Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 455 AM EDT Thu Apr 4 2024 Valid 12Z Thu Apr 04 2024 - 12Z Sun Apr 07 2024 ...Northeast... Days 1-3... Over-amplified upper pattern in the eastern US will feature an ex- parent low over the central Great Lakes with a new coastal low just south of Long Island that will lift past Block Island to Cape Cod (reaching peak intensity/maturity this afternoon) then finally head into the Gulf of Maine this evening. Waves/bands of precipitation will continue to wrap around the low and into coastal New England where temperatures are quite marginal, especially along the coast, but dip below freezing with just a bit of elevation gain. The system will be driven by the parent upper low (still the dominant upper center) as it wobbles southeastward across the Mid-Atlantic later this afternoon, eventually catching up to the surface low in the Gulf of Maine early Friday. This will encourage a pause in the movement of the surface low and a cyclonic loop in the Gulf before slowly pulling away late Friday into Saturday. Precipitation will linger over the Northeast, especially in the terrain, through Saturday and Saturday night. The heaviest snowfall will be early in the period as the storm reaches peak intensity and height falls are strongest with the lead vort max beneath the LFQ of a 155kt upper jet. Strong easterly inflow will bring in milder air to the coast north of Boston, where mixing with rain is likely for the beaches and temperatures hover near and maybe just over 32F. However, just inland, winds will be from the northeast over land which will promote accumlating snow through the day. SLRs are likely to be low, especially during the day, except for within stronger bands of snowfall. Areas across interior Maine into northeastern New Hampshire have the highest probabilities of seeing an additional 12 inches of snowfall after 12Z today (>70% chance). The expansive circulation will yield snowfall back across central/western New York, but with accumulation modulated by surface temperatures, favoring areas along/south of I-90 from the Catskills west to the Chatauqua Ridge. In addition, northeastern NY into the Green Mountains have at least a 50% chance of an additional 8 inches of snow. Snowfall rates will diminish markedly by D2 as the system starts to fill and the strong dynamics move out to sea, but light snow will continue over especially northern New England. Even on D3, there are low to moderate (10-50%) probabilities of at least 4 inches of snow over northern Maine. ...Central Appalachians... Days 1-2... Parent upper low over Ohio this morning will rush eastward today and help to lower snow levels and increase precipitation across the central Appalachians, focused into eastern WV. This will result in accumlating snowfall above elevations of around 2000ft that will be most intense this afternoon/evening but persist through Friday with NW flow around the large circulation over the Northeast. WPC probabilities of at least 4 inches of snow D1-2 are highest (>50%) above 2000ft, but especially 3000ft. ...Western U.S.... Days 1-3... Balancing the deep upper low in the northeastern US will be another deep trough moving ashore the western CONUS this afternoon, with the jet stream dipping through SoCal and lifting northeastward through the Great Basin. The neutrally-tilted trough will become negatively-tilted by Saturday morning over the Rockies, carrying low pressure through the northern Great Basin and across the Divide where it will eventually reform/reorganize in the western High Plains. For D1-2, the upper jet will provide broad lift throughout much of the West, with lowering snow levels as the colder air moves in. In addition, combination of the strongest height falls and upslope flow will maximize snowfall over the Sierra, where more than a foot of snow is likely above about 5000ft. Farther north, generally lighter to locally modest amounts are likely for the Cascades and NorCal ranges eastward to the Blue Mountains and into Idaho (esp by D2). Cold temperatures aloft will penetrate as far south as SoCal into D2, where snow is likely in the Transverse Range (Big Bear Lake) southward through the San Jacinto mountains where WPC probabilities for at least 6 inches of snow are >50%. By D3, subsequent troughing will start to move into the PacNW with additional snowfall for the Cascades. Farther east, next phase of the interior troughing may be yet another closed low onto the High Plains, with surface low developing out of the lee of the Rockies. Upper jet will split over Wyoming, promoting lift over the region along an inverted trough axis into eastern Montana. Snowfall is especially favored over the Bighorns and into the Laramie/Shirley Mountains. WPC probabilities for at least 8 inches of snow are high (>70%) over these ranges but also extending northward through central Montana and southward into northern CO with lower values (moderate, 40-70%). By the end of the period, precipitation will spread eastward into the Plains, but snowfall will be confined to the Black Hills initially due to warmer antecendent temperatures. Fracasso ***Key Messages for Early April Nor'easter*** ---Heavy snow continues Heavy, wet snow and areas of sleet will continue over northern/central New England and northeastern New York. Moderate to heavy snowfall will gradually decrease in intensity overnight tonight, but light snow will continue throug Friday over northern New England. Total snowfall accumulations of 1-2 feet are likely for much of northern New England and the northeast part of the Adirondacks. ---Heavy snow and wind impacts Heavy snow and gusty to damaging winds will result in dangerous travel with whiteout conditions possible on snow-covered roads this morning. The combination of wet snow, a high snow load, and the strong wind gusts will cause some tree damage and power outages. ---Moderate coastal flooding this morning Prolonged onshore winds will result in moderate coastal flooding for parts of the southern New England coast. Impacts include widespread roadway flooding, coastal and bayside flooding, impassable roads, and some damage to vulnerable structures.