Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 420 PM EST Wed Jan 16 2019 Valid 00Z Thu Jan 17 2019 - 00Z Sun Jan 20 2019 ...California/Pacific Northwest/Great Basin/Rockies... Days 1-3... Rounds of heavy snow are expected to continue in the ranges of central to northern California on day 1 with an additional 2-3 feet possible in the Sierra Nevada range. The snow is driven by a Pacific moisture feed being fed into the mountains by a strong deep layer jet and upper divergence maxima. As the jet continues inland in tandem with the upper trough into the Great Basin, secondary maxima of 1-2 feet are forecast across the Wasatch Mountains of UT and ranges of western CO such as the San Juans, as well as ranges of southern ID. 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 300 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 on day 2. 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 south central MT. On Day 3, the focus of heavy precipitation shifts north to the Pacific NW Friday night into early Sat with an atmospheric river pushing onshore and into the northwest, with deep southwest flow transporting moisture but also warm air. The snow elevation will be high, 5000-7000 ft over the Cascades with very heavy snow for the highest peaks. Well defined upper divergence maxima cross the WA Olympics and Cascades, so the primary uncertainties are the exact QPF and how long is it cold enough for snow at different elevations. ...Great Plains/Mid MS Valley/Ohio Valley... Day 2... Upsloping southeasterly flow gets lifted with low level frontogenesis and convergence across southern South Dakota. The NAM and ECMWF indicate a band of enhanced QPF and snowfall as this area is also located in a coupled jet region with 300 mb divergence maxima. This area of coupled low level frontogenesis and upper divergence continues downstream into Iowa by late Friday. Good a heavy snow band threat over SD where moderate Day 3 probabilities for six inches also exist. Day 3... Southern stream energy breaks off a low that 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 air down across the Plains, resulting in bands of snow and wintry mix across northern KS and across northern MO, then downstream across central to northern IL/IN/OH Friday night/Saturday. Further south in parts of central to northern MO east closer to the Ohio River, a precip type transition zone develops with snow changing to a mixture of sleet, freezing rain and rain. The lift over the Oh Valley becomes strong due to a coupled jet region with well defined lower convergence/upper divergence maxima, so the model confidence in a storm developing is strong. The biggest uncertainty is duration of each precip type. The models are in agreement the the strengthening frontal zone and storm leads to a potential heavy snow event downstream into western New York just beyond this period. ...Ohio Valley/Mid-Atlantic/Northeast... Days 1-2... A low amplitude and progressive mid-level shortwave trough crosses the mid MS Valley tonight and up the OH valley and northeast Thursday and Thursday night. 700 mb warm/moist advection produces a stripe of light mixed QPF starting in KS tonight with pockets of freezing drizzle. The threat spreads east across central IL/IN and into OH. 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 snow and 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. 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 low, with locally higher amounts in the Allegheny Highlands. The probability of a quarter inch of freezing rain is less than 10 percent days 1 and 2. Petersen