Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Tue Apr 02 2019 Valid 12Z Tue Apr 02 2019 - 12Z Fri Apr 05 2019 ...The West... Days 1-3... Two separate systems will affect the Western CONUS this period, bringing snow to the mountainous terrain. The first will be a mid-level trough and its associated jet max dropping onto the CA/OR coast today before shifting eastward into the Great Plains on day 2. Onshore flow ahead/south of this feature will allow snow for the high terrain of the Sierra Nevada and CA/OR Cascades with snow elevations around 7000ft per the NBM. The heaviest Day 1 snow will be over highest terrain of the Sierra, Unitas, and Colorado Rockies, where WPC probabilities are moderate for 8 inches. As the upper trough shifts across the Great Basin and Central Rockies to the central High Plains on Day 2, the associated jet diffluence rotates northeast to spread snow inland and towards ID/MT. The forcing mechanisms weak by this time and snow levels remain high, so WPC probabilities for 6 inches are high only in the NW WY mountains, Colorado Rockies, and far northern Rockies towards Canada. A second system pivots onshore late Thursday into Friday bringing renewed moisture into the West. Moisture will advect into CA and spill northeastward as the trough axis remains offshore, spreading precipitation from the Sierra of CA, into Utah, and northeast through the Cascades and Northern Rockies. Snow levels will remain high, 5-7 kft, and WPC probabilities reflect that by remaining less than 50 percent for 8 inches even in the highest terrain of the Sierra. ...Western Carolinas... Day 1... Surface low pressure developing off the GA coast will lift northeast this morning while deepening rapidly. Moisture associated with this feature will spread westward into the Carolinas, aided by increasing divergence ahead of a potent southern stream trough ejecting east from TN. Although temperature profiles are marginal, as precipitation develops across the Upstate of SC, it will fall into a dry surface layer and wet-bulb effects will likely cause p-type to transition to snow. Although QPF is not expected to be significant, briefly moderate snowfall is possible, and a small area of moderate probabilities for 1" exist along and east of I-85 in western SC and SW NC. Although this is below watch/warning thresholds, 1" in April is rare for this area so would be significant if it occurs. ...New England... Days 1-2... A rapidly strengthening surface low pressure will move up the coast to a position SE of the Benchmark Wednesday morning, before lifting rapidly into the Canadian Maritimes during day 2. This low will be strong, and precipitation will spread into eastern New England, with a sharp west-east gradient expected. The antecedent airmass is marginally cold, and the high pressure will retreat into an unfavorable position to lock in cold air. However, height falls associated with an approaching northern stream impulse from the Great Lakes, and dynamic/wet-bulb cooling in an area of enhanced mid-level deformation and frontogenesis will likely cause precipitation to changeover to snow inland from the coast. The guidance has trended a bit westward this morning, and the best overlap of cooler temperatures, deformation, and 850-600mb frontogenesis should occur from the Worcester Hills of Massachusetts into the foothills of the White Mountains in NH tonight, before shifting into eastern and northern Maine on day 2. WPC probabilities have increased, and now feature a slight risk of 4 inches in the hills of NH/MA on day 1, with slightly higher probabilities into northern Maine on day 2. The probability for significant freezing rain (0.25 inches) is less than 10 percent all 3 days. Weiss