Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 456 PM EST Sun Jan 02 2022 Valid 00Z Mon Jan 03 2022 - 00Z Thu Jan 06 2022 ...Pacific Northwest to the Central Rockies... Days 1-3... Widespread snowfall across the Pacific Northwest through the Central Rockies is likely the next three days as a closed low southeast of Alaska slowly opens and pivots onshore WA Tuesday, shedding sheared vorticity eastward into the Rockies by D3. This will be accompanied by modest Pacific jet energy and mid-level confluent flow to drive moisture into the region, with IVT approaching 750 kg/m/s by D3. Snow levels through the period will remain low, generally 1500-2500 ft D1 and D2, before climbing, especially in OR and points east, D3 to 4000 ft as the more significant moisture plume pivots onshore. With nearly continuous moisture and forcing, snowfall is likely in the terrain each day of the forecast period, and there exists a high risk for 12+ inches of snow both D1 and D2 in the WA/OR Cascades, the Olympics, the northern CA ranges and into the Sawtooth of ID. 2-day snowfall could exceed 5 feet in the Cascades, with several feet also likely at the important Cascade mountain passes. Low probabilities for 4 inches extend into most of the Northwest valleys except the Columbia Basin and lowlands around Seattle and Portland. By D3, the best moisture begins to spill eastward as far as the High Plains of WY, but forcing is a little less robust by this time. This will produce moderate snowfall as far east as the Nebraska Panhandle, and WPC probabilities for 4 inches are high in the terrain from the WA Cascades through the Sawtooth, the NW WY ranges, and into the Park Range of CO. Locally more than 12 inches is possible in any of this higher terrain on Wednesday. ...Northern Plains and Great Lakes... Days 2-3... A shortwave dropping out of Alberta will interact with sheared vorticity coming across the Pacific Northwest to produce a closed low over the Upper Midwest on D3. This feature will be accompanied by an increasingly coupled jet structure, and the overlap of this ascent will produce a fast moving wave of low pressure moving through the region on Wednesday. While this low will be fast moving and in a region of modest moisture, a cold column supporting above-climo SLRs will lead to moderate to heavy snowfall accumulations D3. The primary band of synoptically forced snowfall will move from ND Tuesday evening through Michigan by the end of D3, and WPC probabilities are 10-30% for 4 inches in this region. However, as the low ejects eastward through MI, robust LES is likely to develop as flow pinches and CAA sets up over Lake Superior from the north, and then curving off of Lake Michigan into the L.P. This LES could be prolonged, and although inversion heights are only progged to reach 6000-7000 ft, this will likely overlap into the DGZ, and significant LES is likely, especially across the U.P. and the Keweenaw Peninsula. WPC probabilities D3 are high for 6 inches here, with locally more than 1 foot possible. ...Southern Appalachians through Coastal New England... Days 1-2... ...Increasing confidence in a significant winter storm impacting the Mid-Atlantic... Intensifying southern stream shortwave will continue to move across the Southeast while amplifying into a closed low and then lifting across the Mid-Atlantic Monday morning before ejecting to off New England late Monday. This shortwave will be accompanied by an impressive poleward arcing jet streak over southern ME, with wind speeds progged to exceed the 90th percentile at 250mb over Maine according to the SPC sounding climatology. The interaction of these features with an impressive surface cold front and associated baroclinic gradient will drive rapid surface low pressure development. This low will move quickly across the region spreading heavy precipitation northward, and a swath of heavy snow has become much more likely tonight through Monday night. The guidance this aftn have continued to trend stronger, a little slower, and more NW with this system. The exception is the NAM, but this model seems to be initializing a bit too far south compared to reality and is not preferred. This trend suggests another uptick in snowfall potential which is reflected in both WSE mean/NBM 50th percentile, as well as the high end potential of the 90th percentiles of these same ensemble clusters. While antecedent conditions are quite warm, and many places may start as rain before changing over, rapid CAA behind the cold front will quickly transition rain to snow in many areas. This CAA will be aided via impressive ascent within a pivoting deformation axis to drive intense dynamic cooling of the column. Although SLRs will vary greatly, starting very low and then increasing with time as the column cools, notable overlap of -EPV* and ThetaE lapse rates near or below 0 suggest CSI or even upright convection (thunder snow) within the best deformation. This could produce snowfall rates of 2+"/hr, which is reflected both by HREF snow probabilities and the WPC snowband prototype tool, with the highest chances across VA, MD, DE, into southern NJ. It is these rates that will help overcome the low-level warmth and hostile antecedent conditions to produce heavy snowfall. Tonight, precipitation will occur as snow within the deformation axis as far SW as northern AL and central TN where heavy snow rates could produce a few inches of snowfall. However, the heavier precipitation will likely overspread the region from the mountains of NC eastward through southern NJ where WAA and a blossoming TROWAL will expand the precipitation shield, with a changeover from rain to snow occurring from NW to SE, and all snow likely well NW of the 850mb low center. This, again, will have an impact on SLR, and the NBM SLR values were decreased closer to Cobb numbers during transition, but then increased above NBM during the period of heaviest snowfall to account for dynamics. While confidence is lower than usual due to still tremendous model spread in thermal structure, WPC probabilities have increased, and now show a high chance for 4+ inches from the NC/TN mountains northeast through central VA and across southern NJ on D1. Within this axis, there are likely to be pockets of heavier snow, and WPC probabilities for more than 8 inches are above 60% in the NC/TN mountains, with a secondary axis of greater than 50% just SE of the I-95 corridor from central VA through southern NJ where convective rates are most likely. It is possible a few locations could receive double-digit snowfall before snow winds down from west to east Monday evening. Further NE, jet streak enhanced precipitation NW of the low could produce moderate to heavy snow across far SE Long Island and across the Cape and Islands into D2, where WPC probabilities for more than 4 inches are as high as 40%. Weiss ~~~ Key Messages for the Southern Appalachians through Coastal New England Winter Storm ~~~ Rain will likely change over to moderate snow across northern Alabama, central Tennessee, and northern Georgia tonight. A few inches of snowfall will likely produce hazardous travel. Heavy wet snow is likely beginning early Monday from the Southern Appalachians through much of the Mid-Atlantic. Snowfall is expected to exceed 4" in many locations, with local amounts approaching 10" possible. Snowfall rates may reach 2"/hr at times, and thunder-snow is possible. These rates combined with gusty winds will produce dangerous travel conditions. Heavy wet snow accumulating on power lines and tree limbs could create scattered power outages. Very cold temperatures Monday night may allow roads to re-freeze, producing additional dangerous travel even after the snow has ended.