Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 AM EST Wed Nov 29 2023 Valid 12Z Wed Nov 29 2023 - 12Z Sat Dec 02 2023 ...Great Lakes... Day 1... The amplitude of the eastern CONUS trough will weaken today, but one last shortwave and associated vorticity lobe swinging through its base will result in a final day of additional lake effect snow (LES). This shortwave is progged to move from northern lower Michigan this morning into New England tonight, and some moderate synoptic snow will accompany the weak PVA and height falls. However, this feature is transient, so synoptic snowfall accumulations are expected to be minor. More significant snowfall is likely downwind of Lakes Erie and Ontario, but as low-level flow backs ahead of the shortwave trough axis, the bands should shift to a more WSW to ENE direction in response to subtle WAA, which will result in weaker snowfall rates and an axis positioned more to the NE of the lakes. As the trough axis pivots eastward this evening, a brief surge of renewed CAA may occur, but with more marginal thermal structure supporting a mix of rain and snow before secondary shortwave ridging approaches rapidly from the west bringing an end to the heavy LES. WPC probabilities for more than 4 inches of snow are as high as 50%, but focused east of Lake Ontario near Watertown. ...Southwest and Southern and Central Rockies... Days 1-3... A 500mb trough along the CA coast today will dive southeast towards the Four Corners while amplifying into a closed low near NM on Thursday. This low re-open and begin to fill as it advects quickly into the Plains during Friday, but will be immediately followed by a secondary shortwave trough Thursday night into Friday, and potentially a third trough before Saturday morning as spokes of energy rotate through an amplifying longwave trough across the Western CONUS. Aloft, the subtropical jet streak will pivot across northern Mexico and the Desert Southwest, becoming more favorably aligned for diffluence beginning Thursday night. This jet stream evolution will provide additional ascent, while also supplying more significant Pacific moisture into the region. PW anomalies according to the NAEFS ensemble tables are progged to remain modest, however, peaking at +1 to +2 sigma Thursday night into Friday, but this will be wrung out efficiently by the synoptic lift and additional low-level ascent through upslope flow and convergence as a back door cold front sags to the south. This evolution will result in multiple waves of precipitation, with light snow occurring D1, generally above 7000 ft, and more significant accumulations likely D2-3 as a surface low develops in the lee of the Sangre de Cristos to enhance moisture and ascent in an already favorable synoptic pattern. WPC probabilities on D1 for more than 4 inches of snow are modest around 20-30%, highest atop the Kaibab Plateau and into the White Mountains of Arizona. WPC probabilities D2-3 expand northeast into New Mexico and Colorado, favoring heavy snowfall exceeding 6 inches in the Jemez Mountains and San Juans. ...Cascades through the Northern Rockies... Days 2-3... An increasingly wet period is likely beginning Thursday night and continuing through the end of this forecast period. A sharpening shortwave will move onshore WA/OR Thursday night with PVA and height falls to drive ascent, followed immediately in its wake by confluent mid-level flow beneath an intensifying zonally oriented jet streak aloft. Within this confluent flow, additional spokes of shortwave energy and accompanying vorticity maxima will shed onshore, driving multiple weakening cold fronts on shore in tandem. This will manifest as a surge of PW and IVT, approaching +2 sigma at times, which will fuel expanding precipitation, some of which could be heavy, especially late Friday into Saturday. There may be multiple rounds of moderate to heavy snow: the first Thursday into Friday, and the second, likely more impressive, late Friday and Saturday. Snow levels will fluctuate through the period between cold fronts and in-between WAA, but should generally be around 2000 ft north, 5000 ft south. This will likely result in heavy pass-level snow producing significant travel impacts by D3. WPC probabilities for more than 6 inches peak above 60% D2, highest in the Oregon Cascades and above 4000 ft, and then expand eastward to the Northern Rockies, while rising above 90% in the WA and OR Cascades by D3. 2-day snowfall in the Cascades could exceed 3 feet in the higher elevations. ...Central Plains into the Midwest... Day 3... An area of low pressure emerging from the Southern Plains Thursday night will shift northeast through Friday as a potent shortwave opens from the Four Corners and ejects into the westerlies. This shortwave will likely shear out and leave the low behind during Friday as a secondary vorticity lobe swings through the region, and it is this secondary feature that will finally cause the precipitation to exit to the northeast. This evolution will result in two waves of precipitation, one late Thursday night into Friday, with a second area developing Friday night. The first is expected to be more impressive due to more intense synoptic lift and better theta-e advection northward, but the column will be marginally supportive for any wintry precipitation. The guidance has trended a bit wetter and farther north tonight, and with cold high pressure centered over Canada, wet-bulb cooling could result in a stripe of freezing rain from eastern Kansas through southern Iowa. There is a lot of uncertainty with how this will develop, but the setup favors at least the threat for minor icing, and WPC probabilities for 0.1" of ice are as high as 5-10%. The second area of precipitation may be a bit colder and more strongly mesoscale forced as an axis of deformation develops upstream of the second shortwave. This could result in a stripe of light to moderate snow from northeast KS through eastern IA, but at this time WPC probabilities for more than 1 inch of snow are less than 10%. Weiss