Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 640 PM EST Wed Jan 11 2023 Valid 00Z Thu Jan 12 2023 - 00Z Sun Jan 15 2023 ...West Coast... Days 1-3... Only a brief lull in the winter part of the active period over the West Coast is expected Thursday as an atmospheric river orients along/off the northern CA and PacNW coast. However, the next powerful Pacific jet and atmospheric river shifts into California Friday with upper troughs focusing this moisture activity over western terrain this weekend. Days 1 and 2 snow probabilities for 6 or more inches are limited to the Shasta/Trinity/CA Cascades and WA Cascades with snow levels around 7000ft Thursday into Friday before dipping below 6000ft late Friday. As the next upper trough approaches the OR/CA coast Friday night, snow levels dip to around 5000ft across CA/OR/WA. A broad Pacific moisture plume/atmospheric river arrives Saturday across CA/southern OR with those lower snow levels maintained under the upper trough. Heavy mountain snow can be expected with high probabilities for a foot or more for the Shasta/Trinity/CA Cascades and the entirety of the Sierra Nevada in the Day 3 snow probabilities which ends 00Z Sunday. This will be yet another impactful winter storm for CA with little relief anticipated through early next week. ...Central Plains to the Great Lakes and the Appalachians... Days 1-3... A potent mid level low currently over eastern CO will track along the KS/OK border into southern MO tonight before being drawn northeast around a Great Lakes trough later Thursday through Friday. Marginal winter conditions are anticipated over the track of the low level low (KS/MO/IL/IN), but snow banding north of the surface low, over much of KS, is expected tonight with potential for further bands into MO. Localized heavy snow is likely in the best band though current probabilities for over 2" on Day 1 are limited to around 10% over north-central KS and less than 10% southeast of KC. The low expands/redevelops on the interior track west of the Appalachians Thursday with a likely break in snow banding north of the low over IL/IN, but returning over northwest OH/southeast MI Thursday afternoon/evening. Increasing cyclonic flow bringing some lake effect snow to the U.P. and northern IN off southern Lake Michigan with much stronger flow and upper support for lake enhanced snow south of Lake Erie Thursday night/Friday where Day 2 snow probabilities for over 6" are moderate (with low probabilities over The Thumb of the MI L.P. from northerly flow off Huron). Prolonged upslope flow for the central and southern Appalachians is expected to bring locally heavy snow with moderately high Day 2.5 probabilities for 4" or more to western slopes of the Allegheny Plateau in WV and to the Smokies. ...Interior Northeast... Day 2... Ahead of the aforementioned interior track low is significant snowfall on southerly flow across northern New England, primarily the White Mtns of NH and ME (where WSSI is highlighting a particular snow load threat for heavy, wet snow) as well as far northern Maine. Snowfall rates will likely be impressive, exceeding 1"/hr, within the most pronounced WAA thanks to enhanced ascent into the DGZ with an increasingly isothermal layer beneath it to support large aggregates. Day 1.5/2 snow probabilities are moderate for 6 or more inches here with potential for 10" in northern Aroostook Co Maine. The strong WAA that will lead to the short duration of heavy snowfall late Thursday into Friday will surge a warm nose northward to cause a changeover to mixed precip over northern NY/New England where sheltered valleys should retain sub-freezing air and allow for some notable ice accumulations. Day 2 ice probabilities for the eastern Adirondacks and northern VT/NH are 20 to 40% for more than a tenth inch of ice. Jackson ...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.