Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 407 PM EST Thu Jan 16 2020 Valid 00Z Fri Jan 17 2020 - 00Z Mon Jan 20 2020 Day 1... ...Southern High Plains to the Central Plains and Lower Missouri Valley... ...Significant icing expected from western OK across Eastern KS and central to northern MO... Deep layer southwesterly flow is forecast to increase from the southern High Plains into central Plains tonight into early Friday. Mid level energy and favorable upper jet forcing interacting with moisture transport will support precipitation spreading northeast across the Plains into the Missouri Valley and mid-upper MS Valley. Temperatures aloft rise above freezing while surface subfreezing cold air persists, supporting freezing rain with accumulating ice likely from the Texas Panhandle to central/eastern Kansas and southern to northeast MO. The uncertainty revolves around accretion rates being cut back due to moderate rainfall rates from the TX panhandle across western OK and central KS. While event totals up to a quarter inch are forecast, locally higher amounts may occur if the forecast runoff is overdone, as ice to liquid ratios are forecast to be 0.4-0.5 from the freezing rain accumulation model in the region of moderate rain rates. Further north, in the deeper cold air snow develops with 4-6 inches expected in this period in far southeast SD to northern IA and southern MN. ...Western U.S.... A 700 mb trough comes onshore across the Pacific northwest to California. The higher snow amounts tonight are expected in the Sierra Nevada mountains in CA as the peak vertical motions occur over the terrain this evening, that then subside and end Fri as the trough moves further inland. Light amounts are expected downstream in the ranges of ID, western MT, western WY and northern NV as the trough progresses across the region with a brief period of ascent. The 12z ECMWF shows a secondary peak of 700 mb ascent in the ranges of northern UT, so enhanced amounts are forecast in the northern Wasatch and western Uintas. Another secondary maxima is expected over the mountains of northern NM and southwest CO as deep layer moist southwest flow persists with lift tonight, with a lull period near tomorrow morning. Friday afternoon, another period of snow showers in expected in the pre-frontal low level convergence zone crossing southwest CO. The event winds down late Fri as the upper trough passes and drier air aloft advects across the region. Day 2... ...Midwest/eastern Northern Plains/Upper MS Valley/Great Lakes/central Appalachians... Strong mid level warm advection and moisture transport combine with 850-700 mb convergence to produce widespread precipitation moving northeast from the mid Mississippi valley and across the northern Oh Valley and Great Lakes. Precipitation is expected to be mostly snow in WI and central and northern MI and points north. Further south in IA/IL/IN to the MI border and northern OH/northwest PA, precip begins as snow before changing over to a wintry mix as the air column warms aloft. The heaviest snow accumulations are expected to center across the Minnesota arrowhead and in west central lower MI, where the synoptic event is followed by a cold frontal passage and then lake enhanced snow where cross Lake MI trajectories pick up fluxes and moisture and heat to aid in development of instability, with snow showers developing in the lee shore areas of western lower MI. The primary day 2 icing is forecast from northeast MO to west central IL and becomes transitional and lighter across the northern Ohio Valley and midwest. A secondary max is possible in mountainous areas and valleys of the central Appalachians of eastern WV to adjacent northwest VA as the mid level warm air raises 850-700 mb temps above freezing. ...Ranges of Western WA/OR... A period of heavy snow is expected across the WA/northern OR Cascades as a closed low offshore drifts east, with a band of enhanced 700 mb moisture convergence and loft moving onshore in WA/OR and across the Cascades Fri night into Sat. The band progresses inland, although the strength of the lift weakens as the cyclone decays. Consequently, lighter amounts are forecast inland across the ranges of northeast WA, northern ID, northeast OR, and northwest MT. The band of confluent flow continues into Sat night across the northern WA Cascades with corresponding lift, so additional light snow is possible into early day 3 in the northern WA Cascades Sat night, but most guidance has stronger ascent and resultant heavier snow amounts north in Vancouver Island/British Columbia. Day 3... ...Great Lakes/Northeast/Mid Atlantic... The 700 mb wave crosses New York and New England, preceded by 850-700 mb warm advection and moisture transport. Low level convergence supports ascent, which may potentially increase a bit over Maine as a coastal low develops near Downeast Maine Sunday morning. This has the potential to produce a stripe of several inches of snow across northern New York and New England, with enhancements in terrain. Strong westerly flow will support lake effect snow showers east of Lake Ontario, with heavy accumulations possible across the Tug Hill region. To the south, snow changing to a wintry mix is expected to produce some light snow and sleet accumulations from southern New York and southern New England Saturday night and then the event ends. Lake effect snow develops with heavy amounts expected in leeshore convergence areas in the Keweenaw peninsula of MI and in upslope areas of northwest lower MI downstream from traverse Bay MI, where longer cross Lake MI trajectories persist in the low level cold advection. Drier air advecting east from the upper MS Valley may lead to tapering of the event late Sun in the UP of MI. The probability of 0.25 inches or greater icing is less than 10 percent Day 3. Petersen