Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 350 AM EST Mon Jan 16 2023 Valid 12Z Mon Jan 16 2023 - 12Z Thu Jan 19 2023 ...Pacific Northwest... Days 1-3... Onshore flow with embedded weak impulses will maintain some mountain snow of a few inches for D1-2, especially over the Olympics and northern WA Cascades. By D3, a trough from the Gulf of Alaska will push into the region, with WAA-induced snow to the Olympics and WA/OR Cascades ahead of a cold front. Snow levels around 3000ft will sink to below 2000ft post-FROPA as the precipitation ends early Thursday. WPC probabilities for at least 4 inches of snow are moderate (40-70%) above about 3000ft and will impact the passes across the Cascades. Lighter snow will make it across the Cascades and into eastern Oregon, with low probabilities of several inches of snow across the Blue Mountains. ...California/Great Basin/Four Corners/Southern Rockies... Days 1-3... Final in a long series of impactful Atmospheric River (AR) events since late December continues to push through southern CA/northern Baja this morning and extended along the U.S./Mexican border on the south side of a 130+ kt jet across the Four Corners, placing central AZ in the right entrance region. Secondary vort max just off NorCal in the Pacific will slip southeastward through CA D1, with another round of snow for the Sierra. Snow levels will fall through the day over the Sierra as the cold core low approaches, from near 5000ft to around 3000ft as snow tapers off. Farther south, snow levels will be closer to 5000ft for SoCal, giving additional snow to the San Gabriels and San Bernadinos. Nose of the moisture plume will be pointed into AZ today, intersecting the Mogollon Rim nearly orthogonally, increasing ascent and favoring snowfall in excess of 12 inches into the White Mountains on rates of 1-2"/hr. To the north, still above normal moisture along with incoming height falls and modest WAA will help yield a broad area of light to moderate snow (heavier in the higher elevations) over central AZ into UT, where WPC probabilities of at least 6 inches are high. To the east, SW flow will favor the San Juans for another round of snow with a favorable upslope component. WPC probabilities of at least 6 inches are high D1. By D2, upper trough will move through the Four Corners in advance of eastern Pacific ridging, turning the flow to SSW and southerly. This will still favor the San Juans in CO and northern NM as the mid-level vort moves right through the region late Tuesday. WPC probabilities for at least 6 inches on D2 are above 60% over most of the San Juan Mountains. Two-day probabilities of at least 18 inches are greater than 50% over the higher elevations. On D3, the system coming into the Pac NW will also bring a somewhat meager surge of moisture (much weaker AR with IVT values just over 250 kg/ms/s) into NorCal ahead of the cold front. W to SW flow will capitalize on upslope into the northern CA ranges (Klamath/Shasta-Siskiyous/Trinity) and the northern Sierra, where several inches of snow are likely. ...Eastern Colorado/Central Plains... Day 3... The D2 trough exiting the Great Basin/Four Corners will close off over southeastern CO at the start of D3, setting up an expanding area of snow. Moisture tap will reach the west Gulf and surge cyclonically into the developing surface low Wednesday as the upper low moves to the northeast. Increased FGEN on the NW side of the low will help drive higher snowfall rates close to 1"/hr from northeastern CO through NE. Guidance has shifted a bit to the north compared to 12/24 hours ago, but overall setup remains similar. At least modest to potentially stronger lift through the DGZ could drive SLRs above 15:1 in some bands which could translate to more than 6-8 inches. Northeasterly flow across northeast CO will also aide in upslope enhancement along the Front Range as well. WPC probabilities for at least 4 inches of snow are high (>70%) from around Denver northeastward into western Iowa just to the north of an impinging dry slot through 12Z Thu. Probabilities for at least 8 inches of snow are moderate (>40%) in a similar area. ...Maine... Day 1... Complex negatively-tilted upper low southeast of the 40/70 benchmark will continue to lift to the NNE today, with an occluded surface low wrapping into its larger circulation and yielding to its triple point low moving into Nova Scotia late this evening. Warm conveyor belt has wrapped northward to northwestward around the low, driving a warm layer above the sub-freezing surface layer over Maine. Result is a mixed ptype transition period of sleet and freezing rain over the southeastern half of the state and some snow over the Central Highlands into northern areas along the Canadian border. Sleet accumulation near an inch is possible in some eastern areas, with a few inches of snow to the west. WPC probabilities of at least 4 inches are low (<30%). For eastern areas, freezing rain may be the more dominant ptype via a deeper warm layer, and icing could exceed a few tenths of an inch. WPC probabilities of at least 0.25" icing are near 50% along the border near Houlton, and drop off to near 10 percent in the middle of the state. Fracasso ...Key Messages for Mid-January Atmospheric Rivers in California... --Another Atmospheric River into Monday Additional rounds of precipitation are expected 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 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.