826
FXUS21 KWNC 171958
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 17 2025
SYNOPSIS: A PERSISTENT MID-LEVEL PATTERN FEATURING STRONG LOW PRESSURE OVER THE
NORTHEASTERN PACIFIC FAVORS AN ELEVATED RISK OF ATMOSPHERIC RIVER (AR)
ACTIVITY, SUPPORTING MULTIPLE WEATHER RELATED HAZARDS, INCLUDING HIGH WINDS,
HEAVY PRECIPITATION, AND HIGH-ELEVATION SNOW ACROSS THE WESTERN CONTIGUOUS U.S.
(CONUS) THROUGHOUT WEEK-2. THE GREATEST AR POTENTIAL IS EXPECTED OVER THE LOWER
HALF OF THE WEST COAST BEFORE SHIFTING NORTHWARD TO THE PACIFIC NORTHWEST LATER
IN THE WEEK-2 PERIOD. SURFACE LOW PRESSURE OVER THE GREAT PLAINS ALONG WITH
SURFACE HIGH PRESSURE OVER THE SOUTHEAST U.S. ARE EXPECTED TO INDUCE
POTENTIALLY HAZARDOUS WINDS OVER MUCH OF THE GREAT PLAINS, MISSISSIPPI VALLEY,
AND WESTERN GREAT LAKES. FARTHER NORTH, BUILDING MID-LEVEL HIGH PRESSURE OVER
THE BERING STRAIT IS LIKELY TO LEAD TO RENEWED NORTHERLY ARCTIC FLOW, AND
ANOMALOUSLY COLD TEMPERATURES FOR MUCH OF SOUTHERN ALASKA DURING THE PERIOD.
HAZARDS
HIGH RISK OF HEAVY PRECIPITATION FOR MUCH OF COASTAL CALIFORNIA, THU, DEC 25.
HIGH RISK OF HEAVY SNOW FOR THE SIERRA NEVADA MOUNTAINS, THU, DEC 25.
MODERATE RISK OF HEAVY PRECIPITATION FOR WESTERN NEVADA, AND CALIFORNIA,
THU-FRI, DEC 25-26.
MODERATE RISK OF HEAVY SNOW FOR THE SIERRA NEVADA MOUNTAINS, THU-FRI, DEC 25-26.
HIGH RISK OF HEAVY PRECIPITATION FOR PORTIONS OF WESTERN WASHINGTON, SAT-SUN,
DEC 27-28.
HIGH RISK OF HEAVY SNOW FOR PARTS OF THE NORTHERN CASCADES, SAT-SUN, DEC 27-28.
MODERATE RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND NORTHERN
CALIFORNIA, FRI-WED, DEC 26-31.
MODERATE RISK OF HEAVY SNOW FOR THE CASCADES AND KLAMATH MOUNTAINS, FRI-WED,
DEC 26-31.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE WEST COAST AND PORTIONS OF THE
DESERT SOUTHWEST, THU-WED, DEC 25-31.
SLIGHT RISK OF HEAVY SNOW FOR THE CASCADES, KLAMATH, AND SIERRA NEVADA,
THU-WED, DEC 25-31.
MODERATE RISK OF HEAVY SNOW FOR THE NORTHERN ROCKIES AND NORTHERN
INTERMOUNTAIN, SAT-WED, DEC 27-31.
SLIGHT RISK OF HEAVY SNOW FOR THE CENTRAL AND NORTHERN ROCKIES, PARTS OF THE
GREAT BASIN AND NORTHERN INTERMOUNTAIN, THU-WED, DEC 25-31.
SLIGHT RISK OF HEAVY SNOW FOR THE LOWER FOUR CORNERS, THU-SAT, DEC 25-27.
MODERATE RISK OF HIGH WINDS FOR MUCH OF THE NORTHWESTERN CONUS, THU-WED, DEC
25-31.
MODERATE RISK OF HIGH WINDS FOR MANY PARTS OF THE SOUTHWESTERN CONUS, THU-SAT,
DEC 25-27.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE CENTRAL AND WESTERN CONUS, THU-WED,
DEC 25-31.
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MUCH OF SOUTHERN MAINLAND
AND SOUTHEAST ALASKA, FRI-SUN, DEC 26-28.
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MANY PARTS OF THE SOUTHERN
AND CENTRAL MAINLAND AND SOUTHEAST ALASKA, THU-WED, DEC 25-31.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF SOUTHERN ALASKA, INCLUDING THE ALASKA
RANGE, THU-WED, DEC 25-31.
FLOODING POSSIBLE FOR MUCH OF THE WEST COAST.
