Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 353 AM EDT Fri Mar 22 2019 Valid 12Z Fri Mar 22 2019 - 12Z Mon Mar 25 2019 ...CA/Great Basin/Rockies... Days 1-3... Several systems will move into the West through the forecast period bringing rounds of snow, mostly to the high elevations. The first is an upper low that crosses UT into CO Friday. Light to moderate snow are expected along the track of the low and accompanying mid level deformation zone from the UT Wasatch and the ranges of Colorado Southern Rockies. The heaviest snow Day 1 is expected in the Rockies of CO where enhanced jet level diffluence and mid-level height falls occur to produce ascent. WPC probabilities are high for 8 inches from the San Juans northward to the WY border. Moderate to heavy snows are also possible in the highest terrain of the Wasatch and into ID. On day 2, the event in UT and CO winds down as the upper trough continues moving east, with temps too warm on the CO plains to support snow as you head towards the KS border. Modest snows are expected in the cyclonic shear zone and upper divergence maxima cross the ranges of southern ID to adjacent northwest WY. Snow picks up again the Sierra Nevada range in CA as the next in the series of upper trough moves onshore, with upper divergence maxima and low-mid level warm/moisture advection producing snow Sat into Sun before moisture spills eastward and brief shortwave ridging blossoms over California. Although widespread heavy accumulations are not expected, WPC probabilities indicate a slight risk for 8 inches in the Sawtooth range and eastward into NW WY. Several inches are expected in the UT Uintas due to the orographic lift plus synoptic lift from the approaching upper trough. Yet another system will then approach California at the end of day 3. This may be stronger than the previous 2, but will get close enough only very late in the forecast such that snowfall probabilities for 6 inches are modest, and confined to the Shasta/Trinity/Siskiyou ranges. ...Interior New York/Northern New England... Days 1-2... A vertically stacked surface-500mb low just south of Long Island Friday morning will lift northeast into the Gulf of Maine. As this occurs, a northern stream impulse will dive into the mean trough to phase with the system, slowing the surface low progression Friday before it all progresses northeast by Saturday morning. Initially on Friday, WAA snows will be confined to the high terrain of New York and Northern New England, with a few inches of snow possible. However, more significant snowfall is likely later on day 1 as the second impulse phases southward producing cold advection on NW flow. This will have a two-pronged effect on snowfall. It will lower snow levels so that accumulating snow will return to the even the valleys of northern New England and New York. It will also produce more intense snow rates and significant snowfall as upslope enhancement combines with pivoting 850-700mb frontogenesis and a deformation axis. As the low finally pulls away during Saturday, snowfall will wind down from west to east, ending last in northern Maine. Heavy snowfall accumulations are nearly certain in the Adirondacks, as well as the mountains of eastern VT, and extreme northern NH and ME. In these areas, WPC probabilities are high for 12 inches, and some locations may receive close to 20 inches, although it will likely come in two waves, this morning with heavier wet snow accumulating several inches, and then late this evening with the drier snow accumulating more heavily. This event is likely to be very elevation dependent, with the heavier snow in the peaks of the mountains and much lighter accumulations in the valleys. This creates somewhat lowered confidence due to steep snowfall gradients in the terrain, and WPC probabilities are high for 4 inches from western NY into the Berkshires of MA and points NE, but much higher accumulations of isolated 12 inches are possible in the Catskills and Berkshires as reflected by latest HREF mean forecasts. Lesser accumulations are likely as far SE as the Worcester Hills, and accumulations may be minimal or none in the Champlain and Hudson River Valleys. Further south, several inches of snow are likely in the favored upslope terrain of WV where a combination of prolonged NW flow in a slowly drying column exists into Saturday. The probability of significant freezing rain is less than 10 percent. Weiss