Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 315 AM EST Wed Jan 30 2019 Valid 12Z Wed Jan 30 2019 - 12Z Sat Feb 02 2019 ...Great Lakes Days 1-3... The winds turn cyclonincally across the lakes today with boundary layer northwest winds off Lake Superior becoming west south across lower Lake Michigan and Lake Erie to more westerly across lake Ontario. Persistent lee shore convergence in the eastern UP of MI leads to the highest snow totals on the day 1 forecast in the upper lakes, with secondary maxima in northwest lower MI. The models show drier air aloft crossing southern Lake MI so amounts will eventually taper there. The highest snow probabilities will be east off of Lake Ontario due to the sustained fetch along the main axis of the lake with saturation. Lee shore convergence and high moisture content combined with upslope enhancement in the Tug Hill will lead to heavy snow amounts, with a foot of snow likely in the heavier bands. On day 2, The highest snow probabilities will be east off of Lake Ontario due to the continued sustained fetch along the main axis of the lake with strong near shore convergence and upslope flow into the Tug Hill. This, combined with upslope enhancement in the Tug Hill will lead to heavy snow amounts, with a band of another foot a strong possibility, as shown in the NAM Conus Nest output. The continued switch to west winds allow the upper lakes to have less coverage/intensity of snow showers, responding to drier air aloft. Light snows may develop along an inverted trough extending north from the Ohio Valley to the southern shore of Lake MI Thu night to early Fri morning on both the IL side and southwest lower MI side of the lake. ...California Days 2-3 and the Pacific Northwest Day 3... Models are in fairly good agreement with a compact upper low dropping southeast from the eastern Pacific into Southern California is expected to produce some locally heavy, high elevation snows along the higher elevations of the mountains of southern CA on Thursday. Several inches of snow are possible. The threat ends with the 700 mb wave moving out of CA into the southwest, with light snow possible in the higher elevations of southern NV, UT, and AZ as the wave approaches Fri. On Fri, attention turns to the next approaching upper trough Friday night to early Sat. The 18-00z GFS/ECMWF/NAM show a closed low approaching the CA coast Sat, preceded by an enhanced plume of moisture moving onshore and inland. The moisture is propelled by a 300 mb jet maxima moving onshore in inland, with well defined 300 mb divergence combining with the low level convergence and upslope flow to drive heavy snow in the higher terrain of the Shasta/Siskiyous, and Ca Sierra Nevada mountains. The increasing model agreement has led to increasing probabilities of heavy snow in the mountains of northern to central CA. Further north, the northern stream portion of the trough moves onshore from the Pacific into WA State, preceded by deep layer southwest flow that produces deep layer moisture. Lift is focused in the 850-700 mb layer, aided by divergence maxima in the right entrance region of an upper jet forecast to cross Vancouver Island and southern British Columbia. This will likely result in snow in higher elevations of the WA Olympics and Cascades. The long duration of moisture and lift leads to enhanced probabilities of 4 and 8 inches of snow Fri night to early Sat. ...Mid MS Valley to the Ohio Valley Day 2 and Central Appalachians to mid Atlantic Day 3... Several models show a mid level impulse rounding the upper level trough moving from the mid-MS Valley across the Ohio Valley then off the Mid-Atlantic coast for Day 3. The mid level confluent flow results in a 700 mb jet moving west to east across the Ohio Valley, along with a swath of warm/moist advection. High 700mb relative humility combined with sufficient vertical velocity from modest frontogenesis leads to widespread light snow across the Ohio Valley. At the end of day 2 through early day 3 (Fri), the frontal lift gets augmented by orographic lift in the terrain of the central Appalachians. The models indicate potential for several inches of snow across upslope regions of WV. A relative minima of ascent occurs in the lee few the central Appalachians in the mid Atlantic states, so amounts are lower than in the mountains. ...Maine Day 1... As the surface low continues to lift north through northern New England Wednesday morning, the upper level trough will become negatively tilted with mid-level vorticity supporting significant snow amounts across interior Maine, where several additional inches are possible. Given the inland track of the low, this might aid in a transition of snow to rain across coastal Maine reducing overall snowfall amounts. The wintry precipitation should diminish as low pressure departs north out of the state Wed evening. The probability for significant icing is less than 10 percent nationwide days 1-3. Petersen