DETAILED SUMMARY
FOR SATURDAY DECEMBER 20 - WEDNESDAY DECEMBER 24:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY DECEMBER 25 - WEDNESDAY DECEMBER 31: THERE CONTINUES TO BE GOOD
CONSISTENCY AND AGREEMENT IN THE DYNAMICAL MODELS FEATURING A HIGHLY AMPLIFIED
500-HPA RIDGE OVER THE NORTH-CENTRAL PACIFIC, WITH STRONG TROUGHING DOWNSTREAM
OVER THE NORTHEASTERN PACIFIC TO PROMOTE ATMOSPHERIC RIVER (AR) ACTIVITY OVER
THE WEST COAST CONTINUING INTO NEXT WEEK. DURING WEEK-2, THE EVOLUTION OF THIS
ACTIVITY CLOSELY FOLLOWS CHANGES IN THE STRENGTH AND POSITION OF THE MEAN
TROUGHING FEATURE, WHERE THE PERIOD STARTS WITH THE GREATEST AR-RELATED RISKS
OVER THE LOWER HALF OF THE WEST COAST, WITH THESE RISKS SHIFTING NORTHWARD
LATER IN THE PERIOD AS MID-LEVEL SHORTWAVE ENERGY REINTENSIFIES NEAR BRITISH
COLUMBIA. BOTH THE GEFS AND ECMWF HAVE COME INTO BETTER AGREEMENT WITH THE
TIMING OF THIS DEVELOPMENT, RESULTING IN A SLIGHTLY DELETED RETURN OF THE
ENHANCED ONSHORE FLOW OVER THE PACIFIC NORTHWEST THAN PREVIOUSLY ADVERTISED.
HOWEVER, STRONGER MID-LEVEL HEIGHT ANOMALIES ARE ALSO FEATURED, RESULTING IN A
HIGHER RISK OF AR RELATED HAZARDS IN THE OUTLOOK. ANY RENEWED AR ACTIVITY OVER
THE NORTHWESTERN CONUS MAY BE PARTICULARLY IMPACTFUL IN LIGHT OF THE COPIOUS
PRECIPITATION RECEIVED (OVER 20 INCHES IN SOME LOCATIONS OVER A 10-DAY SPAN)
TRIGGERING RECENT FLOODING OVER MANY PORTIONS OF WASHINGTON, ALONG WITH A VERY
WET WEEK-1 FORECAST. LATER IN WEEK-2, ENSEMBLES SHOW LITTLE INDICATION OF ANY
PROGRESSION OF THE LONGWAVE FEATURES OVER NORTH AMERICA, WHICH CONTINUES TO BE
CONSISTENT WITH A NEGATIVE PACIFIC NORTH AMERICA (-PNA) PATTERN. CONSEQUENTLY,
THIS UNFORTUNATELY IMPLIES THE PERSISTENCE OF POTENTIALLY HAZARDOUS WEATHER
OVER THE WESTERN CONUS AT LEAST THROUGH THE END OF 2025.
LATE IN WEEK-1 AND INTO EARLY WEEK-2, THERE REMAINS GOOD AGREEMENT IN THE GEFS
AND ECMWF INTEGRATED VAPOR TRANSPORT (IVT) TOOLS DEPICTING THE SOUTHWARD
SOLUTION OF THE AR ACTIVITY, WITH ENHANCED PROBABILITIES FOR IVT VALUES
EXCEEDING 250 KG/M/S REACHING AS FAR SOUTH AS 30N. BASED ON DAILY MEAN
PRECIPITATION AMOUNTS FROM THE ECMWF, GEFS AND CANADIAN, SOME OF THE HIGHEST
PRECIPITATION AMOUNTS OVER CALIFORNIA (FROM THE BAY AREA SOUTHWARD) NOW APPEAR
TO BE TIMING OFF INTO WEEK-1, THOUGH BOTH RAW AND CALIBRATED PROBABILISTIC
EXTREME TOOLS (PETS) TOOLS CONTINUE TO SHOW ELEVATED CHANCES FOR 3-DAY AMOUNTS
EXCEEDING THE 85TH PERCENTILE AND AND 1 INCH DURING THE FIRST FEW DAYS OF THE
PERIOD. THEREFORE, A HIGH RISK OF HEAVY PRECIPITATION REMAINS ISSUED AND VALID
THROUGH DEC 25 OVER MUCH OF COASTAL CALIFORNIA, WITH A BROADER MODERATE RISK
AREA OF HEAVY PRECIPITATION POSTED THROUGH DEC 26 BEFORE MUCH OF THE ANOMALOUS
TROUGHING ALOFT DEAMPLIFIES. RAW SNOW TOOLS ARE CONSISTENT WITH THIS EVOLVING
THREAT OF ENHANCED MOISTURE OVER CALIFORNIA, AND AN ACCOMPANYING HIGH AND
MODERATE RISK OF HIGH ELEVATION HEAVY SNOW REMAINS POSTED OVER THE SIERRA
NEVADA MOUNTAINS THOUGH DEC 25 AND DEC 26, RESPECTIVELY. IT IS WORTH NOTING
THAT SHOULD THE STRONGEST AR POTENTIAL BE REALIZED OVER THE MORE HEAVILY
POPULATED AREAS OF COASTAL CALIFORNIA, THIS IS LIKELY TO CAUSE MAJOR
DISRUPTIONS TO TRAVEL OVER THE HOLIDAY SEASON.
