Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 436 PM EDT Wed Apr 21 2021 Valid 00Z Thu Apr 22 2021 - 00Z Sun Apr 25 2021 ...Great Lakes/Northeast... Days 1-2... Low pressure currently between the eastern Great Lakes and Long Island will continue to allow for snow to lift across the interior Northeast with most synoptically forced snow occurring tonight from the Northeast Kingdom of VT up through far western/northern Maine in the low-to-mid level frontogenesis maxima west of the deepening surface low. Locally heavy snow is possible this evening in the northern Green and White Mountains and then the ranges of western Maine later tonight into Thursday. Lake effect snow follows the system as northwest winds cross Lake Ontario with less shore convergence leading to snow showers from Rochester to Syracuse and along the southeast shore of the lake. Day 1 snow probabilities are moderate for 2 or more inches southeast of Syracuse. Low pressure exits Maine on Thursday, but it stalls over New Brunswick through Thursday night with continuing mid level deformation/frontogenesis supporting ascent and potential for several more inches of snow in northwest Maine. Overall, the heaviest additional snowfall totals are expected to occur north from far northern VT/NH through far western/northern Maine where Day 1 snow probabilities for 6 or more inches are moderately high. ...Northern Rockies and Northern High Plains... Days 1/2... A shortwave trough and associated cold front rounding a sprawling cold core centered over Nunavut pushes southeast from the Canadian Rockies and over MT/ID Thursday. Post-frontal northeast winds turn upslope for eastern slopes of the front range/Glacier National Park south to the northern Absaroka Mountains of MT. Several inches of snow are expected in favored windward terrain, with moderate Day 1.5 probabilities of 6 or more inches forecast. Expect slightly weaker frontogenesis maxima and more progressive 700 mb convergence maxima and less moisture farther west, so a sharp gradient in snow is forecast with little for the Bitterroots along the MT/ID border and the Clearwater Mountains of ID. Thursday night into Friday, the weak front progresses steadily across the Bighorn Mountains or WY, where enhanced relative humidity near 90 percent can be expected Friday. Lift is provide from the cold front and the subsequent upslope flow in windward terrain. Day 2 snow probabilities are moderate for 4 inches for the Bighorns with warmer conditions over the Plains. ...Pacific Northwest... Day 3... Low pressure currently just south of the Aleutians will eject southeast Thursday night with an associated cold front approaching the WA/OR coast on Saturday. Onshore flow and Pacific moisture reaching the coast Friday night as a ridge that had been off the coast pushes inland. PWs around 0.75" and upper level lift from the left exit region of a jet streak pushing inland over the OR/CA border promotes some moderate precip rates with snow levels around 4000ft for the Olympics and WA/OR Cascades through Saturday. Day 3 snow probabilities for 4 or more inches are low and limited to the higher Olympics and WA Cascades. A blocked pattern causes the low to stall with precip persisting over western WA/OR and expanding south over northern CA through Sunday. For Days 1-3, the probability of significant icing (0.25 inch or more) is less than 5 percent. Jackson