Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 400 AM EST Sun Jan 15 2023 Valid 12Z Sun Jan 15 2023 - 12Z Wed Jan 18 2023 ...Pacific Northwest... Days 1-3... Weakening upper low will drift northward along the OR/WA coasts, bringing some modest snow to the Cascades D1. Snow levels around 4000ft may lower just a bit overnight tonight into Monday, with some accumulations to pass levels. Onshore flow will maintain some snow over the WA Cascades D2 but with light amounts. By D3, troughing out of the Gulf of Alaska will bring in some WAA-induced snow to the Olympics and WA Cascades as a cold front approaches the coast by early Wednesday. Snow levels will rise a bit over 4000ft as the brief surge of milder air attempts to move inland. Highest probabilities for at least a few inches will be at and above pass level. ...California/Great Basin/Four Corners/Southern Rockies... Days 1-3... Lead AR continues to push through CA and the Great Basin this morning, with a focus of the moisture plume into the Four Corners region D1. Mid-level trough will zip eastward today but PW anomalies of +1 to +2 early Sunday, combined with quick height falls and upslope into especially the N-S terrain, will yield several inches to perhaps a foot of snow for the Mogollon Rim to the White Mountains in AZ and also over the southwestern CO (San Juans) where close to two feet is possible. UT ranges and CO Rockies will see several inches of snow as well as the trough moves through. As the system lifts to the northeast over the Plains, light snow and some icing is likely into the Great Lakes, but WPC probabilities for at least 4 inches of snow or 0.25" ice are less than 10 percent. The next and final AR event of this amazingly active run since late December will ride along a 130kt jet into central CA as a surface low moves ashore NorCal early Monday. Moisture plume will be much narrower that the previous AR, focused over SoCal, but enough moisture on its northern edge will still intersect the central and especially southern Sierra D2 to yield 1-2 feet of additional snow. Snow levels will be below 4000ft (central to northern CA) and near 5000ft for SoCal, giving additional snow to the San Gabriels and San Bernadinos. As this AR moves eastward, just like its predecessor, another round of modest snow is likely for southern NV/UT into central/northern AZ on WSW flow. Probabilities for at least 6 inches of snow are highest above about 4000-5000ft to the north and 6000-7000ft to the south. On D3, mid-level trough will slow from a positive to neutral tilt as heights rise over the Plains. SW flow will favor the San Juans on D3 with additional snow over 6 inches, and broader light amounts over much of the Four Corners region as low pressure skips across the terrain and lee cyclogenesis begins in southeastern CO. ...Eastern Colorado/Central Plains... Day 3... The end of D3 will feature the start of the next phase of the AR event through the West, as low pressure starts to form in southeastern CO. Upper trough may start to close off by the end of D3 as moisture from the south wraps around the burgeoning circulation. Increased low-level frontogenesis north of the low will act to ramp up snowfall over northeastern CO by the end of the period, where WPC probabilities for at least 4 inches of snow are low at this point (10-40%), but will increase and expand eastward into D4. ...New England... Days 1-2... Closed low south of the 40/70 benchmark this morning will drift eastward then northward as multiple vorticity centers pinwheel around the centroid, dumb-belling it into Nova Scotia by early Tuesday. The occluded system will wrap its warm conveyor belt up and 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 snow, sleet, and freezing rain east of the CT River Valley D1. WPC probabilities for at least 4 inches of snow are low (10-40%) over southeastern MA with additional light snow up the coast of Maine. By D2, milder oceanic surge aloft will bring a ptype transition to sleet/freezing rain for much of Maine as the surface low slowly creeps north of 40N. Sleet up to and over an inch is possible over eastern Maine, along with icing over 0.10" to 0.25" especially toward Eastport up to Houlton. WPC probabilities for at least 0.25" freezing rain are 20-40% over eastern Maine and at least 20% for 0.10" icing for most of the eastern half of the state. Explicit ptype remains somewhat uncertain but will likely be in transition, then eventually turn back to snow and become much lighter as the system gains latitude past Maine into D3 and another approaches from the west. Fracasso ...Key Messages for Mid-January Atmospheric Rivers in California... --Another Atmospheric River Monday Particularly heavy mountain snow on the Sierra Nevada winds down this morning, 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 early this morning 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.