Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 355 PM EST Sat Jan 14 2023 Valid 00Z Sun Jan 15 2023 - 00Z Wed Jan 18 2023 ...Pacific Northwest... Days 1-2... Upper low off Oregon lifts north along the coast through western Washington through Sunday. Snow levels dip to around 4000ft with moderate to locally heavy snow at higher pass levels across the WA Cascades. Day 1 snow probabilities are moderately for over 6 inches on the OR and southern WA Cascades. Reinforcing troughing Sunday night into Monday continues mainly over the WA Cascades with snow levels still around 4000ft where Day 2 snow probabilities for over 6 inches are moderate. ...California/Great Basin/Four Corners/Southern Rockies... Days 1-3... Ongoing AR with broad onshore flow on a NWly 180kt jet through CA, the Great Basin, and the Four Corners tonight and the southern Rockies Sunday. Further height falls under the trough will allow snow levels to drop below 4000ft over Sierra Nevada with 6000ft to 8000ft over the far southern CA ranges closer to the core of the AR. Day 1 snow probabilities are moderate for an additional 18 inches for the Sierra Nevada, which is mainly prior to 12Z Sunday. From the Great Basin into the Rockies, the AR 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. The next and final AR event of this amazingly active run since late December will be narrower with the upper trough crossing CA Monday into the Great Basin overnight Sunday through Monday. The more southerly track of the low will allow a bit colder profile with snow levels below 4000ft (central to northern CA) and near 5000ft for SoCal, but also will have less moisture flux on lower PW anomalies and a focus into far southern CA and the northern Baja. Still, more than a foot of snow is likely for the central/southern Sierra on Day 2 with moderate Day 3 probabilities for 8 or more inches over southern NV/UT and northern/central AZ and into southwest CO (San Juan Mtns). ...New England... Days 1-2... Closed low over eastern North Carolina this afternoon will drift north off the Mid-Atlantic coast Sunday and then toward New England Monday before lingering over the Canadian Maritimes Tuesday. The surface low will occlude and the warm conveyor belt will wrap back around the surface low and into New England atop a cold surface layer. Precipitation will be locally moderate in banding with a wintry mix of sleet and freezing rain for Down East Maine. Ocean enhanced snow into southeast MA will also occur Sunday into Sunday night. WPC probabilities for at least 2 inches of snow are still about 20-30% over southeast MA. Up the coast, milder E to NE flow at 850mb (40kts) across Atlantic Canada will drive a warm nose into much of Maine, favoring sleet and freezing rain over the area later Sunday until wrap around flow turns more toward snow Monday night. Though there remains uncertainty into the westward extent of the precip shield with increasing probabilities for a glaze of freezing rain and sleet up to an inch or so. WPC probabilities for at least 0.25" freezing rain are still around 20% over eastern Maine and at least 20% for 0.10" icing for most of the eastern half of the state. Jackson ...Key Messages for Mid-January Atmospheric Rivers in California... --Another Atmospheric River Monday Particularly heavy mountain snow on the Sierra Nevada this evening winds down by Sunday, and another atmospheric river is expected to impact the region Monday, bringing additional rounds of precipitation including heavy mountain snow and potential flooding due to heavy rainfall. --Dangerous Mountain Travel Continues Heavy mountain snow and strong winds will lead to blowing snow and whiteout conditions at times, creating dangerous to near impossible travel above 4,000 ft in the mountains and passes of Central and Southern California. Snow rates should reach 2â€+/hr at times both Saturday evening and Monday in the central and southern Sierra Nevada. Road closures are likely. --Additional Flooding Likely With overly saturated conditions still in place, further limited flooding impacts are expected across California through tonight and into Monday. Landslides will remain a threat due to the additional rainfall and saturated soils. --Active Weather Pattern to Conclude Late Week In the wake of a weak frontal passage over northern California on Wednesday, high pressure is expected to build in overhead by Thursday giving the state a much needed break from the multi-week onslaught of heavy precipitation.