Subj : HVYSNOW: Probabilistic Heavy Snow And Icing Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Feb 16 2023 09:44:58 FOUS11 KWBC 160944 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 443 AM EST Thu Feb 16 2023 Valid 12Z Thu Feb 16 2023 - 12Z Sun Feb 19 2023 ....Upper Midwest, Great Lakes, Northeast... Days 1-2... The anomalous upper low over the Four Corners tonight will shift eastward to the Central Plains Thursday morning and begin to shear out to the east as it becomes embedded within increasingly confluent flow between a ridge across the Southeast and a weak shortwave dripping out of Manitoba towards the Great Lakes. Although this will lead to reduced amplitude of the primary wave, strong vorticity within the trough will maintain sharp height falls and strong PVA to drive ascent from southwest to northeast, aided by modest coupled jet streaks surging eastward downstream of the primary longwave trough. This strung out trough and accompanying vorticity lobe will move more progressively eastward through Friday, exiting the New England coast by Saturday morning, with the associated surface low following this progressive trough. Despite the weakening amplitude and faster forward motion, moisture advection will become quite impressive out of the Gulf of Mexico noted by PW anomalies surging to +3 standard deviations above the climo mean from the Ohio Valley to the Northeast D1 into D2. This moisture will surge northward on impressive 290-295K isentropic lift characterized by mixing ratios of 4-5 g/kg into the DGZ. At the same time, the associated WAA will help deepen the DGZ so that the most intense ascent through WAA and associated fgen will occur within this layer. This suggests that bands of heavy snow will likely occur on the NW side of this low, with snowfall rates progged by the WPC snow band tool to reach 1+"/hr from IA through the L.P. of MI, but the progressive nature of these bands should still prevent extreme snowfall totals. Still, WPC probabilities for more than 6 inches are 40-60% from central IA through central lower Michigan, and WSE plumes indicate locally some areas could receive in excess of 8 inches in the most persistent banding. As the system continues into the Northeast late D1 into D2, a rapid drying of the mid-levels should quickly erode moisture within the DGZ across the Great Lakes, but with low-level saturation occurring, a moderate period of light freezing rain/freezing drizzle is likely to occur. Where this additional freezing rain falls atop areas that received mixed precip early in the event due to the WAA/warm nose, substantial ice accretions above 0.1" are likely, especially from near Detroit, MI through the northern Adirondacks of Upstate NY where WPC probabilities for more than 0.1" are 30-40%. Additionally, the strong WAA will yield an expansive precipitation shield across Upstate NY and into central/northern New England, but at least at onset most of this should be rain outside of the higher terrain of the White Mountains and far northern Maine. However, as the low continues to pull away, CAA on the backside will rapidly progress southeastward resulting in a p-type changeover back to snow, aided by some anafrontal ascent such that cold air chasing moisture actually will result in heavy snow for parts of northern New England with snowfall rates of 1+"/hr likely which could accumulate rapidly before precip winds down Friday night. WPC probabilities for more than 6 inches of snow have increased as guidance has trended a bit colder, and despite uncertainty into exactly how the thermal structure will evolve to drive the p-type transition, WPC probabilities for more than 6 inches are above 70% in northern ME, with a few inches of snow possible all the way to the coast and parts of southern New England. Finally, with the strong N/NW flow behind the departing system, the setup looks favorable for at least modest LES south of Lakes Superior, Michigan, and Erie, with some upslope snow accumulation possible across the Appalachians of WV. WPC probabilities for more than 2 inches of snowfall are as high as 50% in the central U.P. and along the Chautauqua Ridge, and 10-30% for the higher terrain of WV. ....Pacific Northwest into the Northern Rockies... Days 2- 3... A closed low dropping southward off the Pacific Coast will shed a lobe of vorticity and an accompanying shortwave into British Columbia and Washington State Friday morning /early D2/, and this positively tilted, overall weak, trough will move progressively southeastward towards the northern Rockies D3 as it remains entrenched within confluent flow along the international border. Weak impulses upstream of the primary trough will drop southward through Saturday and into Sunday morning, bringing rounds of synoptic ascent which will support at least waves of moderate snowfall from the Cascades through the Northern Rockies, with the heaviest snow expected D3 when the final shortwave drops across the region bringing periods of heavy snow above about 2000 ft. WPC probabilities for more than 4 inches D2 are generally 10-30% in the WA Cascades and Northern Rockies, increasing to 30-50% across the Northern Rockies, Bitterroots, Absarokas, and Tetons on D3. Weiss ....Key Messages for Feb 14-17 Winter Storm... --Low pressure moving from the Mississippi Valley this morning into New England on Friday will spread a mixture of heavy snow, as well as sleet and freezing rain, across the Upper Midwest, Great Lakes, and Northeast --Bands of heavy snow will produce snowfall rates of 1+"/hr at times, which when combined with winds gusting up to 40 mph will result in dangerous travel conditions as blowing and drifting produces low visibility and snow-covered roads. --South of the heaviest snow, a corridor of mixed precipitation including sleet and freezing rain is expected to produce moderate ice accumulations. This will produce dangerous travel due to slippery roadways. --A brief period of very cold air will follow in the wake of this system. Wind chills are likely to fall below zero degrees in many areas that receive snow, tonight in the Upper Midwest and Great Lakes, Friday night across New England. $$ = = = To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .