Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 337 AM EST Fri Dec 24 2021 Valid 12Z Fri Dec 24 2021 - 12Z Mon Dec 27 2021 ...Repeating rounds of heavy snow continue for western terrain with multiple additional feet expected while snow levels decrease from north to south... ...Western U.S... Days 1-3... As one shortwave trough moves quickly east into the northern Plains, a second has quickly followed with a closed upper low expected to develop later this morning near Vancouver Island. This will support continued unsettled weather across the Pacific Northwest today, including locally heavy mountain snows for the western Washington and Oregon ranges. Upstream energy digging southwest of the low will establish an axis of enhanced westerly flow into western Washington and Oregon, before aligning farther south into northwestern California later today into the evening. This will bring additional heavy snows back into the Klamath Mountains and the Shasta Cascade region later today into the overnight. Meanwhile to the south, a southern stream shortwave off of the coast of Southern California will accelerate inland later this morning. Deep southwesterly flow ahead of the system will transport ample moisture across Southern California into the Southwest and the central to southern Rockies. This moisture interacting with a low-to-mid level baroclinic zone, overlapped by favorable upper jet forcing, is expected to support periods of organized moderate to heavy precipitation across the region today. This system is expected to move quickly across the Southwest and the Rockies later today with precipitation diminishing from west to east as the system moves into the central Plains this evening. Area impacted by heavy snow are expected to include the Kaibab Plateau and Mogollon Rim in northern and central Arizona, and the Utah, western Colorado and north-central New Mexico ranges. Additional heavy snows are expected for the mountains of northwestern California and northern Sierra early Saturday as an upper trough continues to amplify off of the coast before moving onshore later in the day. As the system moves inland, heavy snows are expected to shift south along the Sierra, reaching into the southern Sierra as well as the Transverse and northern Peninsular ranges during the overnight into early Sunday. This system is expected to draw low snow levels even lower -- dropping to below 2000 ft across much of Northern and Central California. To the north, models show a steady stream of mid level energy diving south through western Canada -- amplifying a trough centered over southern British Columbia and the Pacific Northwest. In addition to heavy snows for the western Washington and Oregon ranges, arctic air pouring into the region is expected to support accumulating snows beginning Saturday and continuing into Sunday across the coastal/lowland regions of western Washington and Oregon. On Sunday, models show this trough continuing to amplify and dig farther south -- bringing this cold air and the potential for even more heavy snows farther south into the mountains Northern California Sunday night into Monday. By early Monday, three day snow totals are likely to exceed several feet across portions of the southern Cascades, the Klamath Mountains, and the Sierra. ...Northern Plains and Upper Midwest... Days 2-3... A shortwave trough emanating from the broader scale trough in the West, along with a low-to-mid level baroclinic zone is expected to support a stripe of light to moderate snow centered across North Dakota into northwestern Minnesota Friday night into Saturday. While widespread heavy totals are not expected, WPC probabilities indicate the potential for some locations across eastern North Dakota into northwestern Minnesota to receive at least a few inches of snow. More widespread significant amounts are expected to develop Sunday night into Monday, as a more pronounced system moves across the central Plains into the Upper Midwest. Widespread snowfall totals of 4 inches or more expected to develop across parts eastern North Dakota and northern Minnesota by early Monday. ...Northeast... Days 1-3... Light, mixed precipitation is expected to spread across the Northeast ahead of an occluded frontal system moving across the Great Lakes Friday night into early Saturday. Then in response to shortwave energy emanating from the western U.S. trough, a surface wave is forecast to develop along the trailing cold front and bring additional mixed and generally light precipitation into the region Saturday. While widespread significant ice accumulations are not expected, areas of accumulating ice appear likely, especially across portions of the Adirondack region and central New England. Widespread heavy snows do not appear likely at this point either, although a changeover to all snow on the backside of the system as it moves off of the northern New England coast may support at least a few inches of accumulation across northern New Hampshire and Vermont Saturday night into Sunday morning. For days 1-3, the probability of ice accumulation greater than 0.25" is less than 10 percent. ~~~Key Messages for Western U.S. Heavy Snow Through Early Next Week~~~ Multiple waves of low pressure will spread precipitation across the West through early next week with periods of heavy snowfall likely for terrain along the Pacific Coast each day through Tuesday, with snow at times spreading across much of the Great Basin and Intermountain West to the Rockies. Heavy snow accumulating to multiple feet is likely during the next several days in many of the mountain ranges, with more than 7 additional feet likely in the Sierra. Light to moderate snow accumulations spread over several days are likely even at sea level over the Pacific Northwest. Travel will become dangerous and is discouraged, especially along mountain passes where long duration closures are likely. Whiteout conditions, downed trees, and power outages are all possible. Dangerous avalanches are likely in the Sierra, Washington Cascades, and Northern Rockies. Extremely cold temperatures will overspread the Northwest early next week, with some record cold possible especially over the Pacific Northwest. Low temperatures below 0F will be common east of the Cascades through the Northern Plains. Pereira