Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 324 AM EST Sat Jan 14 2023 Valid 12Z Sat Jan 14 2023 - 12Z Tue Jan 17 2023 ...Pacific Northwest... Days 1-2... As a strong jet brings another Atmospheric River (AR) event into CA, parent low attached to the front will move slowly eastward then northeastward over the northeast Pacific D1 into D2. Generally lighter snow is expected D1 in advance of the low with snow levels around 4000ft, favoring the Cascades and into the Blue Mountains. On D2, weakening surface low will make its closest approach to the OR/WA coasts with heavier snow and slowly lowering snow levels into D3 to near 3000ft. Several inches of snow are possible at higher pass levels across the WA Cascades with over 6 inches likely in the higher peaks. WPC probabilities for at least 4 inches peak D2 (about 30-40% at pass level) and then fall off by D3 as weak ridging moves into the region. ...California/Great Basin/Four Corners region... Days 1-3... Two back-to-back AR events will move through CA and the Great Basin this weekend into Monday/early Tuesday as the active pattern continues. D1 system will bring a strong flux of moisture to the region (PW anomalies +2 to +3 sigma and IVT values over 500 kg/m/s which is about the 99th percentile) and several inches of QPF into the Sierra. 200kt jet will slip past 140W this morning and weaken just a bit as it noses into SoCal this evening/overnight, promoting additional lift in the left exit region atop the mid-level shortwave. 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 through the region. 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. Second AR event will be a weaker one with a quickly moving low out of the Pacific eastward through CA into the Great Basin overnight Sunday through Monday. The more southerly track will be a bit colder overall with snow levels below 5000ft (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 D3 with additional snow of at least several inches eastward across southern NV/UT and northern/central AZ by the end of the period. ...New England... Days 2-3... Closed low over the Piedmont this morning will wobble eastward offshore D1 as its trough axis takes on a negative tilt. Surface low will occlude and the warm conveyor belt will wrap back around the surface low as it lumbers north-northeastward southeast of the 40/70 benchmark. However, with such a strong surge around its circulation, moisture will push back westward into New England atop a cold or marginally cold surface layer. Precipitation may be on the lighter side, but could still yield several inches of snow over parts of southeastern MA D2. WPC probabilities for at least 2 inches of snow are only 20-30% over the South Shore but there remains upside potential per some CAM runs. 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 as the moisture surge continues D2 into D3. Though there remains uncertainty into the westward extent of the precip shield, agreement has increased on more freezing rain into Downeast Maine this period. Sleet up to an inch or so is also quite probable. WPC probabilities for at least 0.25" freezing rain are no higher than about 20% over eastern Maine but are at least 20% for 0.10" icing for most of the eastern half of the state. Fracasso ...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.