Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 AM EDT Mon Apr 18 2022 Valid 12Z Mon Apr 18 2022 - 12Z Thu Apr 21 2022 ...West Coast to the Northern Rockies... Days 1-3... A parade of storm systems will move into the Pacific Northwest over the next 3 days bringing unsettled and active weather to the region. The first system arrives today with a cold front sweeping through. Anomalous moisture combined with favorable forcing for ascent over the region will bring widespread precipitation during the day 1 period. Snow levels will rise with warm air advection which is expected to confine the greatest snowfall totals to the higher elevations of Olympics, OR/WA Cascades and the northern California mountains. This is where the WPC probabilities for 6 inches are more are high with a moderate signal for 8-12". Brief period of ridging comes for day 2 which will limit snow but is quickly followed by another storm system for day 3. This one is positioned a bit further south with the moisture axis expected to be focused on OR/northern CA. A bit warmer too - so the greatest snowfall will be confined to the highest tops of the southern OR Cascades and northern CA ranges where totals greater than 6" are likely. Some heavy snow will also spread southward across the northern sections of the Sierra. ...Northern MN... Day 3... Strong warm air advection and isentropic lift ahead of low pressure tracking across southern Canada will interact with a marginally supportive environment for wet snow across northern Minnesota Wednesday afternoon/evening. Enough moisture lifting north ahead of the system will allow for the potential for several inches along the North Shore where moist upslope will favor enhanced snowfall. The latest WPC probabilities are moderate (40-60 percent) for 4 inches (near 20 percent for 6") during the day 3 period. ...Eastern Great Lakes and Central Appalachians to the Northeast... Days 1-2... Water vapor imagery early this morning shows split flow with northern and southern stream shortwave energy approaching the Appalachians. Through Tuesday, these features phase, deepen, and take on a strong negative tilt over the eastern Great Lakes and Northeast. As a result, deepening low pressure is forecast to move up the Mid-Atlantic coast through southern New England into Canada. Compared to previous forecast cycles, the strong upper low continues to draw the surface low westward/inland and the 00Z NAM leads the model suite in that direction again though remains the furthest west at this time. As a result, strong warm air advection pulling northward across New England will make for a primarily all rain event. However for locations of PA, central and upstate NY - generally northwest of the low/mid level low - strong banding and frontogenesis supports heavy snow, particularly for the terrain areas above 1500-2000 ft where the impressively strong lift coincides somewhat with the DGZ. This is where the potential for intense snow rates exists with upwards of 2"+/hr snow rates at times as the low lifts through as suggested by the latest HREF. The upward trend in the QPF across the board led to a substantial increase in snow forecast and the latest WPC probabilities for 8" or more are now up to 70-90 percent across northern PA through central and upstate NY with a slight chance for totals exceeding a foot for the higher elevations of the Adirondacks. The probability of significant icing across the CONUS is less than 10 percent. Taylor