Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 332 PM EST Sat Nov 24 2018 Valid 00Z Sun Nov 25 2018 - 00Z Wed Nov 28 2018 ...Central Plains to the Great Lakes... Days 1-3... ...Blizzard and Winter Storm Conditions Expected from the central Plains to the Great Lakes... The models show confluent flow that leads to a band of well defined mid level frontogenesis extending across the central Plains to the Mid MS Valley and Great Lakes. Mid level theta-e advection and convergence near the frontogenesis results in precipitation spreading east from WY across the central Plains and adjacent MS Valley tonight through Sunday. Snow is expected from the Nebraska Panhandle east across central into southeast Nebraska, becoming heavy as the magnitude of the low level convergence increases on Sunday across northeast KS and northwest MO across southeast IA and northwest IL. The precip type transition zone sets up across northern KS to central MO and northern IL, with rain changing to snow as the shortwave moves east, with colder air surging down the backside of the associated surface low. The models have converged towards having a low pressure center near southern IL Sun evening, moving northeast towards Lake Erie Mon morning and beyond. Continuing mid level frontogenesis northwest of the low track supports snow near southern Lake Michigan northeast to Lake Huron. Snow winds down as the low departs on Monday morning. The faster NAM has slightly lower amounts than the SREF and GEFS mean snowfall, so there are differences on the north-south axis of snow in MI on day 2. The 12z ECMWF trended faster on its 12z run, favoring slightly lower amounts than on the prior 00z run. In the system's wake, post-frontal cold advection on northwest flow favors lake enhanced and then lake effect snow showers in the lee of Lake Superior and in the lee of northern lake MI in northwest lower MI. The WPC preference was for a blend of the 12z High Res Ensemble Mean, Canadian Regional GEM/12z NAM QPF and temperature profiles on day 1 and the 12z GEFS Mean on day 2. The SREF Mean was further north with its QPF and snow axis on day 2 following a further north low track, and is considered to be a low probability, following its typical bias. ...Northeast... Day 3... Models show the initial low moving from near Lake Erie to Lake Ontario Monday evening, with a new triple point low developing near the mid Atlantic coast. As the low moves east northeast near the New England coast, low level frontogenesis and convergence supports snow in interior NH and ME, with potential for heavy snow in interior sections. The primary uncertainty is temperature profiles across southeast New England and coastal areas of NH and ME, where onshore flow provides enough warm air to have either predominantly rain or a mix of precip types. Light icing accumulations are possible inland in areas where low level subfreezing cold air is trapped in New England. The models cluster better than average for a day 3 forecast, although the operational ECMWF has wobbled with its low track from 00-12z. The ECMWF ensemble mean clusters well with the 12z GFS and UKMET low tracks. ...Pacific Northwest... Days 2-3... The NAM/SREF Mean/GFS Guidance continues to indicate appreciable ice amounts from the northwestern Columbia Basin region into the mountain valleys of the WA Cascades on Monday through Monday night. Models show the mid levels warming ahead of an approaching shortwave trough, while easterly winds help to maintain subfreezing boundary layer temps across valley and gap areas. WPC probabilities show a Low to Moderate Risk for ice accumulations of 0.25 or more across portions of this region. On later Monday night into Tuesday, heavier snows are possible in the WA Cascades as height falls occur and the 700-300 mb mb jet streams ashore, creating 700 mb convergence maxima in the WA Cascades that combines with continuing moisture advection and 300 mb divergence maxima to support snow in upslope areas. The jet extend south into the OR Blue Mountains on Tue, so several inches of snow may develop there as the operational 12z ECMWF develops 300 mb divergence over the mountains. Petersen