Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 348 AM EST Thu Jan 07 2021 Valid 12Z Thu Jan 07 2021 - 12Z Sun Jan 10 2021 ...Southern Appalachians onto the southern Mid-Atlantic Piedmont... Days 1-2... A well-defined southern stream low pressure system currently shifting from TX to LA. Snow will taper off over the higher elevations of the Ozarks this afternoon. Rain is expected through the heavier precipitation across northern MS and AL with snow limited to the northern margins of the precipitation shield over the northern Tennessee valley into this evening. A changeover to snow is expected to result in mainly light accumulations across middle and eastern Tennessee tonight. Overnight tonight and into Friday, the potential for significant snow will increase across portions of the Cumberland Plateau and the southern Appalachians. The biggest threat for heavy amounts is expected to center across the southern Blue Ridge, with lighter totals across the Cumberland Plateau and farther east into the western Piedmont of mainly NC. As the low-to-mid level center moves across Georgia and South Carolina, easterly flow north of the low-to-mid level center is expected to support moderate to heavy snowfall across the upslope regions. Day 1.5 snow probabilities are moderate for 6 or more inches across mountainous western NC with 10 to 20 percent probabilities spreading east across the Blue Ridge area of northwest NC to the VA border. There is question as to how far north into VA the precip/snow shield will extend with the 00Z ECMWF/GFS/NAM having very limited precip north of NC with Canadian based models as well as some CAMs such as the 00Z HRRR being farther north into VA. A compromised QPF solution continues to display some day 2 precip across southern VA which would be mostly snow. The northern extent of the precip remains a forecast challenge for this event. In regard to the eastern extent of snow it should be noted that Day 2 snow probabilities for 2 or more inches extend east across the Piedmont to the Fall Line though this is for southern VA where precip is less certain among global guidance. ...Pacific Northwest and Northern California... Days 1-2... The next wave coming off the Pacific is a series of shortwaves around a Gulf of Alaska low. The final reinforcing shortwave arrives in the Pacific Northwest late Thursday night before diving southeast across the Great Basin through Saturday and and across the southern Rockies Saturday night. This will be another round of organized precipitation, including locally heavy mountain snows across the southern Cascades and the Klamath Mountains late tonight through Friday. While widespread heavy totals are not expected, there are moderate Day 1.5 probabilities for six or more inches of snow across portions of this higher terrain. ...Southern Rockies on the southern High Plains... Day 3... The amplifying ridge behind the aforementioned next wave from the Pacific cuts off Pacific moisture as it crosses the Great Basin through Saturday. However, by the time it reaches the southern Rockies Saturday night return flow from the western Gulf of Mexico provides precipitation enhancement on the eastern side of the southern Rockies onto the southern High Plains Saturday night. Day 3 probabilities for 6 or more inches of snow are moderate for the Sangre de Christos in CO/NM with low probabilities spreading across northeast NM. For Days 1-3, the probability of icing for 0.1-inch or greater is less than 10 percent. Jackson