Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 316 PM EST Tue Nov 21 2023 Valid 00Z Wed Nov 22 2023 - 00Z Sat Nov 25 2023 ...Northeast... Days 1-2... A multi-stream, positively-tilted trough extending from the Great Lakes to the lower Mississippi Valley will lift into Ontario this evening and then turn eastward across northern New England on Wednesday. High pressure over Quebec has planted cold air over much of the Northeast today with the precipitation shield expanding across the Ohio Valley ahead of a frontal boundary and its parent low moving through the Midwest. As the system tracks northeastward, the evolution will follow a fairly typical Miller-B setup, with mild air surging northward west of the Appalachians via the initial surface low while colder air holds on at the surface along and east of the mountains from the Mid-Atlantic northward. By Wednesday morning, the parent low will weaken and dissipate over the St. Lawrence Valley as a new low deepens along the Mid Atlantic coast and then becomes the new main low as it continues northeastward across southeastern MA and toward Nova Scotia by early Thursday. A mix of snow, sleet, and freezing rain will continue this evening across portions of interior NY including the Adirondacks, but transition to freezing rain and rain as warmer air moves in aloft. As the new coastal low deepens Wednesday, the flow will turn to easterly and northeasterly rather than southerly, allowing snow to remain more the predominant precipitation type across northern New Hampshire and interior Maine. Snow rates could exceed 1-1.5"/hr over northern NY into the Green and White Mountains per the 12Z HREF probs and WPC snowband tool. Total two-day accumulations may be heaviest over northern New Hampshire northeastward across interior Maine. In addition, low SLRs would increase the chance of impacts due to heavier snow load, per the WSSI. There, the WPC probabilities for at least 4 inches of snow are high (>70%) and for at least 8 inches are 20-50%. Over the North Country in NY and portions of the Green Mountains, WPC probabilities for at least 0.25" icing are low (generally near or below 10%), but are moderate (>40%) for at least 0.10" of ice. ...Northern Rockies to the Central Rockies and High Plains... Days 2-3... Mid-level flow out of the northeastern Pacific will dig into the Pacific Northwest and northern Rockies late Thursday and close off into an upper low over the northern Great Basin by early Friday. At the surface, a weaker Pacific system will push inland on Thursday, as a stronger cold front dives southward and eastward through Montana. This front will slow around the Divide, which will enhance low-level convergence on increasing easterly flow out of the northern Plains as the right entrance of the upper jet skirts eastward, providing broader lift to the region. Snow will expand and change over from rain as colder air moves southward on Thursday. Continued upslope flow and 700 mb frontogenetic forcing will maintain and increase snow over much of southern and southeastern Montana into Wyoming Thursday into early Friday. The Overlap of favored DGZ with vertical ascent will likely support rates nearing 1"/hr and totals well over 6 inches in at least the mountains. As the the upper low begins to turn more to the east, into the northern Rockies, the threat for at least light to moderate snow is expected to extend into the central High Plains by late Friday. Meanwhile, a mid-level shortwave and upper jet digging south of the low will begin to produce snow farther south into the southern Colorado ranges. WPC probabilities for at least 8 inches of snow days 2-3 are maximized over the Bighorns and northern Absarokas into south central MT as well as the Wind River Range. Even at lower elevations, accumulating snow is likely for much of Wyoming westward across the southern Idaho ranges into the northern Wasatch and Uintas as the upper low tracks near that region on Friday. Moderate (40%-70%) probabilities for at least 4 inches cover an extensive area extending from the northern Front Range and southeastern WY into the Black Hills and the northern Nebraska Panhandle and northwestward along and east of the Divide up to Glacier NP. Moderate to high probabilities for accumulations of 4 inches or more also cover many areas west of the Divide, including the southern Idaho ranges, the Uintas, northern Wasatch, and the San Juans. Fracasso/Pereira ***Key Messages for Thanksgiving Snowstorm*** --Snow Will Fall Over the Thanksgiving Holiday Accumulating snow, at least 1 inch, is very likely (80-90% chance) to affect large parts of the Northern Rockies Wednesday Night and Thursday, and the Central Rockies and adjacent Central High Plains Friday and Saturday. This may lead to periods of hazardous travel. --Heavy Snow Most Likely in Montana and Wyoming Winter Storm Watches are in effect for portions of southern Montana and much of Wyoming where snowfall in excess of 8 inches is likely (60-80% chance). Blowing snow will locally reduce visibility, including along parts of Interstates 25 and 80. --Impactful Snow Possible Further Southeast Idaho and Utah could see minor to moderate snow impacts Thursday and Friday. There is more uncertainty in snow totals in portions of Colorado, including the Front Range communities, and the adjacent Plains of Nebraska and Kansas Friday night into Saturday. Residents and travelers to these areas should monitor future forecasts for updates. --Flash Freezes May Affect Road Travel An Arctic front may cause flash freezes in Utah, Southern Wyoming, Colorado, and the Nebraska Panhandle Thursday (Thanksgiving) evening as temperatures rapidly drop below freezing.