721
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
..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.
*** 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.
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
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