Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 PM EDT Wed Mar 17 2021 Valid 00Z Thu Mar 18 2021 - 00Z Sun Mar 21 2021 ...Kansas/Missouri... Day 1... An anomalously deep mid-level low will move eastward from OK tonight to TN Thursday evening, accompanied by an upper divergence maxima to drive slowly strengthening surface low pressure. As these features shift east, pronounced warm/moist advection in the form of a TROWAL will rotate cyclonically around the upper low and back to the W/SW near the KS/MO border. While precipitation will initially begin as rain across all of the area, an intense deformation axis W/NW of the low combined with sharpening fgen will cause rapid dynamic cooling of the column to cause a p-type transition from rain to snow. The rate at which this occurs still varies, but the preferred high-res guidance are more aggressive owing to better CSI potential to enhance snowfall rates. With low-level thermals remaining marginal, it will require these heavy rates to accumulate efficient, and the recent HREF guidance has increasing probabilities for 1"/hr during the period of most intense ascent. WPC probabilities for heavy snow have increased, and now indicate a narrow corridor of 10-20% chance for 4", potentially including the Kansas City metro area. ...Northeast... Day 2... Models show the previously described low beginning to shear as it moves east of the central Appalachians and begins to interact with a northern stream trough amplifying over the Northeast Thursday night driving a cold front through New England. Moisture interacting with the boundary will support precipitation spreading across the northern Mid Atlantic and portions of the Northeast on Thursday into Friday. The guidance has trended a bit further north with its precipitation today, likely due to some sheared vorticity spreading northward from a subtly stronger mid-level low to the south. There remains considerable spread in the accumulation forecasts from the models, likely due to different handling of the intensity of the mesoscale forcing. Since the antecedent conditions are unfavorable for snow (warm with rain), the higher res models are preferred since they depict intense ascent through a deformation axis and possible CSI, which would be enough to overcome the antecedent warmth. The heaviest snow is likely in a stripe from near the Catskills eastward across CT and MA, where WPC probabilities are as high as 20-30% for 4 inches due to 1"/hr snowfall rates. Further to the south across PA and potentially even into the higher terrain of WV/VA, light accumulations are possible as the initially colder temperatures in the terrain will allow for more efficient accumulation as the column cools behind the cold front. ...Western U.S.... Days 1-3... A closed mid-level low will pivot slowly eastward across the Pacific Thursday and Friday, moving onshore WA/OR Saturday morning. This feature will be accompanied by a slow moving surface low, while at the same time shedding pieces of vorticity energy eastward into the Northern Rockies. With the broad trough surrounding the low maintaining its axis west of the coast, prolonged 700-500mb SW flow will spread moisture inland, but with relatively high freezing levels. At least one of these vorticity spokes will spawn secondary surface low pressure developing across the Great Basin, but in general waves of precipitation are likely to occur across the region in response to the combination of synoptic forcings. Rounds of snowfall are likely each day, first in the northern CA ranges and then spreading northeast as far as the Olympics and Cascades, as well as the Northern Rockies and ranges of NW WY including the Tetons. High WPC probabilities for 6 inches are confined to the Shasta area on D1, and then expand into the Sierra, Olympics, and Cascades D2, and continue across these ranges while spreading into the Tetons on D3. Total snowfall may exceed 2 ft in the highest terrain of the Olympics and Sierra. For Days 1-3, the probability of significant icing (0.25-inch or more) is less than 10 percent. Weiss