Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 235 PM EDT Wed Mar 20 2024 Valid 00Z Thu Mar 21 2024 - 00Z Sun Mar 24 2024 ...Upstate New York and New England.. Days 1&3... Two distinct systems will bring heavy snow to primarily elevated regions of the Northeast through the weekend. The first is driven by an amplifying closed mid-level low which will drop slowly southeast from Quebec towards eastern Maine by tomorrow morning. These impressive height falls and PVA will combine with modest diffluence along the LFQ of an upper jet streak to rapidly deepen a surface low as it tracks northeast along the coast of Maine. As this low deepens, it will transport robust moisture northward and lift it cyclonically within the accompanying theta-e ridge to expand an area of precipitation across ME and eastern NH. The column ahead of the low within the best WAA will be too warm for snow, but moisture pivoting cyclonically around the low will enter a much colder atmosphere, with some dynamic cooling aiding through a potent deformation axis. This will result in a swath of heavy snow pivoting across NH/ME through D1 before exiting to the east by the start of D2. WPC probabilities for 4+ inches of snow are above 80% in the White Mountains of NH and then across much of eastern/northern ME away from the coast. As this lead system pulls away, brief shortwave ridging will envelop the Northeast bringing one dry day, before the fast flow sends yet another shortwave out of the Northern Plains/Great Lakes and into the Northeast. This shortwave is progged to be a bit weaker/less amplified as it pivots from MI into New York on Saturday, but will begin to interact with some southern stream energy emerging from the Carolinas to help the trough tilt negatively. This evolution produces a rapidly intensifying and poleward arcing jet streak downstream, which will support a fast moving but strengthening surface low pressure surging northeast along the Atlantic coast. The deepening low combined with the robust synoptic setup will draw plentiful moisture northward on increasing 290K isentropic ascent, with mixing ratios progged to reach nearly 6 g/kg in eastern New England. This accompanying WAA surging northward will keep most of the precip as rain for southern and central New England, but into Upstate NY and northern New England, enough cold air in the column will likely yield a shield of heavy snow, with strong fgen helping to drive ascent for heavy rates despite a low SLR /heavy-wet snow/ event. WPC probabilities D3 reach above 70% for more than 4 inches across the Adirondacks, and then much of northern New England. There continues to be quite a bit of latitudinal spread with the guidance as far as the thermal structure for this event, so additional fluctuations in amounts are likely, but at least the higher terrain is likely to receive a significant late-season snow event due to the anomalous moisture accompanying this system. ...Northern Plains through the Great Lakes... Days 1-3... A fast moving wave of low pressure will develop Thursday afternoon in the Northern High Plains and then advect E/SE into IA by Friday morning before weakening as the parent shortwave gets absorbed into more confluent flow over the Great Lakes by Saturday. This low will deepen at least subtly thanks to PVA collocated with the RRQ of a departing upper jet streak, although admittedly the intensity of this feature is also modest. As this low shifts east, increasing WAA lifting northward from the south and downstream of the low will expand moisture into the Northern Plains and Upper Midwest, resulting in an east-west oriented axis of impressive 700-600mb fgen, especially from MN into WI and then MI Thursday into Friday. This fgen will slope favorably into the deepening DGZ, although the most intense WAA may remain below the based of this snow growth region. High-res simulated reflectivity indicates there may be two laterally translating bands of heavy snow embedded within the larger swath of moderate snow, but confidence in placement is low. There will likely be at least one band driven by the mid-level fgen, with a second band farther north possibly persisting within the RRQ of the upper jet. Should these forcing align, the RRQ of the jet could ageostrophically enhance the fgen to drive a more intense single band, but again, confidence is low. Within these bands, snowfall rates above 1"/hr are likely, but with otherwise modest synoptic ascent in place, the snowfall should be more moderate intensity. This suggests that the heaviest accumulations will be D2 and within any banded structures, with more modest accumulations elsewhere as the low races to the east. WPC probabilities feature a bimodal distribution reflecting the potential for dual-bands. For D1-D2, a stripe of WPC probabilities exceeding 50% for 4+ inches exists from central ND through far northern WI, remaining generally north of Minneapolis. The more impressive /fgen driven/ band is likely south of that first area, developing across far SE Minnesota and then expanding within the WAA shield into WI and then eventually the central L.P. of MI D2. In this secondary area, WPC probabilities for more than 4 inches peak above 80% D2 from just north of Milwaukee, WI through central lower Michigan. Local amounts of 6-8 inches are likely within the most intense banding despite the fast progression. ...The West... Days 1-3... An increasingly active period will begin in the west, especially by D3 as a large scale trough develops across the area. Before this occurs, 2 days of persistent moderate to heavy snow will occur across the Northern Rockies. A low pressure moving along the Canadian/US border will be driven eastward beneath a modest shortwave pivoting within the pinched northern stream flow, and will leave a cold front in its wake banked against the terrain from the northeast. Upslope flow, height falls, and increasing moisture will produce heavy snow D1, with persistent upslope flow continuing the heavy snow on D2. WPC probabilities D1 and D2 for more than 6 inches of snow are above 70% near Glacier NP. More widespread snow begins to fill in across the West D3 as an amplified trough with an embedded closed mid-level low digs out of the Pacific and pushes onshore Saturday morning. Around this trough, waves of vorticity will shed E/NE to increase ascent, while moisture surges through confluent flow downstream of the trough axis overlapped with a subtropical jet streak intensifying to 170 kts. This will spread moisture within IVT of 200-300 kg/ms into much of the west, highest across CA and into the Four Corners, resulting PW anomalies according to NAEFS reaching +1 or more sigma in some areas. As heights lower within the expanding and advecting trough, snow levels will fall from around 4000-5000 ft to 3000-4000 ft by the end of the period. The falling snow levels and increasing moisture/ascent will expand heavy snow across the Cascades, northern CA ranges, and Sierra, where WPC probabilities for more than 6 inches reach 70-90%, highest in the Sierra where 1-2 feet of snow is possible on D3. Farther north across the Northern Rockies D3, heavy snow begins to expand more impressive from near Glacier NP into eastern MT along the residual baroclinic gradient and within an axis of increased divergence downstream of the parent trough to the west. Surface low development late in the period will also enhance ascent and moisture, and WPC probabilities D3 for 6+ inches of snow reach 30-50% as far east as the Big Snowy Mountains. Weiss ***Key Messages for Northern Plains Winter Storm March 23 - March 26*** -- Winter storm becoming likely: Confidence is increasing that a large storm system will produce a swath of heavy snow, as well as freezing rain and sleet, across portions of the Northern Plains and Upper Midwest into early next week -- Widespread heavy snow: Heavy snow will likely overspread eastern Montana late Saturday, then expand into the Northern Plains and Upper Midwest by Sunday night. Significant accumulations are likely. Snow should exit the area during Tuesday. -- Significant impacts due to snow, ice, and wind: A combination of heavy snow and gusty winds, as well as freezing rain south of the heavy snow, could result in hazardous travel and impacts to infrastructure. -- Forecast changes anticipated: Uncertainty remains with the timing and location of the storm track, which will affect where the most significant impacts will occur. Keep up to date with the latest forecast as this storm evolves.