Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sat Apr 25 2020 Valid 12Z Sat Apr 25 2020 - 12Z Tue Apr 28 2020 ...Northeast... Days 2-3... A Miller-B type low pressure is set to bring a period of wintry weather to upstate New York and New England Sunday and Monday. The synoptic setup responsible for this late season event involves the interaction of a southern stream and northern stream shortwave, as well as increasingly coupled jet streaks. For the former, a shortwave diving out of the Midwest will rotate within the broad cyclonic flow across the northeast while weakening, allowing a secondary impulse following quickly behind to become dominant and strengthen. This second wave is progged to interact with northern stream energy to produce a negatively tilting trough, which should close off over New England on Monday. As this phasing occurs, jet streak coupling will intensify as well, leading to enhanced ascent and a strengthening (albeit moderately) surface low pressure moving northeast from the Tidewater of VA to the Gulf of ME. On D3, this low may stall in the Gulf of ME as secondary closing of the upper low will capture it and cause it to drift before finally ejecting to the east late D3 into D4. Although synoptic forcing will be sufficient for ascent, the lift due to mesoscale features may become intense. Robust WAA ahead of this low will spread precipitation northeastward from PA into New England Sunday. This precipitation should be entirely rainfall outside of the highest terrain during the daylight hours Sunday owing to marginal thermal profiles. However, as the low pivots to the east, frontogenesis intensifies, especially from near the Catskills and along a swath to the E/NE towards coastal ME. This fgen will occur concurrently with strengthening mid-level deformation pivoting across the region, and mid-level omega becomes maximized in a region of high DGZ RH. This, on top of ageostrophic cooling from a favorably placed Canadian high pressure, suggests p-type will transition from rain to snow Sunday night in all but the lowest terrain and parts of Southern New England. While the heaviest accumulations should be confined to the terrain above 1500 ft, where the most intense ascent occurs, dynamic cooling should bring light accums all the way to the valleys, and even along the coast of Maine. WPC probabilities for 6 inches are low to moderate, and confined to the southern Adirondacks, the southern Greens, and much of the Whites. However, probabilities for 2 inches are moderate to the coast of Maine, and in the Berkshires, Worcester Hills, and Catskills as well. As the low pivots nearly in place Monday, wraparound snow due to continued deformation, continued cold air drainage, and modest upslope ascent should produce additional light snow. As temperatures warm in the late April sun during the aftn, snow should struggle to accumulate and may change back to rain before ending late D3. However, guidance does indicate a period of enhanced fgen collocated with the leading edge of some colder air /fgen/ which could produce a burst of moderate snow anywhere across the region. WPC probabilities for 2 inches on D3 are generally confined to the higher terrain of ME, but will need to watch for this pivoting band for any additional light accums across the area. ...Pacific Northwest... Days 1-3... A series of troughs will swing south of a gyre over the Gulf of Alaska, bringing rounds of mostly light snow to the highest terrain of the Washington Cascades, and stretching into the Northern Rockies by D3. The most significant of these impulses is likely to cross Monday, with ascent through PVA/height falls and better LFQ diffluence tapping some enhanced moisture thanks to mid-level confluent flow from the Pacific. D2.5-D3, WPC probabilities for 4 inches are high in the WA Cascades, with low probabilities extending into the terrain around Glacier NP. Otherwise, 4" probabilities are generally moderate and only in the northernmost reaches of the Cascades. For Days 1-3, the probability of significant icing (0.25-inch or greater) is less than 10 percent. Weiss