Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 415 PM EST Sun Jan 15 2023 Valid 00Z Mon Jan 16 2023 - 00Z Thu Jan 19 2023 ...Pacific Northwest... Days 1-3... Modest mountain snow in onshore flow continues through Monday. Snow levels just below 4000ft will further lower a bit overnight tonight into Monday, with some accumulations to pass levels with moderately high Day 1 probabilities for 6 or more inches. After a bit of reprieve in ridging Monday night, a trough from the Gulf of Alaska will approach the WA coast Tuesday and push into the PacNW Wednesday. WAA-induced snow to the Olympics and WA Cascades ahead of the cold front will bring heavy precip Wednesday to the WA/OR/far northern CA coast. Snow levels linger around 3000ft, below pass level with moderate Day 3 probabilities for 6 or more inches along the length of the Cascades and the Olympics. ...California/Great Basin/Four Corners/Southern Rockies... Days 1-2... The final CA atmospheric river event of this amazingly active run since late December will ride along a 130kt jet into southern CA as a surface low moves ashore in central CA late tonight. The strength of the moisture plume will be much narrower than the previous AR, but enough moisture on its northern edge will still intersect the central and southern Sierra into Monday to yield 1-2 feet of additional snow. Snow levels will be below 4000ft over central to northern CA and near 5000ft for SoCal, giving additional snow to the San Gabriels and San Bernadinos. As this AR moves eastward another round of snow will occur in terrain for southern NV/UT into central/northern AZ on WSW flow. It is noted that this second wave is a bit less than the previous wave that is ending this afternoon. Day 1.5 probabilities for at least 8 inches of snow are moderately high over Four Corners terrain. ...Eastern Colorado/Central Plains... Day 3... An amplified trough over the Desert SW Monday night will continue pumping Pacific moisture into the southern Rockies through Tuesday as it closes into a low as it moves over the CO/NM border. Lee-side troughing over the southern High Plains Tuesday night allows Gulf moisture to stream up the southern Plains and fgen snow bands develop over central High Plains that then spreads east as the surface low ejects east over the KS/OK border with the dry slot spreading northeast from the southern Plains. The combination of cold air spreading in from the northern Plains and Ely/NEly flow which will have upslope enhancement up the Plains and likely up the Front Range foothills and eastern slopes. The 12Z GFS/NAM stand out from the 12Z consensus as being farther south while the 12Z ECMWF/CMC are farther north and the 12Z UKMET is between. Overall a bit of a north push in the QPF/snow axis was noted with the forecast today. Day 3 snow probabilities are moderate for 8 or more inches a bit east of the Denver metro and near the KS/CO/Neb borders. A key factor for heavy snow, particularly through the Denver metro will be the residence time for upslope bands to wrap in. Guidance like the 12Z ECMWF suggests that residence time into the foothills will be longer. ...New England... Days 1-2... Energy rounding the strong low off the northern Mid-Atlantic will swing into Nova Scotia tonight/Monday with the warm conveyor belt up and around the surface low and across Maine 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. WPC probabilities for at least 2 inches of snow are 20 to 30% over southeastern MA and moderate over interior Maine. A quick ptype transition to sleet/freezing rain for much of Maine will occur as the surface low slowly creeps north of 40N. Sleet over an inch is possible over eastern Maine, along with moderate probabilities for over 0.25" over Down East Maine. Explicit ptype preference is shifting toward freezing rain which continues through Monday evening. Jackson ...Key Messages for Mid-January Atmospheric Rivers in California... --Another Atmospheric River into Monday Additional rounds of precipitation are expected late tonight into Monday, including heavy mountain snow and heavy rainfall that could cause flooding. --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 3â€+/hr at times late Sunday night into Monday morning in the central and southern Sierra Nevada. Road closures are likely. --Additional Flooding Possible With overly saturated conditions still in place, further limited flooding impacts are possible across California through 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 frontal passage over northern California on Wednesday, high pressure is expected to build in overhead by Thursday night, giving the state a much needed break from the multi-week onslaught of heavy precipitation.