Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 350 PM EST Thu Jan 12 2023 Valid 00Z Fri Jan 13 2023 - 00Z Mon Jan 16 2023 ...Pacific Northwest... Day 1... An upper level trough over the eastern North Pacific will continue the AR (atmospheric river) into the PacNW, allowing for a short break in the precipitation for the southern 3/4 of CA. The AR is being driven by a 110 kt S to SSW jet. With snow levels at around 7,000 feet in the northern Washington Cascades, expect the 2-4" of liquid equivalent rain expected just on Day 1 to result in snow totals of around 2 feet above the snow level. For Days 2 and 3 starting Friday night, expect significantly less snow in the Washington Cascades as the moisture plume moves south back towards CA, even as snow levels return to 3,000 to 4,000 feet. ...California and the Southwest... Days 1-3... A shortwave rounding the base of the trough over the Pacific will drive it into the CA coast, which in turn will direct the AR initially over the Pacific Northwest back southward into central and northern CA Friday into Saturday. This will result in the opposite trend from the Washington Cascades as far as snow totals in the Sierra Nevada and Mt. Shasta regions goes. Relatively little snow falling on Day 1 tonight into Friday turns around and accumulates to multiple feet of snow on Days 2 and 3 Saturday and Sunday. Snow levels starting out around 6,000 feet on Day 1 fall to around 4,000 feet Saturday night going into Day 3. A strong upper level low moves into the coast of OR on Saturday, as additional shortwave energy digs the trough south across all of CA. This will result in another round of precipitation for essentially the entire state, allowing the aforementioned multiple feet of snow in the Sierras to accumulate. The upper level trough moves inland early Sunday as yet another piece of energy initiates yet another round of precipitation starting in northern California Sunday afternoon and persisting through Sunday night. The wave moving inland Saturday night into Sunday will spread mountain snows into the Intermountain West, with amounts over a foot possible in the higher terrain from the Mogollon Rim of AZ northward through UT and CO. ...Great Lakes into the Northeast... Days 1 & 2... A low moving across the OH Valley will be centered near the OH/PA border at the start of the Day 1 period this evening. Much of the associated wraparound snow is likely to remain on the Canada side of the Great Lakes, however the wraparound cold air is already starting off a renewed round of lake-effect snow on the upper Lakes, which will eventually spread to the lower Lakes as the low moves up the St. Lawrence Valley by Friday morning. Northerly flow over the upper Lakes will favor the U.P. of Michigan, and areas due south of Lake Michigan into northwestern IN, where a few inches of accumulation are possible. Heftier snowfall totals are likely on the southern end of Lake Erie where additional help from the terrain from northwest OH through southwest NY will help wring out 6-10 inches of snow from the lake-effect. More significant snowfall totals are possible along the northern tip of St. Lawrence County, NY where around 6 inches of snow are possible, and amounts closing in on a foot are expected for much of northern ME where the cold air is likely to hang tough as the low center passes nearby. Lingering lake-effect off of the lower Lakes may total another couple inches into Friday night before drier air moving in on northerly flow ends the lake-effect altogether as the fetch over the lower lakes minimizes. On the warmer east side of the low, expect warm air advection to override cold air currently in place in sheltered valleys and low-lying areas of the northern Adirondacks, northern VT & NH, and up into northern ME. This will make for a moderate probability of freezing rain in these areas, resulting in anywhere from one tenth to one quarter inch of ice accumulation. From NY into NH, the cold air will eventually scour out making for a brief period of plain rain before cold air returns on the back side of the low, while the transition to plain rain is unlikely further north into northern ME, where any freezing rain/sleet will change back over to snow when the cold air returns. ...Southern and Central Appalachians... Days 1 & 2... Once the low over the OH Valley moves into New England, cold air on the backside of the low will take advantage of lingering moisture and the upwind Great Lakes to upslope into the central and southern Appalachians. The big snowfall winner with this round will be the Great Smokies along the TN/NC border, whose orientation orthogonal to the mean northwesterly flow will optimize the upslope in this region once the cold air arrives. Expect 2 day snowfall totals to approach 2 feet of snow in the heaviest snows immediately along the state line. Much lower snowfall totals are expected further north with the flow far less optimally directed. The central Appalachians can expect a general 2 to 4 inches of snow with locally higher amounts into the Allegheny Front of PA/MD/WV. Similar to the upstream Great Lakes, drier air and changing wind flows will end the snow on Saturday as the upslope diminishes. Wegman ...Key Messages for Mid-January Atmospheric Rivers... -- Heavy Precipitation Spreads Inland Friday An atmospheric river will impact Northern California and the Central California coast on Friday. This will be followed by a second, more widespread system that will bring heavy precipitation, including heavy mountain snow, across much of California on Saturday and Saturday night. -- Dangerous Mountain Travel Friday through Saturday Heavy mountain snow and strong winds will lead to blowing snow at times, creating dangerous to near impossible travel above 5000 ft in the mountains and passes of Northern and Central California. -- Additional Flooding is Expected With saturated conditions already in place, limited flooding impacts are expected across Northern and Central California through next week. Limited flooding impacts are expected in portions of south-central and Southern California this weekend into early next week. -- Additional Atmospheric Rivers into Next Week Another strong atmospheric river is expected to impact the region late Sunday into Monday, bringing additional rounds of heavy precipitation including heavy mountain snow.