Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 236 PM EST Mon Feb 17 2025 Valid 00Z Tue Feb 18 2025 - 00Z Fri Feb 21 2025 ...Great Lakes... Days 1-2... Impressive winter storm exiting into the Canadian Maritimes today will leave cyclonic flow in its wake and across the Great Lakes. This will promote continued CAA through D1, with 850mb temps plummeting to as low as -25C (Lake Superior) and -20C over Lake Ontario. This will support bands of lake effect snow (LES) especially D1 during the most intense CAA before the mid-level flow pattern relaxes downstream of a deepening trough centered over the Northern Plains/Upper Midwest later Wednesday. The generally W/NW flow will support heavy LES bands, likely in the form of single bands within unidirectional shear, and strong ascent focused into the low-level but deep DGZ will support heavy rates that could reach 2-3"/hr at times D1, heaviest east of Lake Ontario where an upstream moisture connection to Lake Huron will exist. These bands should be prolonged and intense D1, leading to WPC probabilities that are high (>70%) for more than 6 inches east of Lake Ontario and the far eastern U.P. Although LES will wane during D2, additional snowfall exceeding 4 inches is possible (30%) in these same areas. Downstream of Lake Ontario, total snowfall of 1-2 feet is possible before LES winds down. ...Pacific Northwest through the Central Rockies... Days 1-3... Two rounds of moisture spreading inland through enhanced IVT will spread wintry precipitation across much of the coastal and interior West. The period begins with a longwave positively tilted trough extending from the interior Northwest through the Northern Plains. This trough will deepen with time, diving into the Great Basin Tuesday morning and then pivoting into the Four Corners by the end of D1 as shortwave energy traverses through it. At the same time, a stationary front will be aligned from inland WA state southeast into the CO Rockies, as onshore flow from the Pacific spreads inland and lifts isentropically producing some modest enhancement of lift. Shortwave ridging expanding along the Pacific Coast late D1 will cut off Pacific moisture by Wednesday morning, but a axis of moderate to heavy snow is likely before this occurs, primarily along the stationary front and into downstream terrain. WPC probabilities are high (70-90%) for more than 6 inches in the terrain above generally 3000 ft from the Sawtooth into the Tetons and as far southeast as the Park Range of CO. After a brief respite, a more pronounced system will approach the Pacific Coast as a potent shortwave closes off over the Pacific Ocean and sheds eastward. This feature is progged to come onshore near the WA/OR border Wednesday evening, and there is good model consensus in this evolution despite modest differences in amplitude. The core of this trough coming onshore WA/OR will manifest as impressive height falls as far south as central CA, and as the downstream ridge erodes, this entire feature will elongated and pivot into the Great Basin/Central Rockies by the end of the forecast period. Modest jet level ascent through the development of a jet streak rotating around the base of this trough will enhance lift, but in general forcing is expected to be broad and driven primarily by height falls through D2-D3. Despite that, some enhanced IVT progged to exceed the 90th percentile within the CFSR climatology will spread inland within WAA downstream of the trough axis, leading to rising snow levels but enhanced wintry precipitation prospects. During this time, WPC probabilities for more than 6 inches climb above 30% in the Cascades D2, but with snow levels climbing above pass levels, with additional moderate chances (30-50%) for 6+ inches on D3 in the same general areas. ...Central Plains through the Mid-Atlantic States... Days 1-3... A significant winter storm begins tonight across the Central Plains and then spreads almost due east to the Mid-Atlantic on Wednesday. There is still considerable uncertainly as to how this evolves off the coast Wednesday night, but at least moderate snow and significant icing is likely for many areas. The system begins to take shape tonight as a shortwave ejects from the Four Corners and traverses to the east, reaching the Central Plains Tuesday evening. This will occur just north of a surface cold front that will surge south into Texas, and the interaction of this impulse with the elevated baroclinic zone will help induce some backing of the low-level winds, leading to enhanced moist isentropic ascent emerging from the Gulf, especially along the 285-290K surfaces where mixing ratios will rise to 3-4 g/kg. At the same time, a jet streak angled into the Northeast will leave its favorable RRQ for ascent atop the region, resulting in an overlap of synoptic and mesoscale ascent. With moisture continuing to surge into the region, this will result in an expanding axis of heavy snow, especially from eastern KS and OK into northern AR, southern MO, and western TN/KY. Here, the overlap of ascent and moisture will combine with a deepening DGZ (SREF probs for 100mb of depth exceeding 70% Tuesday evening and Tuesday night, resulting in an axis of heavy snowfall. Additionally, a secondary jet streak intensifying downstream of the sharpening trough will produce an environment supporting modestly coupled jet streaks, and the resultant ageostrophic response to the already impressive fgen will likely (>80% chance) drive snowfall rates to 1-2"/hr as shown by both HREF probabilities and the WPC prototype snowband tool. This system will become increasingly progressive to the east, but an 18-24 hour period of moderate to heavy snow with fluffy SLRs in the cold environment will support rapid and significant snowfall accumulations, especially Tuesday night from the Central Plains into the western Tennessee Valley. Here, WPC probabilities for more than 8 inches reach as high as 70%, with locally 12+ inches possible in the vicinity of the most intense banding. South of this heavy snow, an axis of mixed sleet and freezing rain is likely, with locally more than 0.1" of ice possible (30%), highest across southern AR. This shortwave begins to race eastward towards the Mid-Atlantic Wednesday as it gets squeezed into more zonal flow downstream of a more impressive northern stream trough (500mb heights falling below the 2.5 percentile anomaly according to NAEFS) digging out of the Northern Plains. This is where the the forecast begins to get more challenging. This northern stream trough will dive southeast towards the Ohio Valley while continuing to deepen into a potent closed low, resulting in downstream (south/east) jet development which will intensify beyond 150 kts. The overlap of the impressive height falls and LFQ of this jet streak will drive pronounced ascent into the Mid-Atlantic states, concurrent with continued moist isentropic lift from the Gulf into the area. This suggests that after a modest "break" in intensity of precipitation in the vicinity of the central Appalachians, renewed heavy precip will develop, especially as secondary surface low pressure develops offshore. There has been a steady trend in guidance to keep this precipitation suppressed south across the Carolinas and Virginia, with the most uncertainty now involving the speed and placement at which the secondary low develops and moves northeast up the coast. With the southern stream less amplified and faster, phasing of energy is progged to occur much later which should spare the northern Mid-Atlantic and Northeast much if any snow. However, heavy snow and mixed precipitation is still likely from eastern TN/KY into NC and VA, with a swath of the heaviest snow most likely across eastern VA. There is still a lot of uncertainty, however, and clusters indicate the primary EOF is dependent on the amplitude of this trough, with the GEFS members appearing to be a bit under-dispersive with a flatter wave, while the EC/CMC are a bit deeper in general. However, these deeper waves are also generally slower, again suggesting the threat for a phased major winter event are quite low. However, with the low eventually deepening rapidly offshore, a potent deformation axis collocated with the greatest moisture will result in heavy snow accumulations, for which WPC probabilities indicate a high risk (70-90%) for 6+ inches centered along the Norfolk/Virginia Tidewater region. Surrounding this maximum, WPC probabilities for 4 or more inches are 10-30% from eastern KY through WV and east towards coastal Delaware and far northern NC. Southeast of the snow, a mix of sleet and freezing rain is expected, and freezing rain potential has increased beneath the warm nose lifting northward. WPC probabilities for 0.1" peak above 70% D2 into D3 in parts of eastern and southeastern NC, with locally 0.25-0.35 inches of ice possible. Weiss ...Winter Storm Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_2.png