Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 344 PM EST Wed Dec 26 2018 Valid 00Z Thu Dec 27 2018 - 00Z Sun Dec 30 2018 ...Central and Southern High Plains through the Western Great Lakes... Days 1-2... Significant winter storm will impact much of the Plains north of Oklahoma before shifting towards the western Great Lakes on Day 2. Strong closed upper low evident on water vapor imagery this afternoon will continue to deepen as it lifts northeast through the Central Plains by Thursday evening. This feature will begin to fill on Friday as it shears out and becomes embedded in the northern stream by the end of Day 2. In response to this upper low, surface cyclogenesis will begin in earnest over the TX Panhandle before deepening while lifting northeast towards Lake Superior Friday afternoon. Strong warm and moist advection will produce tremendous moisture, with PWATs climbing to near +4 standard deviations above the climo mean. Most of this moisture will be confined to the warm sector, but a significant amount will wrap cyclonically around the low and into the cold air within the mid-level deformation zone and TROWAL. This coupled with jet level diffluence will produce synoptically forced ascent and heavy snowfall. Additionally, a sharply sloped band of 925-700mb frontogenesis is likely to align SW to NE from eastern CO through eastern SD, and within this region EPV will drop below 0 suggesting convective snowfall rates of up to 2"/hr are possible. The most likely location for these intense snowfall rates will be across NW KS, central NE, and into eastern SD, but some uncertainty still exists into the exact placement of these mesoscale features. Despite that, the guidance has shown a considerable shift eastward today with the heaviest snowfall, and nearly all available guidance including today's high-res ARW/NMM/RAP suggest a similar placement of the heaviest snow axis. This has necessitated a shift in the highest probabilities for snowfall, but the northern edge across portions of SD and ND remain similar as the deformation pivots overhead producing a longer period of snow despite somewhat lesser rates. Although some of this precipitation will begin as rain, especially in western KS and NE, cold advection and dynamic cooling will rapidly transition any precipitation to snowfall, and WPC probabilities now show a high risk for 8 inches of snow in a stripe from extreme SE Colorado through central Nebraska, the eastern Dakotas, and into western Minnesota. In many locations, especially in NE/SD/MN, over 12 inches is likely, with up to 18 inches possible in isolated spots. A secondary maximum of snow is also likely NW of Lake Superior in the Arrowhead of MN where SLRs will be slightly higher, and some upslope enhancement into the Iron Ranges is likely. In this region, WPC probabilities are also high for 8 inches, with more than 12 inches possible. On Friday, as the upper low begins to shear out to the northeast ahead of an amplifying trough over central Canada, the larger scale heavy snow threat is expected to diminish across the upper Midwest. However, cold air advection with northerly to northwesterly flow is expected to support lake effect snow, which may produce some additional significant amounts across parts of the U.P. of Michigan through Friday night.. ...Northern New England... Day 2... Warm advection associated with a warm front southeast of a potent surface low moving into southeast Canada will produce overspreading precipitation into New England on Friday. Initially, the presence of cold high pressure will keep the column cool enough for snow across the Adirondacks of NY and points east through VT/NH/ME. The strong WAA will quickly cause a p-type transition to sleet/freezing rain and then rain as the 850mb temps climb above 0C. By Friday afternoon, all precipitation should be liquid except far northern Maine and NH where sub-freezing surface air will persist freezing rain a little longer. Despite a quick transition from freezing precip to liquid, strong enough ascent in the WAA regime will produce snowfall which may accumulate to a few inches in the terrain of Northern New England, and WPC probabilities are around 30 percent for 4 inches. After the snow, a period of freezing rain is likely, and although accretion is forecast to be up to one tenth in the sheltered high elevation valleys, the probability for significant (0.25 inches) accretion is minimal. ...Southwest and Southern Rockies... Days 1-3... One negatively tilted trough will eject from the Southwest early on day 1, to be replaced by a robust and positively tilted trough late Friday into Saturday. Some light snow will persist early on day 1 into New Mexico, but the majority of the snow this period will occur with the forcing associated with the second trough Friday into Saturday. As this feature digs into the Four Corners region, it will produce height falls from Utah through AZ/NM/CO, which will aid in ascent created by strong diffluence within a coupled jet structure around the base of the trough. At the same time, strong moist advection is likely as increasing 700mb winds from the SW advect Pacific Moisture into the region, and significant QPF is likely. The highest snowfall is expected where upslope enhancement occurs in the San Francisco and Black Mountains, as well as the southern Sangre De Cristos. In these ranges, WPC probabilities feature a moderate risk for 8 inches of accumulation, and 2-day totals may exceed 12 inches. Elsewhere, WPC probabilities are moderate for 4 inches from the mountains of the eastern Great Basin, southeast through Colorado and New Mexico, with the highest probabilities above 6000 feet. ...Pacific Northwest... Days 1-3... Persistent moisture being channeled into the region atop a mid-level ridge will produce periods of mountain snows from the coastal ranges of OR/WA through the northern Rockies. The strongest forcing will occur Thursday due to diffluence within an embedded 120kt jet streak and a surface cold front pushing inland. On Thursday, snowfall is likely as the front pushes inland, with WPC probabilities moderate for 4 inches from the Oregon Cascades through the Bitterroots of Idaho. A secondary push of moisture will bring an additional round of light snow on Friday, but accumulations should be confined to the high terrain of the Washington Cascades and Northern Rockies. Weiss