Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 250 PM EST Sat Dec 16 2023 Valid 00Z Sun Dec 17 2023 - 00Z Wed Dec 20 2023 ...Great Lakes to the Central and Southern Appalachians... Days 2-3... Amplifying and intensifying cyclonic flow across the eastern CONUS behind a powerful coastal low pressure will result in an expansion of both lake effect snow (LES) downwind of the Great Lakes, and upslope snow into the central Appalachians. The primary mechanism for this intensifying cyclonic flow will be an amplifying mid-level trough stemming from a potent shortwave dropping out of Manitoba Sunday evening and then racing southeast to phase with a southern stream vort shedding up from the Carolinas. This will result in a negatively tilted trough with a closed center developing over the Mid-Atlantic Monday night, with this feature then shearing out as it reaches the coast of New England late in the forecast period. Although the primary low pressure, which will be intense, will be accompanied by only rainfall due to a warm column, rapid CAA in its wake and behind this sharpening trough will produce an increasingly favorable environment for both LES and upslope snow. LES is likely to begin first south of Lake Superior after 00Z Monday as CAA plunges 850mb temperatures to as low as -20C, producing significant instability over the lake and lowering the DGZ to efficiently align the best ascent with the snow growth region. This will likely manifest as heavy snow multi-bands over much of the U.P. on D2, with LES extending south and east from there to impact parts of western lower Michigan and eventually northeast OH and along the Chautauqua Ridge southeast of Lake Erie. Although the fetch direction is less than ideal in most areas except maybe SW Michigan, effective fetch and enhanced moisture from upstream connections could enhance LES, especially south of Lake Erie. WPC probabilities on D2 for more than 4 inches of snowfall are highest in the western U.P. where they reach 30-40% in the Porcupine Mountains, with probabilities around 10% across most of the other favored NNW snow belts. The highest LES probabilities shift east on D3, peaking above 70% for more than 4 inches along the Chautauqua Ridge, with locally up to 8 inches possible. Then beginning late Monday and especially Monday night into Tuesday, increasing NW winds in the wake of the low and upper vort will drive a more favorable environment for upslope snow, especially from the Laurel Highlands of PA southward along the WV Appalachians, and even into the higher terrain of NC/TN. Soundings Monday night become quite extreme across WV with steep lapse rates from the surface through nearly 700mb beneath the cold core low, and a cross-hair signature of omega into the lowering DGZ. Additionally, theta-e lapse rates show some evidence of going to < 0, suggesting convective potential within the already favorable environment. Winds in the DGZ are quite strong which could fracture snow growth, but otherwise this could be a significant event for upslope region, and WPC probabilities peak above 40% for more than 8 inches in the highest terrain of the WV Allegheny Mountains. While lesser impact due to amounts, it is also possible that a swath of convective snow showers or even isolated snow squalls may occur behind the primary vort and the surface cold front. This is reflected by high NAM snow squall parameter values shifting from ND Sunday night into the Ohio Valley Monday night. Limited low-level fgen and modest 0-2km theta-e lapse rates suggest the squall threat is modest, but brief heavy snow rates producing limited visibility and hazardous travel are possible. ...Sierra... Days 2-3... A closed low dropping southeast across the Pacific will result in downstream confluent mid-level flow, directing modest IVT of around 250 kg/ms onshore the CA coast Monday night into Tuesday. Within this pinched flow, ascent will be driven via pockets of PVA ahead of subtle vorticity maxima rotating eastward, which will also be collocated with the LFQ of an approaching Pacific jet streak. As PWs ride to above +2 sigma according to the NAEFS ensemble tables, this ascent will result in increasing precipitation coverage and intensity across CA late Monday through Tuesday. With snow levels progged to rise to as high as 9000 ft before slowly falling late D3, this will confine any snow impacts to the higher terrain of the Sierra. Here, WPC probabilities for more than 4 inches on both days 2 and 3 reach 30-50%, with 2-day snowfall maxima of 12-18" possible in the highest terrain. Weiss