Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 343 AM EST Wed Feb 27 2019 Valid 12Z Wed Feb 27 2019 - 12Z Sat Mar 02 2019 ...Pacific Northwest/Mountains of Northern-Central California to the Northern Rockies/Northern Plains... On day 1 (Wed), models show a well-defined shortwave moving inland into southern Oregon and northern California Wednesday morning. This will bring a fresh surge of deeper moisture into a quasi-stationary baroclinic zone extending from near the Oregon-California border through the northern Rockies. This will support additional heavy snows from northern California eastward into the northern Rockies through Tuesday night, with snow spreading farther south along the Sierra as the upper trough moves inland Wednesday morning. WPC Day 1 probabilities (ending 00Z Thu) show a High Risk for accumulations of a foot or more for portions of the California Cascades and northern Sierra. WPC probabilities also continue to indicate a High Risk for a foot or more over the Sawtooth of central Idaho. The upper low along the Northwest coast is expected to produce enhanced moisture and lift into the ranges of the Pacific northwest, with several inches of snow expected in the WA Cascades. On Day 2 (Thu), the upper trough off the Pacific northwest coast moves onshore and begins moving inland, with additional heavy snows possible ahead of the associated wave pushing east from the Pacific Northwest to the northern Rockies and along the low level front in the area. A 700 mb wave peels off and moves east out of the Rockies on to the northern Plains of MT. 700 mb convergence, along with increasing low level frontogenesis, is expected to support a swath of light to moderate snows from central to southeastern Montana. On Day 3 (Fri), Confluent flow produces bands of 700 mb convergence and an upper level jet streak that moves east across northern and central CO. This will likely result in widespread snow, including locally heavy accumulations across the Colorado Rockies to the WY border. The jet continuing east on to the Plains of northern CO should allows snow showers to develop further east as well, but with lighter amounts than in the mountains. ...Great Lakes/Northeast... Day 1... The models indicate a pair of upper level jet streaks will move across the Great Lakes, New York and New England on Wednesday. Increasing low-mid level theta-e advection into a low-mid level baroclinic band underneath upper divergence maxima in the right entrance region of an upper jet maxima will support a stripe of light to moderate snows currently centered in Michigan to move across western and central New York and central and southern New England Wednesday and Wednesday night. Locally heavy snows are possible where 700 mb theta-e advection and convergence maximize this afternoon/evening over western NY. A tight QPF gradient leads to lower amounts in northern PA east across northern NJ and Long Island NY. The event winds down as the 850-700 mb convergence moves off the New England coast Thu morning. ...Upper Ms Valley/Upper Great Lakes... Day 3... A 700 mb wave bringing snow to the northern Plains on day 2 (Thu) continues to progress downstream across the Upper MS Valley and upper Great Lakes on Day 3. Enhanced moisture advection and modest 700 mb vertical velocities lead to light snow across the region, which tapers once the wave passes. With the progressive wave, no one location has a along duration snow, keeping odds of a heavy snow event low. Differences in the amplitude and phasing of the 700-500 mb wave persist, leading to slight QPF and resultant snow differences that remain to be resolved in later forecasts. The probability of a quarter inch of icing is less than 10 percent days 1-3. Petersen