Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 412 PM EST Sun Dec 26 2021 Valid 00Z Mon Dec 27 2021 - 00Z Thu Dec 30 2021 ...Northern Plains and Great Lakes... Days 1-3... An upper trough rounding a developing cyclonic gyre over western Canada will lift northeast from the central Rockies this evening, but then slow as it encounters a blocked pattern downstream resulting in a trough/low over northern MN/Lake Superior through Monday despite the very strong WSWly jet (180kt at 250mb) just to the south across the north-central CONUS. Snow developing this afternoon over the eastern Dakotas/southern MN into IA in the leading warm air advection will continue to increase under improving jet dynamics as it spreads northeast across the rest of ND and north-central/northern WI tonight and northern MN Monday. An inverted surface trough developing on the northwest side of the low will focus heavy snows with extra enhancement for the North Shore of MN from long fetch in easterly flow over Lake Superior maximizing snowfall on the escarpment there where there are moderately high Day 1 snow probabilities for a foot or more with potential for 20 inches. Day 1 snow probabilities for 8 or more inches are moderate over the northeastern part of ND and over most of northern MN (but not the Boundary Waters). South of the heavier snows, a period of mixed precipitation, including freezing rain producing light ice accumulations generally under a tenth inch, is expected from eastern SD across southern MN through the IA border, southern/central WI to the IL border, and much of lower MI into northern IN and OH. Shortwave ridging behind this system will provide a cold and quiet before the next low (which is stronger, but farther south) tracks from KS to lower MI through Tuesday night. Late Monday night light snow in advance develops over the Dakotas with this expanding southeast to the Midwest under the upper trough. Day 2 snow probabilities for 4 or more inches are around 30 percent along the southern MN/WI border, expanding north across eastern MN and northern WI and the UP for Day 2.5 with again enhancement off Lake Superior allowing a maximum over the North Shore where Day 2.5 snow probabilities for 6 or more inches are around 20 percent. Mixed precip and a light ice glaze are once again likely on the south end of the snow shield with Day 2 probabilities for 0.01" or more of ice moderate over much of IA, along the WI/IL border and over the central LP of MI. ...Northern Mid-Atlantic to the Northeast... Days 1-3... Cold antecedent conditions under the warm advection pattern ahead of the northern Plains/Great Lakes system will allow mixed precip to develop late tonight and through Monday over the central Appalachians and northern Mid-Atlantic. Moderately high Day 1 probabilities for ice accumulation over 0.1" is centered on the Laurel Highlands of PA dipping into the Potomac Highlands of MD and covering much of west-central PA with at least low probabilities. Widespread probabilities for light icing then expand across northeastern PA/northern interior NJ and Upstate NY for Days 1.5/2. Then a similar set up occurs with the next system with Tuesday afternoon/night mixed precip and low to moderate Day 3 WPC probabilities for light icing over west-central and northern PA through NY into central New England. ...Western U.S... Days 1-3... A broad upper trough with several embedded shortwaves and an Arctic airmass will continue to support unsettled winter weather across much of the western U.S. through the rest of the work week. The last of a string of quickly repeating troughs descends from another developing low over western WA this evening, digging to the SF Bay area through midday Monday before swinging inland over the Four Corners and onto the central High Plains Monday night. Ample Pacific moisture and increasing jet dynamics from an intensifying northerly jet off the Pacific NW shifts yet another axis of heavy precipitation from the OR/CA border this evening down through the central CA coast through Monday with snow levels down to around 2000ft in this heaviest axis. The heaviest totals will once again be along the western slopes of the Sierra Nevada with an additional 2 to 4 feet expected for the High Sierra and significant snows of 6 to 12 inches down to around 2000ft, much lower than normal for heavy Sierra snows (that are usually associated with warmer Atmospheric Rivers). Additional heavy accumulations are expected again Southern California's Transverse and northern Peninsular ranges Monday afternoon/evening. The axis of heavier snow will extend across the northern Great Basin to northern UT/western WY where Day 1 snow probabilities are moderately high for 8 or more inches. This heavy snow axis then shifts south with the trough Monday night with Day 1.5/2 snow probabilities shifting to southwest UT terrain, the western slopes of the CO Rockies and across northern AZ terrain north from the Mogollon Rim where continued southwesterly flow allows prolonged snow with moderate snow probabilities for Days 2/2.5. The next shortwave around the mean trough is off the Pac NW Tuesday/Wednesday, but the presence of the cold air does bring threats for low elevation snow over much of the west south of WA with Day 3 snow probabilities for 4 or more inches light to moderate over the OR Cascades/Klamath and Sierra Nevada and Transverse Ranges as well as the northern Great Basin. ~~~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 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 along 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