will spread from the mid-MS VLY through the Mid-Atlantic, reaching the Great Lakes and Northeast shortly thereafter. Where the primary low tracks into the Great Lakes, a region of enhanced deformation will develop NW of the 850mb low, and as this occurs, especially during surface low deepening, a p-type transition from rain to snow is likely as CAA surges southward, aided by cold ageostrophic drainage into the low. The setup looks less than ideal for an intense band across the MS VLY and into the Great Lakes due to an unfavorable high position, but there could still be a rapid changeover with heavy snow and at least moderate accumulations as reflected by WPC Probabilities from southern Iowa through the "thumb" of Michigan reaching 10-30% for 4+ inches, with some local enhancement possible due to lake moisture near Chicago and Milwaukee. However, there remains above normal spread in both the deterministic models and ensemble plumes, providing lower than typical confidence at this time range. Farther to the east, the strong WAA will drive a warm nose northward, and while guidance still differs considerably as to where the warm nose >0C will lift, the result either way will be an area of heavy snow, transition to sleet, freezing rain, and eventually rain as far north as NYC. As the secondary low develops, an inverted surface trough will likely stretch west to east across Upstate NY and into New England, which will be collocated with higher theta-e air to provide a band of heavy snowfall just north of the mixed precip/transition zone. Confidence in where this occurs is low however, both due to highly variable thermal structures among the guidance and the potential for a dry slot to rotate northward into Upstate NY, and much of this will be dependent on where, and how rapidly, secondary cyclogenesis occurs offshore. At this time, the greatest risk for heavy snow exceeding 6 inches is stretched from near the Tug Hill Plateau eastward through Portland, ME and including much of the higher terrain from the Catskills through the Greens, Worcester Hills, and Monadnock region of NH where WPC probabilities for more than 4 inches are above 80% D3. South of here, at least modest ice accumulations are possible for the Laurel Highlands on D2-D2.5 where WPC probabilities for more than 0.1" peak above 30%. Weiss ...Key Messages for Plains to Northeast Winter Storm... --A potent winter storm will spread periods of heavy snow and mixed precipitation, including sleet and freezing rain, from the Central Plains through New England late this week into the weekend. --A swath of heavy snow is likely which could result in significant accumulations, especially from the Great Lakes through New England. --The combination of heavy snow rates exceeding 1"/hr and strong winds may result in hazardous travel as well as impacts to infrastructure. --A corridor of significant mixed precipitation, including sleet and freezing rain, is also possible south of the heaviest snow. --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. Keep up to date with the latest forecasts as this storm evolves.