Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 314 PM EST Fri Dec 01 2023 Valid 00Z Sat Dec 02 2023 - 00Z Tue Dec 05 2023 ...Cascades to the Northern and Central Rockies... Days 1-3... An atmospheric river is forecast to bring heavy precipitation, including mountain snow, to the Northwest and the northern Rockies this weekend. Low snow levels at the onset are expected to rise markedly by the latter half the weekend. Impactful snows are likely downstream as ample moisture and energy moves into the central Rockies. The next in a series of well-defined shortwaves is forecast to dig into the Pacific Northwest on Saturday. This will be the wettest of the systems so far, as ample moisture and enhanced lift along the left-exit region of a 130+kt upper level jet support moderate to heavy precipitation from the Pacific Northwest to the northern Rockies on Saturday. For the Cascades, snow levels are expected to remain low through Saturday, impacting travel through the passes. WPC guidance shows high probabilities (greater than 70 percent) for accumulations of 12 inches or more, with heavier amounts expected for most of the major passes on Saturday. Several feet can be expected by late Saturday along the higher peaks of the Olympics and the Washington and Oregon Cascades. As energy and moisture moves east of the Cascades, heavy amounts of a foot or more are possible across portions of the Blue Mountains and along the Rockies from northern Idaho to northern Utah on Saturday. On Saturday night and Sunday, this initial shortwave will continue its dive southeast. The associated upper forcing and baroclinic zone will support areas of moderate to heavy precipitation spreading farther southeast into the central Rockies, with the probabilities for heavy snow increasing over the northern Utah and the south-central Wyoming to the northwest Colorado ranges. Meanwhile, a compact low intensifying over the Gulf of Alaska and the northeastern Pacific will focus a plume of deeper moisture into the Pacific Northwest, fueling heavy precipitation especially along and west of the Oregon Cascades. However, snow levels will begin to climb significantly beginning Saturday night, limiting the threat for additional heavy snow apart from the higher peaks. Snow levels are forecast to fluctuate but remain relatively high through Sunday, before additional increases are expected as a ridge builds over the Northwest and northern Rockies on Monday. ...Northern New England... Days 2-3... A multi-system event will unfold over the Northeast late day 2 into day 3, as a main shortwave lifts through the Upper Ohio Valley interacts with a lead baroclinic zone along the Mid-Atlantic coast. Canadian high pressure over Quebec will mostly stay in place to the north of New England, allowing a supply of colder air to maintain itself. However, temperatures will still be marginal over central New England with the T850 0C line into northern VT/NH Sunday morning as low pressure starts to deepen over the Mid-Atlantic once the Ohio Valley shortwave gets close enough to the Appalachians. By Sunday evening into Monday, combination of the strengthening upper jet into Southern New England and sharp mid-level height falls will lead to continued cyclogenesis across southeast Mass. The pattern will remain quite progressive so the system will exit rather quickly, but there will be a window where a deformation band could produce some heavier snow rates on the northwest side of the surface low in the deeper colder air (most likely northern NH into far interior Maine) as well as the higher elevations. There, WPC probabilities for at least 4 inches of snow are high (>70%), with a broader footprint of low probabilities (>10%) from the Adirondacks eastward through the Green Mountains and to the I-95 corridor in Maine (but not quite to the coast). Pereira/Fracasso ***Key Messages for Pacific Northwest Winter Storm*** --Prolonged winter storm in the Northwest A long-duration winter storm for the Pacific Northwest will continue through this weekend. Expect periodic heavy precipitation with rain for the valleys and snow at many mountain passes. --Major snow for the Cascade passes Through Sunday, there is a high chance (>90%) of at least 24" of snow for Cascade passes, including I-90. Snow levels remain below 4000 feet through Saturday before rising above pass level Saturday night. Travel will be difficult and hazardous. --Heavy snow and blowing snow in the Rockies Increasing wind with periods of heavy snow will produce areas of blowing snow over the higher terrain of the northern Rockies. There is a high chance (>70%) of 1-3 feet of snow in the higher terrain through Sunday. --Potential for river flooding into next week Several inches of rain in the valleys and the rising snow levels Saturday night into Sunday will likely produce minor to moderate river flooding late this weekend into next week in western WA/OR.