Subj : HVYSNOW: Key Messages are To : All From : Mike Powell Date : Mon Dec 22 2025 09:10:20 FOUS11 KWBC 220827 QPFHSD Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 327 AM EST Mon Dec 22 2025 Valid 12Z Mon Dec 22 2025 - 12Z Thu Dec 25 2025 *** Strong Atmospheric River to impact California and the Sierra Nevada over the next several days *** ....Sierra Nevada/Northern California/Cascades... Days 1-3... Maximum impact level from WSSI: Major to Extreme A long-lived Atmospheric River event will contain periods of precipitation as embedded systems move inland from the Pacific over the next several days (and beyond). For Day 1 (ending 12Z Tuesday), the ongoing AR will lift north into northern CA and weaken with limited snowfall impacts across CA. Meanwhile, much lower snow levels and a potent shortwave entering southeast British Columbia will allow for heavy snow across the Olympics and Cascades today into early Tuesday. Here, snow levels will vary between 1500-2000ft, which puts accumulating snow below the major WA Cascade passes. By late Day 2 into Day 3, the next surge of moisture impacts the West Coast and continues beyond into late this week. Deepening height falls off the West Coast and a potent shortwave lifting on the eastern periphery of this deep upper trough rapidly surges moisture inland Tuesday night. IVT directed into the Sierra will be strong (>800 kg/m/s) Tuesday night, but relatively short-lived as broad onshore flow and moderate precipitation continues on Day 3. Snow levels during this strong AR are expected to generally remain around 8000ft (closer to 8500ft in the core of the AR across the central/southern Sierra Nevada), but quickly fall to between 5000-6500ft (lower north and higher south) across CA on Day 3. This will allow for heavy snow to impact all major passes by Wednesday. For the most impactful day (Day 3) WPC probabilities for at least 12 inches of snowfall are 70-99% above around 7000ft and >70% for at least 24 inches of snowfall above 8000ft in the Sierra Nevada. Strong winds combined with very heavy-wet snowfall could make travel difficult to impossible at times in the high terrain of the Sierra Nevada, with damage to infrastructure also possible. This very active and impactful weather pattern across California is expected to continue through much of this week, with additional high elevation heavy snow likely into the end of this week. See our latest Key Messages that cover the event through the end of next week, and CPC's Key Messages into week 2. ....Northeast... Days 1-3... Maximum impact level from WSSI: Minor to Moderate Upper level pattern over CONUS by the end of Day 1 includes an anomalous ridge centered over the central U.S. and strong jet soaring to the north along the U.S-Canadian border before aiming southeastward across the Northeast, where slight troughing remains. An embedded shortwave crossing north of the Great Lakes on Tuesday within a divergent left-exit region of a strengthening 150kt jet streak will aid in blossoming precipitation across the Northeast by early Tuesday. Broad WAA ahead of the system over the still cold Northeast (especially just inland) will bring generally light snow to much of the Northeast and northern Mid-Atlantic, but some light freezing rain on the southern edge of the precipitation shield (central PA and perhaps into parts of WV). The highest QPF associated with the strongest low-level WAA will lie over Northern/Upstate/Central NYS into central New England as the surface warm front lifts into the region. With the fast flow, the parent low over Canada will get left behind as the front occludes and stretches across New England, ultimately forming a new low in the Gulf of Maine. This will likely lead to a separate and higher maximum in QPF due to an inverted trough or quickly developing TROWAL maximizing snow potential across southern/coastal Maine, but some uncertainty remains with the details as this trough could still shift somewhat east or southwest along the Gulf of Maine. This setup has the potential to produce a narrow corridor of very heavy snowfall given the strong low- level convergence potential from roughly Portland to Bar Harbor, ME. In fact, the NBM 75th percentile currently sits around 12 inches, highlighting the higher end potential. The system will pull away from eastern New England late Wednesday afternoon. WPC probabilities for at least 4 inches of snow are >50% over portions of the Finger Lakes region northward to the Tug Hill and Adirondacks, Green Mountains, and into the White Mountains and Maine Midcoast. The highest probabilities (40-70%) for at least 6 inches of snow extend between the southern and midcoasts of Maine. The National Weather Service in Gray, ME has issued Winter Storm Watches in this region for the potential for total snow accumulations greater than 6 inches. Lighter, festive snowfall is expected elsewhere in New England on Tuesday and even as far south as northeast PA/northern NJ for the Tuesday morning commute. ....Interior West & Rockies... Days 1 & 3... Maximum impact level from WSSI: Moderate to Major Heavy snowfall continues to impact the high terrain of the Rockies and Intermountain West, but will see the overarching pattern begin to shift away from the recent west-east oriented Atmospheric River spillover towards more southerly flow and approaching height falls by midweek. With that said, the Day 1 period starts with the aforementioned AR and anomalous PWs extending from the northern Sierra Nevada into the Northern Rockies, but this moisture stream is expected to wane today as an associated shortwave ejects eastward with ridging building in its place across the Rockies through Tuesday. Snow levels will remain high within the axis of greatest precipitation located across central ID into southwest MT and western WY, with levels generally in the 7000-8000ft range. The most impact snowfall with amounts measured in feet continue to be located across the Wind River Range, Tetons, and Absarokas. A brief lull in heavy snowfall is expected on Tuesday as IVT weakens and reorients into a more north-south direction into the northern Great Basin as height falls approach the West Coast. However, by Day 3 a broad surge of tropical moisture lifts throughout the interior West due to a near-record deep upper trough off the West Coast. Given the moisture source it isn't too surprising that snow levels across the Great Basin and Rockies will be rather high, above 9000ft in Utah and above 10,000ft in western Colorado, with slightly lower values around 7000-8000ft in the Northern Rockies during this time period. WPC 72-hr snowfall probabilities for at least 12 inches of snow are highest across the Sawtooths of ID and Wind River/Absarokas of WY, with 70-90% chances mainly above 8500ft. The probability of significant icing is less than 10 percent. Snell ....Atmospheric River Key Messages are in effect. Please see current Key Messages below... https://www.wpc.ncep.noaa.gov/key_messages/LatestKeyMessage_1.png $$ --- SBBSecho 3.28-Linux * Origin: Capitol City Online (618:250/1) .