Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 327 PM EST Sat Jan 07 2023 Valid 00Z Sun Jan 08 2023 - 00Z Wed Jan 11 2023 ...Western U.S... Days 1-3... With longwave troughing favored over the eastern North Pacific for the next several days, the parade of Atmospheric River (AR) events will continue, focused over California and western Nevada. The three-day forecast period will feature two separate AR events, with the lead system Sunday a more modest event focused over NorCal but the second system is forecast to be much more significant (and wetter) with more spillover east of the Sierra crest. For Day 1, a modest surge of PW values to +1 sigma and IVT to 500 kg/m/s on SW to southerly flow will favor the northern California terrain (Shasta/Trinity/Siskiyou) where snow levels will be around 4000ft or so. Snowfall will exceed one to three feet in the most favored areas north and northwest of Redding near Mount Shasta but WPC probabilities of at least a foot are high (>70%) into the northern Sierra as well. With the shortwave lifting southwest to northeast, additional snowfall is likely over the Cascades and especially the Blue Mountains and into the Sawtooth/Salmon River where WPC probabilities for at least 6 inches of snow are greater than 30 percent. For Day 2 into 3, an intense AR will move into California and focus over the northern and central Sierra, with lesser but not insignificant affects astride the core. Amplifying mid-level trough/ridge couplet over the West Coast/Rockies and incoming 130kt jet streak will kick off a strong influx of moisture with IVT values above 750 kg/m/s (99th percentile / +4 sigma) directed into the Sierra. With such a strong SW flow off the Pacific, warm surge aloft will drive 700mb temps to above -5C to the north and -2C to the south, driving snow levels up to 7000-8000ft and SLRs down below climo values for January. Result will be a very heavy/wet snow during the core of the AR trending a bit less dense as colder air comes in post-FROPA. This is reflected in the high probs of extreme impacts from snow load per the pWSSI. Two-day snowfall in the Sierra will be several feet along the Sierra but spillover east of the crest will bring appreciable snowfall to western Nevada by Day 3 as well. To the northeast/east, broad SW flow in the mid-levels will bring adequate moisture to Idaho/western WY/Utah where several inches of snow is likely. WPC probabilities for at least 6 inches of snow are highest over central Idaho with over a foot possible. The probability of significant icing greater than 0.25 inches is less than 10 percent. Fracasso ...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.