Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 443 AM EDT Sun Apr 07 2019 Valid 12Z Sun Apr 07 2019 - 12Z Wed Apr 10 2019 ...Pacific Northwest to the Northern Rockies and Northern Great Plains... Days 1-3... An ongoing atmospheric river allows snow elevations to continue to rise as a broad ridge spreads inland tonight with ample Pacific moisture ahead of the next trough. That next trough will cross the Pac NW coast Monday night and amplify as it digs to the Desert SW through Tuesday night. The northern Rockies see snow elevations rise from around 5000ft this morning to around 8000ft by this evening. Moderate to heavy precip is heavy snow for the higher portions of ID/MT/Yellowstone where Day 1 and 2 probabilities for eight inches are moderate. Snow elevations drop markedly with the trough on Tuesday to the 3000 to 5000ft range across the Pac NW and Intermountain West. Expect moderate to heavy snow to shift east with the northern Rockies again in the moderate probability range for 8 inches as well as UT and the rest of higher WY. A developing surface cyclone in the lee of the CO Rockies allows TROWAL development over the northern Plains - most likely over SD as of this time with moderate probabilities for four inches over that state on Day 3. ...Northeast... Days 1-3... Strong low level theta-e advection into a region of confluent flow aloft and exiting surface high pressure will support mixed precipitation developing across northern Vermont and New Hampshire late tonight, with primarily snow farther to the east across northern Maine. As low pressure sliding south of the region and then off the coast helps to maintain the cold air, significant snow accumulations are expected across northern Maine with Day 2 probabilities moderately high for eight inches in a stripe across northern Maine. Guidance has come into good agreement among the main global deterministic models with the precip axis along the stationary front across southern Maine and northern VT/NH. Day 1.5 ice probabilities are moderate for 0.25 inches over northern VT/NH along this frontal zone. The upper trough axis shifts east across the Northeast Tuesday and offshore Tuesday night with coastal low development allowing a second round of wintry weather (mainly snow) farther south than the first. Day 3 probabilities for four inches are 20 to 30 percent in the White Mountains to western Maine. Wrap around snow after the trough axis passes should reach south into MA. Jackson