Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 PM EST Mon Dec 13 2021 Valid 00Z Tue Dec 14 2021 - 00Z Fri Dec 17 2021 ...A Major Winter Storm is Ongoing over The West into Wednesday with the Next System reaching the Pacific Northwest on Wednesday... ...Western U.S.... Days 1-3... A strong and amplifying trough south of a closed low off WA will swing into CA through Tuesday before ejecting east to the Four Corners Tuesday night and then turning northeast to the Central Plains as it takes on a negative tilt. Confluent low/mid-level flow and WAA ahead of the trough will combine with a strong Pacific jet streak to drive ample moisture onshore noted by PWs reaching +1 to +2 standard deviations above the climo mean. The Atmospheric River will continue over central and southern CA through Tuesday. Snow levels tonight will drop below 1000ft under the trough over the PacNW and will remain 5000 to 6000 ft within the AR and east to the Central Rockies. However, the approach of the trough and the eastward progression of the associated cold front will drop snow levels down to near sea level in the Pacific Northwest, and to just 1000-2000 ft elsewhere across most of the West by Wednesday. For tonight/Tuesday, the heaviest snow is expected over the CA Cascades southward through the Sierra Nevada where the most intense ascent will produce snowfall rates that may reach 3"/hr, especially in the Sierra Nevada due to the nearly orthogonal flow within the AR driving intense upslope into that range. WPC probabilities are high for 18 additional inches across the higher Sierra Nevada, with locally in excess of 5 ft likely above 6000 ft for the central and southern portions of that range. Heavy snow exceeding 12 inches is also moderate to high in the northern CA ranges, and also into the Sawtooth of ID east to Yellowstone. Also, southward expansion continues Tuesday with mountain snow into the Peninsular and Transverse ranges of Southern CA where snow levels drop from 7000ft to 5000ft with snowfall of 1 to several feet in the higher terrain of the San Bernardino Mountains. By late Tuesday, the heaviest precipitation is likely to shift inland over the Great Basin to Central Rockies as the system accelerates east. WPC probabilities for at least 12 inches on Day 2 are moderate from Mt. Charleston in NV, to the southwest UT ranges along with the Wasatch, Uintas, as well as the Wind Rivers in WY and the San Juans in CO. Due to the speed of the system, the western slopes of the other ranges in CO are low to moderate for 8 or more inches on Day 2. While this trough shifts northeast over the Plains late Wednesday, the next closed low will reach the WA coast before opening as it shifts eastward over the northern Rockies Thursday. This will be accompanied by another surge of moisture into the Pacific Northwest, with moderate to locally heavy mountain snow for the OR Cascades/northern CA Klamath/Trinity Alps and Cascades before expanding south over at least the northern and central the Sierra Nevada on Thursday. Snow levels rise Wednesday night, reaching roughly 2000ft over WA, 3000ft over OR, and 4000ft over CA. Day 2.5 WPC probabilities for 6 or more inches are high over these ranges, expanding through the central Sierra Nevada for Day 3. 4-day total snowfall across the Sierra Nevada may reach 8 feet in the highest terrain. ...Northern Plains... Days 2/3... A closed mid-level low lifting rapidly northeast out of CO Wednesday will work in tandem with intense upper diffluence within the left exit region of an amplifying jet streak to drive surface cyclogenesis with rapid deepening of the low expected as it tracks northeast across central Neb to southwestern MN with further deepening as it tracks up to the western Lake Superior Wednesday evening. 12Z Guidance still feature above normal spread in the spatial and temporal track of this low, along with warm thermal conditions ahead of the low. Therefore, the heavy snow threat looks brief and within the normal six hour time windows for day 3. Synoptic ascent will become strong, with enhancement likely through a pivoting deformation axis collocated with a pronounced TROWAL wrapping cyclonically to the west of the surface low. Where the most intense ascent occurs, a band of moderate to briefly heavy snow is is likely somewhere over MN. Day 2.5 WPC probabilities for more than 4 inches remain just 10-30% near the WY/MT/SD borders and on Day 3 are also just 10 to 30 percent from central MN and north. Localized amounts of 6 inches are possible. For Days 1-3, significant icing is not expected. ~~~Key Messages for Dec 11-14 Winter Storm across the Western U.S.~~~ ...Heavy mountain snow... Snow over California will move eastward into the Great Basin tonight into tomorrow, with several feet accumulating in total across the Sierra. Snowfall rates may reach 2 to 3 inches per hour. ...Widespread impacts to travel and infrastructure... Travel may be impossible, especially across the mountain passes, and some road closures are expected. Whiteout conditions, downed trees, and power outages are all likely. Avalanches are possible across the Sierra Nevada. ...Heavy rain for portions of California... Several inches of rainfall will create the potential for flash flooding and debris flow impacts, particularly near recent burn scar areas and along the central and southern California coast. Jackson