Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 324 PM EST Thu Feb 20 2020 Valid 00Z Fri Feb 21 2020 - 00Z Mon Feb 24 2020 ...North Carolina/Virginia... Day 1... Surface low pressure developing along a baroclinic gradient off the Southeast coast will combine with WAA ahead of a positively tilted mid-level trough to spread precipitation across the area through early Friday. While this is ongoing presently, and is mostly in the form of rain, the column is expected to cool through the evening and into the overnight as the mid-level front sags southward and the surface low pulls off to the east. The guidance has come into better agreement that a modest deformation band will pivot eastward behind the surface low, which will enhance snowfall rates across eastern NC and SE VA. The heaviest snow is likely in E-Central NC where WPC experimental snow-band probabilities indicate several hours of 0.5"/hr snowfall tonight. Antecedent conditions are unfavorable for accumulations due to prior warm temperatures and rain, and drying in the upper half of the DGZ during CAA will erode at least some of the potential. Still, WPC probabilities are moderate for 4 inches in northeast NC, with lower amounts possible all the way to the coast which will see a later changeover before the precipitation ends. ...Southern Great Basin... Days 2-3... An upper low will lift onshore Southern California with its associated surface low moving into the southern Great Basin beneath it. Ahead of this system, deep and robust moist advection will spread northward on low-level southerly flow out of the Pacific and into the Four Corners region. PWAT anomalies become quite impressive during this time, reaching as high as +5 standard deviations above the climatological mean. While snow levels will be high on the tropical moisture plume, and synoptic forcing looks to be progressive, the extreme moisture will wring out as heavy snow, especially in the mountains of the Mogollon Rim, Wasatch, San Juans, Sangre De Cristos, and Colorado Rockies. WPC probabilities for 4 inches on day 2 are moderate, but ramp up and feature a high risk for 8 inches in many of these ranges on day 3. WPC snowfall is on the high end of the guidance envelope at this time, but the extremely anomalous moisture and potential for persistent upslope low-level flow aiding the synoptic ascent supports heavy snow accumulations in these ranges. ...Pacific Northwest... Day 3... Pacific jet streak will advect towards the Washington State coast Sunday as a potent mid-level shortwave drops southeastward from the Gulf of Alaska. Confluent mid-level flow, height falls, and LFQ jet level diffluence will provide ascent as column moisture increased as noted by PWAT anomalies of +0.5 standard deviations reaching the coast. Snow levels will be moderately high, so the heavy snow will be confined to the Olympics and Cascades, with WPC probabilities featuring a high risk for 6 inches from Mt. Rainier and points north. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Weiss