Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 415 PM EST Fri Jan 13 2023 Valid 00Z Sat Jan 14 2023 - 00Z Tue Jan 17 2023 ...Pacific Northwest... Days 1-3... Upper troughing over the eastern North Pacific will continue to shift east to the PacNW into Monday, allowing snow levels currently above 5000ft to drop below 4000ft (reaching many mountain pass levels) on Sunday. Ample Pacific moisture streams in ahead of a surface low that reaches the OR coast Saturday evening. The increase in rainfall and lower snow levels allows snow probabilities for 6 or more inches to increase from moderate over the higher WA/OR Cascades to moderately high for Day 2 along the OR/WA Cascades as well as Olympics. The next wave Monday is farther south into CA, so Day 3 snow probabilities in the PacNW are rather limited with generally light precip. ...California/Great Basin/Four Corners region... Days 1-3... The wave/atmospheric river (AR) that had been oriented along the CA coast and shifting inland will continue to weaken this evening with focus turning to the next wave/broad AR ahead of a powerful NWly jet coming across most/all CA late tonight into Saturday night, spreading over the Great Basin and reaching the Four Corners late Saturday night. A vort max in the left exit region of a 170kt jet will bring this broad AR moisture surge (PW anomalies +2 sigma) into the Sierra and points east. Height falls under the trough will keep snow levels at or below 6000ft (a bit lower than most of the recent ARs). Several feet of snow are expected through the period for much of the Sierra Nevada, with dangerous 3"/hr rates most likely Saturday afternoon as the cold front crosses. Heavy snow is also expected in the higher parts of the southern CA mountains (San Gabriel and San Bernardino) where snow levels will be 6000ft to 7000ft. From the Great Basin into the Rockies, the AR will lose a bit of its punch but will still yield PW anomalies around +1 sigma through much of the Four Corners region and closer to +2 sigma over the Mogollon Rim. Height falls, approaching/extended jet across the region, frontal convergence, and upslope into the terrain will drive snowfall well over 6 inches in the terrain over Utah (Wasatch/Uintas but especially the southern UT ranges near the best dynamics) and into northern AZ into the White Mountains. As the system moves eastward over the Four Corners, focus will be into southwestern CO (San Juans/Mesa Verde Region) which will capitalize on favorable upslope. One to two feet of snow are likely in the higher terrain. Another wave, likely final significant one into CA over this particularly active stretch, will cross CA Sunday night/Monday. This is a colder system with snow levels below 5000ft over central/northern CA and around 5000ft for SoCal. The AR ahead of the wave will mainly be directed into the Baja CA with broad moderate to locally heavy snow over the Sierra Nevada and far northern CA terrain where Day 3 snow probabilities for over 8" are high. ...New England... Days 2-3... Southern end of an upper trough over the Eastern CONUS close into a low over the Carolinas tonight before sliding offshore and lifting north through the weekend. Precip associated with the inverted trough offshore will extend over Cape Cod into southeast MA Saturday night into Monday. Ocean enhanced snowfall is likely on the outer edge of this precip shield where enough cold air is present in the frontogenesis zone. A couple inches are possible, particularly in the Day 2.5 range southeast of Boston. A wave rounding the offshore low likely reaches Nova Scotia Monday with a prominent warm nose extending over much of eastern Maine. A mostly sleet and freezing rain event is likely near the coast (snow farther inland) for Monday with moderate Day 3 probabilities for a tenth inch of freezing rain. ...Southern and Central Appalachians... Day 1... Upslope NNW flow will continue to focus heavy snow into the Smokies along the TN/NC border with additional snowfall (after 00Z this evening) of 6 or so inches. Storm total snowfall totals may approach 2 feet along this border. Additional upslope snow showers over the Allegheny Highlands of West Virginia should yield a couple more inches tonight. Jackson ...Key Messages for Mid-January Atmospheric Rivers in California... --Heavy Precipitation This Weekend A strong atmospheric river will bring widespread precipitation to much of the region this weekend, including heavy mountain snow and lower elevation heavy rainfall. The peak of the storm is expected Saturday through Saturday night. --Dangerous to Near Impossible Mountain Travel Heavy mountain snow and strong winds will lead to blowing snow and whiteout conditions at times, creating dangerous to near impossible travel above 5000 ft in the mountains and passes of Northern and Central California. Snow rates should reach 3â€+/hr at times Saturday afternoon in the Sierra Nevada. Road closures are likely. --Additional Flooding is Expected With saturated conditions already in place, further flooding impacts are expected across California through this weekend into Monday. Landslides will remain a threat due to the additional rainfall and saturated soils. --Another Atmospheric River Monday Another atmospheric river is expected to impact the region Monday, bringing additional rounds of heavy precipitation including heavy mountain snow and potential flooding due to heavy rainfall.