Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 303 AM EST Sun Jan 08 2023 Valid 12Z Sun Jan 08 2023 - 12Z Wed Jan 11 2023 ...Western U.S... Days 1-3... Extremely active pattern will continue across the West as spokes of vorticity and associated shortwaves shed off a closed low spinning south of the Gulf of Alaska and move onshore the Pacific Coast. These will be accompanied by periods of enhanced upper diffluence as the Pacific jet remains busy and multiple jet streaks shift eastward downstream of each shortwave. This will drive multiple atmospheric rivers (AR) into the region, each of which have a high probability of exceeding IVT of 500 kg/m/s, with the most intense AR affecting CA late D1 through D2. While each subsequent AR will be accompanied by WAA driving upward snow levels, they will remain in general 3000-5000 ft, but climb to as high as 8000 ft in CA D2. On D1 a shortwave trough will lift northeastward from central CA through the Northern Rockies, providing sufficient height falls within the increased moisture to produce heavy snowfall across the higher elevations, especially where flow becomes more orthogonal to terrain features. This trough is relatively transient, and becomes more removed from the greater moisture anomalies as it shifts northeast, but WPC probabilities for more than 6 inches are moderate to high for the Blue Mountains, Sawtooth/Salmon River Ranges, and higher terrain of the WA/OR Cascades and into the Olympics. However, the greatest snowfall D1 is expected in the Sierra and Shasta/Trinity/Siskiyou region as the departing shortwave interacts with greater moisture, and WPC probabilities are moderate for more than 12 inches, with locally 2-3 feet of snow possible, especially near Mt. Shasta. Late D1 into D2 the most intense AR pivots into CA bringing copious moisture onshore and spilling over into the Great Basin and as far northeast as the Northern Rockies. With rising snow levels, SLRs will collapse even in the higher terrain of the Sierra, likely falling to around the 25th percentile from the Baxter climatology. Despite this, exceptionally heavy snow is likely with rates of 3"/hr above 7000 ft accumulating to more than 5 feet in the central and southern Sierra. This will result in extremely impactful snow noted by high pWSSI probabilities for extreme impacts due to snow load. Additionally, heavy snow adding up to multiple feet is forecast for the northern Sierra and northern CA ranges. Farther to the east, the core of the AR will be displaced, but WPC probabilities above 30% for 6+ inches extend into the Uintas, NW WY ranges, Sawtooth, and higher terrain of the Pacific Northwest. Although the intensity of snowfall will begin to wane D3 and the coverage will shift southward, WPC probabilities for more than 6 inches reach 30-50% across the Great Basin terrain and as far east as the San Juans, with an additional 1-2 feet of snow likely on Tuesday across the Sierra, furthering what will be an extreme snowfall event across that area, with heavy snow also spreading into the higher peaks of the San Bernadinos and San Gabriels. The probability of significant icing greater than 0.25 inches is less than 10 percent. Weiss ...Key Messages for Early January Atmospheric Rivers... --Threat of Heavy Rain, Flooding Continues This Weekend: The cumulative effect of successive heavy rain events will lead to additional considerable flood impacts, including rapid water rises and mudslides, over northern and central California, and portions of far western Nevada. Minor to isolated major river flooding is expected through the weekend into next week, with record high river levels possible. --Dangerous Mountain Travel Resumes Today: 1-3 feet of additional snowfall this weekend, with blowing snow at times, will lead to dangerous travel above 4000 feet in the mountains and passes of northern and central California. --Increasing Risk of Infrastructure Impacts and Avalanches: Mountain communities will be at increasing risk of infrastructure impacts and avalanches as snow load continues to increase. By Sunday, two week snow totals should approach or exceed 100 inches in many of the higher elevation areas of the Sierra Nevada. --Stronger Atmospheric River Arrives Monday: A more intense atmospheric river will arrive over California and western Nevada Monday, with additional heavy precipitation and damaging winds up to 60 MPH expected Monday and into Tuesday. Flash flooding and debris flows are possible over burn scar areas. Scattered power outages are also likely. Another round of heavy precipitation is likely later next week. --Emergency Preparedness: Never drive across flooded roads †turn around, donâ€t drown †and be sure to have both an emergency preparedness kit and evacuation plan in place.