Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 PM EST Fri Dec 27 2019 Valid 00Z Sat Dec 28 2019 - 00Z Tue Dec 31 2019 ...Southwest U.S.... Day 1... Increasingly vertically stacked low comprising the southern end of a full latitude trough will begin to close off east of the Four Corners, leading to lee cyclogenesis in the CO High Plains Saturday. Strong forcing due to height falls and PVA combined with robust 700mb warm/moist advection will spread heavy snowfall along the Mogollon Rim, Sacramento Mountains, and into the Sangre De Cristos/San Juans/Colorado Rockies. Snow levels will range from near 5000 ft near the Mexico international border, to as low as the surface in northern CO. The heavy snow will be above these levels, and the highest snowfall is likely in the eastern Mogollon Rim, and the mountains of northern NM and much of CO where WPC probabilities are high for 8 inches, and many places may exceed 12 inches of snowfall. ...Central and Northern Great Plains... 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 early Saturday morning from the Colorado High Plains and then lift northeast into Minnesota Sunday morning. As this occurs, a full latitude upper trough will move eastward, gradually tilting negatively and closing off, causing the sfc low to stall over MN while it occludes into Monday. This will spawn secondary low pressure into the Great Lakes Monday along the occluded front to the east. This low will intensify rapidly beneath the height falls and coupled 150+ kt jet streaks. At the same time, an open Gulf of Mexico will allow anomalously high PW to be drawn northward, and IVT reaches 2 standard deviations above the climo mean into the Northern Plains. Along and east of the low track, p-type will be rain or a mix, while points northwest will be a mix and snow. As this moisture wraps cyclonically around the low, a pronounced TROWAL is progged to develop driving the rich theta-e air into the Dakotas and Nebraska. Where this TROWAL overlaps with the best deformation, a swath of heavy snow is likely. While fgen is forecast to be modest in this event, limiting the duration and intensity of 1"/hr+ snow rates, prolonged moderate snowfall is likely, accentuated within the TROWAL/deformation/comma overlap. This is most likely from far northern NE through eastern SD and into eastern ND. Here, WPC probabilities for 12 inches are moderate, highest on D2 (00z Sunday - 00Z Monday). Surrounding this heaviest snow region, a large swath of moderate to heavy snow is likely from the High Plains of CO/WY, northeast through Minnesota and Wisconsin. WPC probabilities are high for 4 inches here. A precip type transition zone will occur along the 850 mb low track, resulting in freezing rain accumulations given the cooling and drying antecedent conditions. The day 1 axis of freezing rain extends from southwest to central NE, southeast SD, and to northwest IA/southern MN, possibly pushing into WI on D2. WPC probabilities have increased for significant accretions of freezing rain, and although the highest chance for 0.25" is along the Buffalo Ridge, anywhere in this stripe has a low to moderate chance for 0.1" to 0.25". ...Northeast... Day 3... A warm front lifting ahead of a triple point in the Ohio Valley will spread precipitation on WAA into New England. This precipitation will initially overspread an airmass that is below freezing due to cold High Pressure over SE Canada. However, strong WAA will gradually push a warm nose northward into central and northern New England, transitioning p-type from snow, to freezing rain, and eventually rain far SW portions. A few inches of snow are possible before changeover, with the highest snowfall likely in NH/ME where WPC probabilities are low for 4 inches. Further SW into the terrain of the Adirondacks, Catskills, Berkshires, and Worcester Hills, a more pronounced freezing rain event is becoming more likely. Some guidance is quite aggressive with accretions, however, heavy rain rates and increasing mid-level divergence to displace the confluence to the northeast should limit accretions. However, WPC probabilities have increased for 0/25", and feature a moderate risk in the terrain of MA, VT, and eastern NY. There remains considerable spread in the forecast for this event, and this could end up more as rain, or more as snow, depending on the track of the front and developing low along it. ...Pacific Northwest to California... Day 2-3... A progressive upper trough with weakening surface low pressure moves from the northeast Pacific towards the northwest US Sunday. The surge in moisture advection and lift crosses the Pacific northwest early Sunday, continuing downstream into northwest California as the day progresses, and then across central CA Sunday night. At the same time, a 700mb closed low will dig down the coast of CA Monday, driving a weak surface low southward beneath it. The combination of this forcing and moisture, along with snow levels lowering to 3000-4000 ft, will bring a round of heavy snowfall along the Cascades of WA/OR, and down through the Siskiyous and Sierra of CA. by the end of the period, snow is likely even into the transverse ranges of southern CA. WPC probabilities for 6 inches are high in the Sierra and southern transverse ranges, and 6-12" of snowfall is possible. Elsewhere, WPC probabilities suggest snowfall will be generally less than 6 inches. Higher elevations along this trajectory are expected to receive several inches of snow, including the OR Cascades, CA Siskiyous, and northern to central CA Sierra. The lack of a long duration snow limits the event potential, . Weiss