Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 316 AM EST Thu Dec 23 2021 Valid 12Z Thu Dec 23 2021 - 12Z Sun Dec 26 2021 ...Western U.S... Days 1-3... A highly-amplified, positively-tilted upper trough will drop southeast from the eastern Pacific into the western U.S. on Thursday. Deep southwesterly flow along the base and leading edge of the trough will support strong moisture advection from central and southern California, through the Great Basin and Southwest, into the central Rockies. The 00Z GFS shows PWs 3-4 standard deviations above normal across central and southern California later today, with highly anomalous values extending east through the Great Basin and Four Corner Region by this evening. This will support widespread precipitation, including mountain snows, with orographically focused moderate to heavy amounts across the region. The heaviest amounts on Thursday into early Friday are expected to fall across central and southern California. In additional to the anomalous moisture and orographic forcing, guidance shows right-entrance region upper jet forcing supporting strong ascent across region. As snow levels drop below 8000 ft during the day, very heavy snowfall accumulations are likely for the Sierra Nevada, with several feet possible by early Friday along the southern Sierra Nevada. As the deep moisture plume extends east, moderate to heavy snow accumulations are expected across parts of the Great Basin into the central Rockies. This includes the Wasatch and Uintas, where guidance shows a frontal boundary and overlapping upper jet dynamics interacting with the anomalous moisture plume to support heavy accumulations tonight into Friday. Meanwhile, unsettled weather, including locally heavy mountain snows is expected from the Pacific Northwest into the northern Rockies on Thursday as well. Models show a shortwave embedded within the northern portion of the broader scale trough separating, leaving a southern branch shortwave to linger along the coast of Southern California into early Friday. Models show the northern stream wave moving progressively east across the Northwest into the northern Rockies on Thursday. However, this will quickly be followed the next in a series of shortwaves diving south from the Gulf of Alaska. This next system is expected to amplify as it approaches the Northwest, with a closed low developing near Vancouver Island Friday morning. Several inches of snow are likely for the Northwest ranges and portions of the northern Rockies, with 1-2 feet likely for portions of the Olympics, northern Cascades, the Sawtooth, and the northwestern Wyoming ranges. On Friday, the remaining shortwave positioned along the Southern California coast is expected to accelerate inland, moving quickly across the Southwest and central/southern Rockies on Friday -- bringing some additional moderate to heavy snows across the region. Back to the north, the upper low centered near Vancouver Island Friday morning is expected to fill. However, upstream energy over the Gulf of Alaska is forecast to dig to its west, establishing deep westerly flow into western Washington and Oregon, and northwestern California -- supporting additional moderate to heavy mountain snows. Energy along the coast will continue to dig southeast, moving into California late Saturday into early Sunday. This will likely bring additional heavy snows back into the Sierra Nevada. This is expected to push three day snowfall accumulations over 100 inches across portions of the higher terrain. Meanwhile, yet another shortwave is expected to dig south and amplify over southwestern British Columbia and Pacific Northwest. This will bring already low snow levels even lower, with accumulating snows likely for the valleys and coastal regions of western Washington and Oregon late Saturday into Sunday. ...Northeast... Days 2-3... Warm advection precipitation associated with an occluded system moving across Ontario and the Great Lakes is expected to bring some light precipitation from the eastern Great Lakes into the Northeast on Saturday. As energy emanating from the western U.S. trough moves across the Midwest, models show low pressure developing along the trailing cold front and moving northeast across the Ohio Valley into the Northeast on Saturday. Models continue to show a fair amount of spread with this system, with the GFS remaining the amplified outlier. WPC preference was for a more suppressed solution, similar to the NAM and the ECMWF. While the details are far from certain, a wintry mix, including accumulating ice, is expected across portions of Upstate New York into central and northern New England on Saturday, with perhaps a changeover to mostly snow across northern New England as the low begins move off of the coast Saturday night. For days 1-3, the probability of ice accumulation greater than 0.25" is less than 10 percent. Pereira