Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 315 PM EST Mon Jan 09 2023 Valid 00Z Tue Jan 10 2023 - 00Z Fri Jan 13 2023 ...Western U.S... Days 1-3... The ongoing atmospheric river-heavy precipitation event will continue into this evening, with the associated core of anomalous moisture supporting additional very heavy snow along the Sierra. Snowfall rates exceeding 3"/hr are expected to continue well into the evening before diminishing as a leading shortwave moves east of the region. Snow levels initially around 8000 ft are expected to drop beginning this evening and continue into Tuesday. Across California, a brief break is expected very early Tuesday before an amplifying upstream trough and associated plume of deeper moisture delivers the next round of organized, heavier precipitation later in the day. While less impressive than the previous plume, moisture will likely be sufficient for additional heavy snows along the Sierra. Day 1 totals of multiple feet are likely, especially for elevations above 7000 ft in the Sierra. Meanwhile, the previously noted leading shortwave, along with an axis of enhanced frontogenesis, will help support an initial round of heavy snow spreading east into the higher elevations of the Great Basin and the central Rockies early Tuesday. The next system will move quickly on its heels, impacting the Great Basin late in the Tuesday before spreading into the Southwest and Rockies Tuesday night. Heavy snow may continue along portions of the Sierra into Tuesday evening before diminishing as the second wave moves east and a shortwave ridge begins to build along the coast. Some warm advection precipitation, including high elevation snow is likely Wednesday into early Thursday, however the threat for widespread additional heavy snow across California is expected to wane as backing flow pushes snow levels higher and directs deeper moisture farther north. By early Thursday, western Washington will likely be the focus for the deeper moisture advection and the greater threat for heavy precipitation. Although snow levels will be on the rise, significant accumulations are possible across the northern Cascade passes. The probability of significant icing greater than 0.25 inches is less than 10 percent. Pereira ...Key Messages for Early January Atmospheric Rivers... --Strong Atmospheric River Continues into Tuesday Heavy rain, heavy mountain snow, and damaging winds up to 60 MPH will continue over California and western Nevada today as the focus of the atmospheric river moves slowly southeastward. Flash flooding and debris flows are possible over burn scar areas. Scattered power outages are also likely. --Widespread Considerable Flood Impacts The cumulative effect of successive heavy rain events will lead to rapid water rises and landslides over northern and central California and portions of far western Nevada. Isolated major river flooding is expected through early this week, with record high river levels already observed, and may continue into Tuesday. --Dangerous Mountain Travel Through Tuesday 1-4 feet (localized amounts greater than 6 feet possible above 7000 feet) of additional snowfall through Tuesday, 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. Two week snow totals have reached or exceeded 100 inches in many of the higher elevation areas of the Sierra Nevada. --Additional Precipitation Tuesday Another round of rain and mountain snow will move through the region on Tuesday. Some flash flooding is possible along with heavy snowfall in the mountains.