Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 323 PM EST Sat Dec 25 2021 Valid 00Z Sun Dec 26 2021 - 00Z Wed Dec 29 2021 ...Northern Plains and Upper Midwest... Days 1-3... Cold Canadian high pressure and some fresh snow from today will be over the region tonight (with some locally moderate LES in nly flow off Superior tonight) into Sunday as an amplified shortwave trough shifts ENE over the northern Plains late Saturday into Monday. Gulf-sourced moisture streams ahead of the trough and left-exit jet dynamics aid lift through Sunday with NW-SE oriented snow bands developing in earnest over central ND to far northeast SD Sunday afternoon before slowly pushing ENE through Monday. Day 1.5/2 WPC snow probabilities for 6 or more inches are moderately high over the eastern 2/3 of ND and across northern MN. Easterly flow off Lake Superior will enhance snowfall for the North Shore where there are 30 to 50% Day 2 probabilities for a foot or more at the top of the terrain right above the lake. A mixed transition zone is expected to set up Sunday night over south-central MN with a few hundredths ice glaze possible from the MSP metro and south through the rest of MN and across central WI. Subtle changes in the sfc low track will further determine where this transition zone sets up. Furthermore, the warm conveyor belt lifting over the cold conveyor belt will allow snow bands to set up over northern WI and the UP Sunday night into Monday with Day 2 snow probabilities for 4 or more inches high near Duluth reducing to 20% on the western shore of Green Bay. The next wave spreads northeast from the central Plains Tuesday with broad cyclonic flow around a weakening surface low. Ample moisture and upper forcing will spread another round of at least moderate snow to the eastern Dakotas, MN, and the central Great Lakes by Tuesday afternoon. As of now Day 3 snow probabilities for 4 or more inches are generally less than 10% and contained to southern MN with ice probabilities in Iowa. ...Northeast... Day 1... Light freezing rain continues this evening over southern NH where a few hundredths inch additional glaze after 00Z is likely. ...Western U.S... Days 1-3... An upper level trough extends from low pressure over western WA to the central CA coast this evening and ejects ENE across the WY/CO border by late Sunday. Then a reinforcing shortwave from BC dislodges the low over western WA Sunday night with the resultant positively tilted trough sweeping across the rest of the West Coast to central CA again Monday before it ejects across the central CO Rockies by late Monday night. Additional shortwave energy then tracks mainly off the Pacific Northwest coast Monday night before reaching the SF Bay area by late Tuesday. **The next 48 hours will be extreme for the Sierra Nevada with two additional rounds of particularly heavy snow expected - tonight and again late Sunday night through Monday - mountain travel is highly discouraged. 12Z HREF mean snow rates of 2 to 4"/hr are forecast to continue to spread down the Sierra spine through about 09Z tonight which should produce 1.5 to 2.5' tonight alone (with up to 3 ft in the southern Sierra). Then, continued onshore/upslope flow should allow an additional 6 to 12" along the Sierra Sunday before the next wave brings more very heavy snow with an additional 2 to 4' expected for Day 2. Please see local forecasts from WFOs STO and HNX for further details on this situation.** Otherwise mainly moderate snow rates will continue for the OR Coast Ranges and Cascades and far northern CA ranges in onshore flow between the waves with high Day 1 probabilities for a foot or more and again for Day 1.5. Locally heavy accumulations of a foot or more are also likely for Day 1 for the highest terrain of the Transverse Ranges of southern CA, northern Nevada mountains, across UT and western WY and the western CO Rockies. Across the Northwest, snow levels will continue to drop to sea level tonight supporting accumulating snows across the lowland area of western Washington and Oregon with moderate Day 1 probabilities for 2 or more inches in Seattle and Portland, along with low to moderate probabilities for an additional 6 inches in surrounding terrain including the northern OR coastal ranges. The Day 2 heavy snow threat is mainly on the Sierra Nevada with additional probabilities for 8 or more inches over higher northern NV and southern/central ID terrain. Then Day 3 the threat is inland across the Southwest with the fast moving system bringing moderately high probabilities for 8 or more inches to the southern CA Transverse Ranges, Wasatch, southwest UT ranges, the Kaibab Plateau and along the Mogollon Rim in AZ, and western slopes of the CO Rockies including the San Juans into far northern NM. Also of note on Day 3 is the low/to sea level snow levels with light to moderate probabilities for 2 or more inches over much of western OR (including the Willamette Valley), into Mendocino County, CA with cold conditions, but weak forcing from the wave being offshore. ~~~Key Messages for Western U.S. Heavy Snow Through Tuesday and Cold Through Midweek~~~ Periods of very heavy snow will continue through Monday for the Sierra Nevada with heavy snow north through the Oregon Cascades and Coastal Ranges. Heavy snow continues across the Intermountain West through the central/southern Rockies through Tuesday. Increasingly cold air will allow for locally moderate snow accumulations to sea level across the Northwest through Sunday night. The foothills and elevated valleys of California and the Great Basin will also see light to moderate snowfall through Monday. Travel will remain dangerous and is discouraged, especially across the Sierra Nevada and along other mountain passes where long duration closures remain likely. Whiteout conditions, downed trees, and power outages are all likely. Dangerous avalanches are likely in the Sierra Nevada, Washington Cascades, Northern through Central Rockies, and Wasatch. Extremely cold air will overspread the Northwest through midweek, with daily record cold temperatures likely for many coastal locations. Low temperatures below 0F will be common east from the Cascades through the Northern Plains through Wednesday. Jackson