Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 431 AM EDT Tue Apr 2 2024 Valid 12Z Tue Apr 02 2024 - 12Z Fri Apr 05 2024 ... Great Lakes into the Northeast... Days 1-3... ...Significant late season winter storm this week... A well-defined southern stream shortwave continues to lift northeast across the central Plains this morning. This system is expected to continue tracking northeast into the mid-to-upper Mississippi Valley and western Great Lakes region, where it will begin to interact with an equally defined northern stream trough diving out of central Canada. As the two streams phase, models show an upper low rapidly developing over the upper Mississippi Valley and western Great Lakes this afternoon and evening. Strong upper forcing overlapping low level frontogenesis will support rain quickly changing to heavy snow on the backside of the associated surface low as it tracks out of the mid Mississippi Valley toward the Great Lakes this afternoon and evening. HREF guidance shows heavy banded snow with rates of 1-2 inch/hour developing across southern Wisconsin by the late afternoon, before gradually expanding north into Upper Michigan during the evening into the overnight hours. Following a significant western shift in the guidance from the 00Z to the 12Z runs earlier today, the latest 00Z guidance has shown overall better run-to-run continuity. However, some models, including the GFS, continue trend west, raising probabilities for heavy snow further west across Wisconsin. Snow will continue to fall across the region through Wednesday, however overall rates are expected to diminish as the low begins to drop back to the southeast. A primary exception will likely be the central to western U.P., which will be the focus for strong easterly flow off of Lake Superior as the low meanders over southern Wisconsin on Wednesday before drifting southeast toward the Ohio Valley. WPC PWPF probabilities for snow accumulations of 8 inches or more are above 70 percent from southern Wisconsin to the U.P. The highest probabilities are centered over the central U.P., where the PWPF even shows some 50 percent or greater probabilities for 2 feet or more before the snow ends on Thursday. Meanwhile, additional southern stream energy phasing with the low will support the development of a triple-point low that will start to become the primary surface feature as it tracks from the Mid Atlantic to Long Island and southern New England late Wednesday into early Thursday. Precipitation will begin to spread across eastern New York into New England on Wednesday, with mixed precipitation at the onset for much of the interior. Low-to-mid level frontogenesis along with divergence aloft will support stronger ascent and a transition to snow across northern New York and interior central to northern New England. It is unclear how much an intrusion of dry air and warm air aloft may impact amounts. Probabilities for heavy snow have retreated a bit further to the north with latest run across from the Catskills eastward into central Massachusetts, southern Vermont and New Hampshire. Relatively lighter QPF and the potential for a more prolonged period of sleet are contributing to the lower probabilities. Heavy, wet snow still appears likely for at least portions of the Adirondack, Green, and White mountains. WPC PWPF shows high probabilities for 8 inches or more extending from these areas into central Maine. Snow will continue across the region into Friday, however rates should begin to diminish by late Thursday as the low lingers but weakens over New England. ...Western U.S.... Days 2-3... An amplified upper trough associated with a low dropping south from the Gulf of Alaska will begin to dig into the northwestern U.S. on Wednesday. Backing flow ahead of the advancing trough will support deepening moisture and the increasing potential for snow along a low-to-mid level front extending northeastward from the southern Cascades on Wednesday. While widespread heavy accumulations are not expected, locally heavy amounts are possible from the southern Cascades to the Blue Mountains late Wednesday into early Thursday. By early Thursday, models show a deep upper low centered along the Northwest coast that will then dig further south into northern California by early Friday. This will bring heavy snows into the Sierra Nevada and parts of central and northern Nevada. Meanwhile, energy lifting east of the low and a lingering frontal boundary will support additional heavy amounts across northeastern Oregon. For portions of the Sierra Nevada, southern Cascades, and the Blue Mountains, WPC PWPF shows high probabilities for 8 inches or more. Pereira *** Key Messages for Early April Nor'easter *** ---Long duration winter storm A large storm system will produce gusty winds and late-season heavy snow across portions of the Great Lakes and the Northeast beginning later today and continuing through midweek. ---Western Great Lakes heavy snow Tue night Heavy snow will likely develop over Wisconsin by this afternoon and expand into Upper Michigan overnight. Snow will continue over much of the region through Wednesday, with additional heavy snow across portions of Upper Michigan, before ending on Thursday. ---Northeast snow and wind Wednesday-Friday Secondary low pressure development along the Mid-Atlantic coast will likely bring heavy, wet snow and some sleet to the Northeast Wednesday afternoon through Friday. Significant snow accumulations are likely over parts of northern New York and New England. ---Significant impacts from heavy snow and wind The combination of heavy snow rates and gusty winds will likely result in hazardous travel due to low visibility and snow-covered roads. The wet snow and high snow load may cause tree damage and impact infrastructure.