Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 238 PM EST Sat Jan 01 2022 Valid 00Z Sun Jan 02 2022 - 00Z Wed Jan 05 2022 ...Pacific Northwest to the Northern Rockies... Days 1-3... Longwave ridging over the West will quickly be displaced by a trough digging out of the Gulf of Alaska, bringing a return to widespread precipitation across the region. The trough axis likely will remain offshore until Tuesday when it shifts eastward, but ahead of this, increasing mid-level confluence beneath an intensifying jet streak will advect enhanced moisture into the Pacific Northwest noted by PWs increasing to +1 standard deviation above normal on IVT of 500 kg/m/s. This will spread moisture from the Pacific Northwest late D1 southward into northern CA D2, and then spilling over into the Northern Rockies D3 while a secondary surge of moisture shifts onshore. Snow levels will remain quite low east of the Cascades through the period, likely peaking only around 1500 ft, but will rise within each plume of moisture closer to the coast. Snow levels D2 with the first surge could rise above 3000 ft, and then over 4000 ft, especially in northern CA, D3. With the late onset of precipitation D1, WPC probabilities for 8 inches are modest and confined to the Olympics and northern WA Cascades. However, by D2 the moisture plume becomes much more impressive and overlaps with intensifying ascent. 700mb flow increases quickly out of the W/SW which will then upslope favorably into the Cascades, and heavy snowfall is nearly certain above 1500 ft in WA, and 2000-3000 ft in OR and CA. WPC probabilities are high for 12 inches in the Olympics and along the Cascades from WA to OR, with locally more than 3 feet possible in the highest terrain. Snowfall at pass level will likely reach 1-2 ft on D2 at Snoqualmie, Stevens, and Santiam Passes. Lighter snows are likely spreading into the northern CA ranges and into parts of the Northern Rockies. Late D2, the trough axis shifts onshore to maximize ascent and drive a surface cold front onshore, but this is followed quickly by an approaching warm front and secondary surge of moisture into the region. As this next plume or moisture surges onshore with only subtly higher snow levels, WPC probabilities indicate a high chance for 12 inches in the OR Cascades, with moderate probabilities into the Olympics and WA Cascades, as well as into the Sawtooth of ID. ...Central and Southern Appalachians... Day 2... A southern stream shortwave will be moving overhead the MS VLY Sunday night to begin D2 as it amplifies towards a negatively tilted trough, with some guidance suggesting a closed low over SC Monday aftn. The models continue to trend a bit slower and stronger with this feature, which in conjunction with the RRQ of an impressive nearly 200kt jet streak arcing over New England should lead to a potent surface low developing across the Southeast and then moving quickly northeast to off the Mid-Atlantic coast Monday aftn. Initially, the column will be warm enough for all rainfall. However, strengthening of the low and CAA behind the cold front could lead to a rapid changeover from rain to snow, and timing of this changeover as well as where this will occur most robustly is the critical challenge to the forecast. The guidance still shows a wide disparity in the track and intensity of this low, with the GFS farther north and snowier, while the NAM is south with more rain. Using the WSE as a guide, the solution will likely end up somewhere in the middle, and this suggests the potential for several inches of snowfall in the mountains of NC/TN where WPC probabilities are moderate for 2 inches. Further to the east and to the coast, antecedent warm temperatures and rain should limit accumulation even after changeover, but the HREF does have some low-end probabilities for 1"/hr snow rates which could occur within the pivoting deformation axis NW of the low. These rates will likely be required for any accumulation, and WPC probabilities for 1" are as high as 20% near Richmond, VA and across southern MD. ...Central Plains through the Great Lakes and into Northern New England... Days 1-2... ...Significant winter storm will continue from the Upper Midwest through Northern New England... Snow has been overspreading the Central Plains and Midwest this morning associated with a low pressure moving through the Southern Plains. As this low lifts northeast through Sunday, a swath of moderate to heavy snow will persist NW of the low beneath an overlap of moist advection and impressive RRQ jet-level diffluence. The guidance has sharpened the northern gradient a bit this aftn with a sharp cutoff likely due to very dry air across the Northern Plains, but a band of snow with rates around 1"/hr will likely move northeast tonight into Sunday stretching from far eastern IA through the Chicago metro, across the L.P. of MI and then streaking into Upstate New York and Northern New England. The heaviest snow may occur near the lake shore of Lake Michigan where synoptic ascent will be amplified by at least modest LES as a band develops over Lake Michigan and pivots southward, but a secondary maxima of snowfall is also likely in the northern Adirondacks. In these two areas, WPC probabilities are moderate for 6 inches on D1, with low to moderate probabilities for 4 inches surrounding the maxima from eastern IA through northern ME. Snowfall is likely to end from west to east across Maine Monday morning. Weiss ~~~Key Messages for the Central Plains to Great Lakes Winter Storm~~~ Significant snow impacts are expected from northern Kansas to the Chicago metro area. Snowfall rates in excess of 1"/hr combined with strong winds will lead to dangerous travel and near zero visibility at times. Significant mixed precipitation including freezing rain and sleet is likely to the south of the heavier snow. Ice accretions up to 0.1", and sleet accumulations near 0.5", are possible from eastern Kansas into western Illinois. Bitterly cold temperatures will follow with dangerously cold wind chills tonight into Sunday morning. Repeating thunderstorms today from Arkansas northeast to the Ohio Valley are expected to produce heavy rainfall totals of 2-4" with isolated higher amounts. This heavy rainfall is likely to produce numerous instances of flash flooding.