Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 358 PM EST Tue Dec 14 2021 Valid 00Z Wed Dec 15 2021 - 00Z Sat Dec 18 2021 ...Western U.S.... Days 1-3... An amplified trough off the central CA coast will eject east, reaching the Four Corners by late tonight before turning northeast across southern CO Wednesday morning, with brief mid-level ridging filling in its wake. The next deep cold-core low off WA then shifts into the WA/OR coast late Wednesday, bringing yet another round of heavy precipitation to The West, with heavy snow expected for the Olympics, OR Cascades, the Klamath/Trinity Alps and CA Cascades, and then down the Sierra Nevada again. This second wave will impact the Sierra Nevada with heavy snow Wednesday night, a mere 36-48hrs after this most recent and more significant heavy snow event. Ample moisture has spread ahead of the first wave across the Great Basin/Four Corners and the moisture plume associated with the next wave/low will also provide ample moisture second PW moisture plume advecting onshore D3 with the next system. Snow levels are generally 6000 to 8000ft at onset ahead of the initial wave and drop down to about 4000ft prior to snow cessation. Snow Levels do not recover much in the brief ridge and will remain about 1000ft in western WA through Wednesday, 2000 to 3000ft at onset of the next round of heavy snow in OR and far northern CA Wednesday, rising to around 4000ft for the Sierra Nevada Wednesday night. Tonight into Wednesday, the impressive overlap of moisture and forcing with the lead wave will allow heavy snow to shift east from southern CA terrain across ranges of eastern NV (and Mt Charleston near Las Vegas), ranges of southwest UT up through the Wasatch and Uintas of northern UT and the Wind Rivers of WY as well as the San Juans of CO where Day 1 WPC snow probabilities for 8+ inches are greater than 70%. In the lower elevations of the Great Basin and points east, lighter snow accumulations are also possible as intense forcing within the DGZ could produce some heavy snow rates to dynamically cool the column and allow accumulating snow into the valleys. WPC probabilities are moderate for 4+ inches across much of the Intermountain West (including terrain in northern AZ and around Yellowstone), with shadowed valleys east of the terrain least likely. Starting early Wednesday, the next system and associated AR spreads heavy snow to WA, OR and CA. With low snow levels, much of the precipitation away from the coast and outside of the lowest valleys will be snow, with Portland, OR and Seattle, WA having about a 10% chance for greater than 0.1" of snow on Day 1.5. Generally 1 to 2 feet of snow are expected for the OR Cascades, far northern CA ranges and down the Sierra Nevada which will give follow up impacts to alpine areas that just got 4 to 6 feet of snow. Heavy snow then shifts to the Sawtooth and Tetons, where Day 1.5/2 WPC probabilities for 8 inches are moderately high. Steep ridging from the Pacific spread across the West Coast late Thursday, spreading the end of precipitation across the West through Friday morning. ...Northern Plains and Upper Midwest... Days 1.5/2... A closed mid-level low racing out of the Four Corners Wednesday morning will interact with a second shortwave dropping from Alberta over the Northern Plains/Upper Midwest Wednesday evening before lifting into central Ontario by late Wednesday night. These features will be accompanied by impressive upper diffluence within the left exit of a poleward arcing jet streak of Pacific origin, which will also spread increased PW noted by anomalies of +5 standard deviations lifting into WI, but still +1 to +3 in the cold sector over ND/MN. These together will drive surface cyclogenesis in the lee of the Rockies near the western Neb/KS border by midday Wednesday with rapid deepening then expected as the low track over northeast Neb Wednesday early evening and over Duluth by midnight. 12Z Guidance continues to suffer from much greater than usual spread both in time and space, leading to lowered confidence in the forecast for snowfall with most CAMs farther northwest with the main precip swath than the global models. WPC QPF in this area was generally created from a blend of the agreeable 12Z ECMWF/GFS/UKMET. There are likely to be two areas of snow over the northern Plains/MN. The first is associated with the Alberta shortwave which will bring light snow Wednesday across eastern MT, and much of the Dakotas - Day 1.5 WPC probabilities are now less than 5% for more than 4 inches of snow there. The second region of heavy snow will be just W/NW of the surface low starting late Wednesday afternoon near the SD/Neb/MN borders where an intense deformation axis pivots along the western edge of a pronounced TROWAL wrapping cyclonically around the low. This should result in a snow band coming quickly on the heels of rain ahead of the sfc low center. Guidance remains to the east-west location of this feature, but confidence is increasing on a track over western MN/eastern Dakotas and across northern MN where a brief, likely less than 6 hour period exists where heavy snowfall rates are possible, and Day 2 WPC probabilities for more than 4 inches are 20 to 40% across central/northern MN high as 40%. There remains some 10% risk for 6 or more inches in central MN, though this may be artificially enhanced by issues assigning more QPF to snow that should be in such a rapidly evolving case. ...New England... Day 2... WAA ahead of the potent low pressure in the Midwest will spread precipitation northeast across New England late Wednesday through Wednesday night. As this occurs, cold Canadian high pressure will slowly retreat in response to mid-level divergence ahead of the approaching trough. Despite the cold high pressure lifting off to the east, cold air will likely remain trapped in some of the elevated valleys across NH and Maine, and a period of freezing rain is likely. Day 2 WPC probabilities are 10-30% for 0.1" of accretion in northern NH and far western Maine. ~~~Key Messages for Dec 11-14 Winter Storm across the Western U.S.~~~ ...Heavy mountain snow... 1-2 feet of snow is likely in much of the higher terrain of Utah as well as the San Juan Mountains of Colorado and the Wind River Range of Wyoming. A second wave of low pressure likely produces 1 to 2 feet of heavy snow across far northern California Wednesday, shifting south down the Sierra Nevada Wednesday night ...Widespread impacts to travel and infrastructure... Travel restrictions are likely in heavy mountain snow, with further impacts Wednesday night in the Sierra Nevada. Whiteout conditions, downed trees, and power outages are all likely. Avalanches remain possible across the Sierra Nevada. ...Heavy rain ends in Southern California... Heavy rain comes to an end by this evening in Southern California. Jackson