Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 335 AM EST Wed Jan 16 2019 Valid 12Z Wed Jan 16 2019 - 12Z Sat Jan 19 2019 ...California/Pacific Northwest/Great Basin/Rockies... Days 1-3... Rounds of heavy snow are expected in the ranges of central to northern California on days 1-2 with multi-day totals of 4-5 feet possible in the Sierra Nevada. Secondary maxima of 1-2 feet are forecast across the Wasatch Mountains of UT and ranges of western CO such as the San Juans. The active weather pattern will continue through Thursday night across The West as a cold front from a low off the OR coast brings multiple rounds of heavy snow, particularly for the Sierra Nevada. The upper jet pushes over northern CA Thursday night, with sustained upper divergence and bands of 700 mb convergence aiding upslope flow in producing heavy snow. The jet continues into central NV with terrain aided maxima expected in windward portions of the NV ranges, with several inches expected. As the upper trough moves further inland from Nevada into Utah and then CO, the upper level jet and 30 mb divergence maxima cross the ranges of UT and CO, resulting in heavy snow there. The highest probabilities are in the ranges of western CO Fri due to the longer duration snow. Furthermore, surface high pressure shifting south over the Canadian Prairies provides MT with upsloping southeasterly flow with upper level divergence downstream from the northern upper trough aiding in producing ascent. This brings moderate probabilities for 4 inches to the ranges of western to central MT. Moderate to high probabilities are also indicated where a higher magnitude 300 mb divergence maxima crosses the ranges of ID to western WY mainly on Day 2. The focus of heavy precipitation shifts north to the Pacific NW Friday with an atmospheric river pushing onshore and into OR through Friday night. As expected with tropically sourced atmospheric rivers, the snow elevation will be rather high, 5000-7000ft over the Cascades with very heavy snow for the highest peaks. Cold air over the Columbia Basin drains down the Columbia Gorge today with arriving precipitation tonight forming patches of freezing rain. Moderate to high Day 1 WPC probabilities for a tenth inch of ice over the Columbia Gorge exist. Southerly flow then limits freezing rain potential in the Columbia Basin for Days 2/3. ...Ohio Valley/Mid-Atlantic/Northeast... Days 1-3... A weak mid-level shortwave trough crosses the central Plains to the mid MS Valley Wednesday night into Thursday and then up the OH valley to the northeast through Thursday night. 700 mb warm/moist advection produces a stripe of light mixed QPF starting in KS tonight where Day 1 WPC probabilities are moderate for a tenth inch over northern MO. The threat spreads east across central IL/IN and into OH where Day 2 WPC probabilities for one hundredth of an inch are moderate. The northern portions of this area should have light snow and the southern portions have snow changes to sleet and freezing rain. Most models have light icing accumulations with low QPF while soundings are favorable for freezing drizzle, which is a result of moist low level conditions with dry air aloft in the snow crystal growth region aloft. On Thursday night/Fri, the mid-level warm/moisture advection progresses steadily downstream across the Mid-Atlantic and New York and New England. The low amplitude and fast motion of the wave keeps QPF and resultant snow showers limited in intensity and duration, resulting in light amounts. In the central Appalachians and adjacent valley areas of the Mid-Atlantic, light icing is expected as the mid-level wave crosses as the initial light precip is followed by drying aloft with low level moisture and lift continuing, allowing for light freezing drizzle to occur. A wintry mix is likely over the central Mid-Atlantic with potential for light snow to transition to mixed sleet and freezing rain, keeping amounts of each precip type less than four inches (outside the Allegheny Highlands) per Day 2 WPC probabilities. The wave shifts northeast and crosses New England Friday. Continued high probabilities for low snow amounts continues across the northeast inland from the coast. ...Great Plains... Day 3... Southern stream energy breaks off a low over the Pacific NW Thursday and ejects southeast to CO into Friday. A surface low develops near the OK/TX Panhandles Friday and shifts east across the southern Plains through Friday night. A northern stream shortwave trough crossing the northern Great Plains brings cold Canadian air down to the converging on the north side of the southern stream low resulting in bands of snow and wintry mix across northern KS and across northern MO into IL Friday night. Enough Gulf moisture wraps around the southern system to warrant a heavy snow threat in the TROWAL with moderate Day 3 WPC probabilities for six inches spanning northern MO. Just south of the higher snow probabilities are low probabilities for a tenth inch of ice from freezing rain over central MO. Upsloping southeasterly flow under the northern stream shortwave also allows a heavy snow band threat over SD where moderate Day 3 probabilities for six inches also exist. Jackson