Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 243 AM EST Sat Dec 02 2023 Valid 12Z Sat Dec 02 2023 - 12Z Tue Dec 05 2023 ...Cascades to the Northern and Central Rockies... Days 1-2... An atmospheric river is forecast to continue bringing 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 impacting the Pacific Northwest this morning and is forecast to dig further into the region and eventually towards the northern Rockies on Sunday. 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 today. For the Cascades, snow levels are expected to remain low until early Sunday morning, 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 of storm total snowfall can be expected by late tonight 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. Tonight and Sunday, as this initial shortwave continues 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 (above 7000 feet) beginning into the early Sunday morning hours, 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. Any impacts associated with heavy precipitation early next week across the Pacific Northwest are expected to be associated with rain. ...Northern New England... Days 2-3... A shortwave racing northeastward within strong southwesterly upper flow between troughing over the central U.S. and ridging in the western Atlantic will send a developing surface low pressure system from the Midwest to the Lower Great Lakes on Sunday, eventually reaching northern New England. Meanwhile, a potential coastal low slides northward from the Mid-Atlantic to the Gulf of Maine by Monday. Cold air will be supplied from a Canadian high pressure system over Quebec that will mostly stay in place to the north of New England, allowing a persistent supply of colder air to maintain a winter weather threat. 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. Guidance has trended slightly colder and is supported by the later half available CAM guidance. 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 and strong 700 mb fgen entering New England. The pattern will remain quite progressive so the system will exit rather quickly, but there will be a window where an initial thump of heavy wet snow is possible from strong warm air advection over parts of New Hampshire and Vermont into Sunday night. Heavy snow is also possible along a potential deformation band on the northwest side of the developing 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 (>80%) and medium (40-50%) for at least 8 inches, with a broader footprint of low probabilities (>20%) for at least 4 inches from the Adirondacks eastward through the Green Mountains and to the I-95 corridor in Maine (but not quite to the coast). Snell ***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 lower elevations 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 total snowfall 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 lowlands 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.