Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 420 AM EST Sun Feb 24 2019 Valid 12Z Sun Feb 24 2019 - 12Z Wed Feb 27 2019 ...Pacific Northwest into the Northern Rockies... Days 1-3... Heavy snows are expected the next few days in the mountains of OR into southern and central ID, southwest MT, northwest WY, and northern to central CA. Multiple feet of snow are expected in favored windward terrain and long duration snow are expected. Highest amounts for the 3 day period are likely over the OR/CA Cascades and the Shasta and Trinity Ranges of northwest CA, where 3 to 5 feet of snow are possible over the multi-day period. On Day 1, confluent flow through a deep layer leads to an enhanced jet stream, including a 50 kt maxima streaming onshore from the northeast Pacific into western OR. The slow movement of the upper jet mean upper divergence maxima from OR inland across southern ID and southwest MT and northwest WY should have prolonged bouts of snow. Ascent is aided with low level frontogenesis and 700 mb convergence as well. Enhancements are also due to upslope flow in windward terrain. On Day 2, the amplifying upper trough off the Pacific northwest coast send a low level and surface wave onshore and east across OR along the frontal zone. Persistent coupled upper divergence/lower convergence leads to a likely period of heavy snow in the southern OR Cascades to the Shasta/Trinity ranges of northwest CA. The 300 mb jet max extending inland across northern ID and MT places central ID and western MT in the favored right entrance jet region for sustained lift and likely heavy snow in windward terrain. The jet drifting south into northern CA allows heavy snows to extend south into the northern CA Sierra. With the snow levels north of the low level front falling to sea level, light to moderate snow accumulations are possible in the Portland OR metro area on Monday as WPC probabilities are moderate for 4 inches. Uncertainty continues as the models/means have differing timing wit the surface wave, which modifies the snow duration and resultant amounts across the valleys of northwest OR and southwest WA. On Day 3, the 300 mb jet crossing OR into central ID and MT places the favored right entrance region divergence and resultant lift and heavy snow to the south across the northern CA Sierra and Shastas/CA to southern OR Cascades inland to the Boise Mountains and Tetons. An additional couple of feet of snow are possible in these ranges due to long duration moisture fluxes coming onshore and inland across northern Ca/southern OR into southern ID and northwest WY/southwest MT. Like on day 2, the timing of low level waves and surges of lift and snow differ. ...Great Lakes... Day 1-2... NW of the surface low crossing northeast from northern Lake Michigan northeast across the UP of MI into Ontario, a stripe of heavy snowfall is expected. Early today, synoptic snow within the 700mb deformation/convergence axis continues following recent trends of snow spreading into the western UP of MI. After the low passes northeast from northern Lake Michigan into Ontario, the synoptic snow decreases and northwest flow across western Lake Superior results in strong lee shore convergence underneath a band of mid level deformation. With high relative humidity and lift, several inches of snow should occur until starting to taper Sunday night once drier air aloft moves in from the northwest and the lee shore convergence weakens. A secondary max is expected over northwest lower MI in the lee of Lake MI. ...New England and New York... Day 1... Warm moist advection on increasing southerly winds will spread precipitation across the Northeast ahead of a deep low pressure moving through the upper Great Lakes into Ontario. During this time, the atmospheric column will be cold enough for snow only across northern New England, with the warming aloft atop a cold air wedge of high pressure producing mixed precip in interior portions of southern New England. Snowfall should be heaviest across interior Maine, where WPC probabilities are high for 8 inches, with lower probabilities for 8 inches into the terrain of NH. Elsewhere, snow amounts will be light. South of the snow/mix line, freezing rain accretion is likely in the Berkshires and Greens, and White Mountains. WPC probabilities are over 50% for 0.1 inches, with low probabilities also existing for 0.25 inches. In New York in the lee of Lake Ontario, the cold frontal passage this evening is followed by winds veering to the west. This produces a long cross Lake Ontario fetch, with downstream upslope flow into the Tug Hill and adjacent western Adirondacks. Locally heavy snow is expected tonight over this area with several inches likely. The snow should wind down Monday as drier air aloft moves in from the west. The probability of a quarter inch of freezing rain is less than 10 percent days 2-3. Petersen