Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 332 PM EST Sun Feb 23 2020 Valid 00Z Mon Feb 24 2020 - 00Z Thu Feb 27 2020 ...Pacific Northwest/Northern Rockies/Northern Plains... Days 1-2... A 500mb trough digging into Washington State will continue to dive southeast into the inter-mountain West and then strengthen into a deep closed low over the Northern Plains by Tuesday. This feature will be accompanied by a 130kt Pacific Jet streak racing southeastward, and the combination of LFQ diffluence and height falls/PVA will produce ascent across the terrain from the Olympics/Cascades/Northern Rockies on D1, through the central Rockies and parts of the Northern Plains on D2. In the mountainous regions, the large scale synoptic ascent will tap increased moisture on 700mb moist advection to produce heavy snow. WPC probabilities are high for 8 inches in the terrain on days 1 and 2. Late on D1 into D2, a low-level front will develop providing some enhanced frontogenesis from MT into the Dakotas. This will cause a a stripe of moderate to heavy snow, and WPC probabilities are 20-30% for 4 inches from eastern MT through the central Dakotas. ...Upper Midwest/Great Lakes... Days 2-3... Binary interaction of northern stream and southern stream impulses will lead to a deepening and slow moving low pressure system likely to bring heavy snow to the region. The lead shortwave, the southern stream energy, will eject out of the Southwest and towards the Ohio Valley through Tuesday morning. As this occurs it is likely to weaken. At the same time, a strengthening northern stream impulse will dig quickly into the Northern Plains while closing off and becoming the dominant upper feature. The stretching of the 700-500mb layers between these features will result in a swath of heavy snow due to deformation, which has the potential to pivot slowly, nearly in place, as the low gets captured before shifting to a triple point off to the east. At the same time, rich theta-e air is likely to rotate cyclonically into a robust TROWAL NW of the low-level center, and a swath of heavy snow with rates to 1"/hr or more is becoming more likely. Initially, the precipitation may be a mix of rain and snow, but as cold air digs into the column N/W of the low, the p-type will transition to all snow and become heavy on Tuesday. The guidance has featured a slow southward trend today, and while spread remains considerable, there has been at least a pinching of the envelope towards the ECMWF and GFS ensemble mean track. The WPC super-ensemble has shown a jump in its mean forecast snowfall across parts of the Upper Midwest today as well, and confidence has increased that significant snow accumulations will occur. The highest WPC probabilities for 6 inches on D2 are across eastern IA, shifting into N IL, S WI, and the northern U.P. of MI on D3. In these areas, snowfall may exceed 12 inches in a few locations, including on the SW lake shore of Lake Michigan where enhancement is possible due to LES from the northerly flow down the lake. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Weiss