Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 329 AM EST Thu Jan 12 2023 Valid 12Z Thu Jan 12 2023 - 12Z Sun Jan 15 2023 ...West Coast... Days 1-3... Amplified eastern North Pacific trough/ridge couplet will focus the moisture surge out of the SSW to S with the strongest IVT values just off (and paralleling) the coast as a frontal system stalls and weakens just offshore D1-2. On the edge of this latest Atmospheric River event, the Washington Cascades will see significant snow above most pass levels as much milder Pacific air is drawn northward. A trailing shortwave will deepen into the mean longwave trough and likely close off just west of Oregon on Sunday, helping to drive a stronger front inland D2 and especially D3. Mid-level vort max on the left exit region of a 160kt jet will move through central/southern CA and focus the moisture plume (PW anomalies around +1.5 sigma) squarely into the Sierra. With a colder flow aloft, snow levels will be near 5000ft and fall to near 4000ft as the front passes. Heavy snow in excess of 1-2 feet is expected for a large portion of the mountain chain into D3. ...Great Lakes... Day 1... Caught between a developing surface cyclone over the OH River Valley and a banana-shaped high from Ontario to the Upper Midwest/Corn Belt, north to north-northeasterly flow across Lake Superior will bring in lake effect snow to the northern U.P. of Michigan near the Porcupine and Huron Mountains. Several inches are likely in some favored areas before high pressure moves in for D2. ...Southern and Central Appalachians... Day 2... Surface and mid-level low over the Mid-South early Thursday will move northeastward/eastward D1 as colder air filters in behind it. By D2, a trailing mid-level vort max will dive into the Southeast/southern Appalachians and close off over the Piedmont. With northwesterly flow, upslope will be maximized into the Smokies where, once rain turns to snow, accumulations could be significant above 2500-3000ft. Farther north, additional mid-level height falls and vorticity will wrap into the developing upper low just to the south, crossing over the central Appalachians and promoting increased upslope on northwesterly flow, where two-day snow totals could reach closer to a foot. WPC probabilities for at least 6 inches of snow are highest (>60%) over the higher elevations in eastern WV and along the TN/NC border with lighter amounts stretching back to the northwest toward the long-fetch moisture source off the Great Lakes. ...Interior Northeast... Day 2... Surface low moving out of the Ohio Valley will track across the Finger Lakes/Southern Tier of NY and into VT/NH and northern ME with a mild southerly flow ahead of the system. Colder surface temperatures will support snow closer to Canada with a wintry mix farther south as a warm nose aloft brings sleet/freezing rain northward just ahead of plain rain. Highest snowfall totals will be across far northern ME, driven by modest UVVs into the DGZ atop a near isothermal sub-freezing layer, favoring larger aggregates. Here, two-day WPC probabilities of at least 8 inches are high (>70%). A bit to the south, from the White Mountains northeastward through the Central Highlands, two-day probabilities of at least 4 inches of snow are at least 50%. Mixed ptype phase may be more transient in the warm sector but could be longer-lasting just to the north and northwest of the surface low track. This favors the North Country/Adirondacks and into the Green Mountains where colder surface temperatures may hold without a strong surge of milder air to the ground from aloft. Icing could be moderate, and WPC probabilities for at least 0.10" of ice are >40%. Farther west, northwest flow off Lakes Erie and Ontario post-cyclone will kick off some multi-band lake effect snow which could add up to several inches near Cleveland, OH and Erie, PA. Fracasso ...Key Messages for Mid January Atmospheric Rivers... --Heavy Rain Along the Northern California Coast A strong atmospheric river will continue to focus heavy rain along the Northern California coast through Thursday night. --Heavy Precipitation Spreads Inland on Friday The strong atmospheric river will shift east, impacting Northern California and the Central California coast on Friday. This will be followed by a second system that will bring heavy precipitation, including heavy mountain snow, across much of California on Saturday. --Dangerous Mountain Travel Expected Friday and Saturday Heavy mountain snow, blowing at times, is expected, which may lead to dangerous 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 The series of atmospheric rivers is forecast to continue, producing additional periods of heavy precipitation across California next week.