Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 419 AM EST Sat Dec 28 2019 Valid 12Z Sat Dec 28 2019 - 12Z Tue Dec 31 2019 ...Central and Northern Great Plains/Upper Great Lakes... Days 1-3... ...Major winter storm forecast to produce widespread heavy snow and light to moderate icing across portions of the northern Plains... Lee cyclogenesis will emerge this morning from the Colorado High Plains and then lift northeast into Minnesota Sunday morning, then staling over southwest MN into Monday. This low will intensify in the left exit region of a forecast 150 kt jet streak. Enhanced 300 mb divergence persisting across central NE to SD today combines with low-mid level deformation to produce moderate to locally heavy snow. Along the sfc-850 mb low track, p-type will be a mix of snow, sleet, freezing rain, and rain. The best odds for significant icing are in southwest MN and southeast SD, where lift occurs over existing below freezing temperatures. The mid level warm advection and possible downward mixing of warmer air may limit ice accumulation potential, though. On Sunday, the stalled low over southwest MN leaves a persistent axis of low-mid level deformation and upper divergence maxima from central SD and into eastern ND. This axis should receive several additional inches of snow. WPC probabilities for 12 inches over the total event are moderate, across much of central SD and adjacent north central NE and southeastern ND. On Day 3, the upper level wave rotates east of the upper low from the mid MS Valley towards the upper Great Lakes. New low pressure develops and moves northwest across MI. Low level convergence along the low track should result in snow developing in northern WI and the UP of MI. Several inches are expected, but the progressive low limits potential for a prolonged heavy snow event. ...Northeast... Days 2/3... A reverse cold air damming episode is expected as high pressure builds from eastern Quebec south across eastern New England. Mid level warm advection over the top of the building low level old air results in a mixed phase precip event with freezing rain developing over portions of the NY Catskills and Adirondacks into the Berkshires Sunday. Low pressure develops over the mid Atlantic Monday and moves off the coast. Low-mid level frontogenesis over southern NY and southern New England leads to a prolonged precipitation event. Deepening cold air leads to the possibility of precip changing over to snow in southern Maine to southern NH/VT and adjacent northern MA. Freezing rain could mix or change to sleet over portions of eastern NY into western Ma and northern CT. The greatest overlap of significant icing among the 00z NAM/Canadian Global/ECMWF/ECMWF ensembles is over the Catskills of NY and and Berkshires of MA. ...Pacific Northwest across California... Day 2-3... A progressive upper trough moves southeast on day 2 from the northeast Pacific southeast across central CA Sunday night. Upper divergence maxima are forecast along and in advance of the upper trough. The combination of this forcing and moisture will bring a period of snowfall along the Cascades of OR, and down through the Siskiyous and Sierra Nevada mountains of CA. Snow is likely into the transverse ranges of southern CA on Monday as the upper trough passes through. Several inches of snow are expected in each of the CA ranges. The lack of a long duration snow limits the event potential. Petersen