TIED TO THE RELOADING 500-HPA TROUGH OFF BRITISH COLUMBIA BY DAYS 9 AND 10,
THERE IS GOOD AGREEMENT INTO THE DAILY RAW TOOLS SHOWING THE RETURN OF
INCREASING PRECIPITATION AMOUNTS OVER THE NORTHWESTERN CONUS THROUGH THE MIDDLE
OF WEEK-2. THE GEFS CONTINUES TO BE THE DRIEST WHEN COMPARED TO THE OTHER
MODELS, THOUGH IT INDICATES A SUBSTANTIAL UPTICK IN THE PROBABILITIES FOR 3-DAY
AMOUNTS EXCEEDING 2 INCHES FOR DAYS 10-12 (DEC 27-29) RELATIVE TO YESTERDAY.
SIMILARLY, THE ECMWF AND CANADIAN HAVE ALSO TRENDED WETTER, WITH THE LATEST
ENSEMBLE SOLUTIONS FAVORING GREATER THAN 50% CHANCES FOR DAILY AMOUNTS
EXCEEDING AN INCH OVER WESTERN WASHINGTON AND NORTHWESTERN OREGON, WITH HIGHER
CHANCES (>70-80%) EXCEEDING 2 INCHES OVER A 3-DAY PERIOD FOCUSED OVER WESTERN
WASHINGTON. ALTHOUGH PETS ARE LESS ONBOARD WITH THE HEAVY PRECIPITATION
POTENTIAL, MAINTAINING ONLY 20-30% CHANCES OF AMOUNTS EXCEEDING THE 85TH
PERCENTILE, THE MORE AMPLIFIED TROUGHING ALOFT FAVORED AND THE CORRESPONDING
PRECIPITATION RESPONSES IN THE RAW TOOLS OVER ALREADY SATURATED, AND FLOODING
SUSCEPTIBLE AREAS OF THE PACIFIC NORTHWEST SUPPORTS THE ADDITION OF A HIGH RISK
OF HEAVY PRECIPITATION AND HIGH ELEVATION SNOW FOR WESTERN WASHINGTON WHERE WET
SIGNALS ARE STRONGEST FOR DEC 27-28. BROADER MODERATE RISKS OF HEAVY
PRECIPITATION AND HIGH ELEVATION SNOWFALL REMAIN ISSUED OVER THE PACIFIC
NORTHWEST WITH ADDED COVERAGE INTO NORTHERN CALIFORNIA, WHERE THERE IS
INCREASED TOOL SUPPORT FOR THE HEAVY PRECIPITATION AND SNOW RISKS AGAIN
SHIFTING SOUTHWARD WITH TIME, BOTH REMAINING VALID THROUGH THE END OF WEEK-2.
TO CAPTURE THE EVOLUTION OF THE AR ACTIVITY FAVORED, BROADER SLIGHT RISK AREAS
OF HEAVY PRECIPITATION AND HIGH ELEVATION HEAVY SNOW REMAIN POSTED OVER THE
WEST COAST FOR THE ENTIRETY OF WEEK-2, WITH THE HEAVY PRECIPITATION SLIGHT RISK
ALSO EXTENDING INTO THE DESERT SOUTHWEST BASED ON CONTINUED SUPPORT OF 3-DAY
AMOUNTS EXCEEDING THE 85TH PERCENTILE AND HALF AN INCH IN THE PETS. BASED ON
THE AFOREMENTIONED ANTECEDENT CONDITIONS AND THE WEEK-1 PRECIPITATION FORECAST
FROM THE WEATHER PREDICTION CENTER (WPC QPF), THE POSSIBLE FLOODING HAZARD
REMAINS ISSUED FOR MOST OF THE ENTIRE WEST COAST EXCLUDING AREAS SOUTH OF LOS
ANGELES, CA. CONTINUED PRECIPITATION ACCUMULATIONS ON SATURATED SOILS ALSO
BRINGS THE ADDITIONAL RISK OF LANDSLIDES AND DEBRIS FLOWS IN STEEP TERRAIN OR
BURN SCARS.
THE INCOMING PACIFIC MOISTURE ASSOCIATED WITH CONTINUED AR ACTIVITY DURING
WEEK-2 IS FAVORED TO OVERSPREAD MUCH OF THE INTERIOR WEST, LEADING TO AN
ELEVATED RISK OF HEAVY SNOW FOR MANY PARTS OF THE ROCKIES, GREAT BASIN AND
NORTHERN INTERMOUNTAIN. THIS CONTINUES TO BE SUPPORTED BY BOTH THE
PRECIPITATION AND SNOW WATER EQUIVALENT PETS, AS WELL AS THE RAW SNOW TOOLS
FROM THE GEFS AND ECMWF. A SLIGHT RISK OF HEAVY SNOW REMAINS POSTED FOR A
BROAD AREA OF THE INTERIOR WEST, ALONG WITH A MODERATE RISK EMBEDDED WITHIN
THIS AREA OVER PORTIONS OF THE CENTRAL AND NORTHERN ROCKIES, NOW VALID FOR DEC
27-31 WHERE SIGNALS FOR AMOUNTS EXCEEDING A FOOT ARE THE HIGHEST IN THE TOOLS.
IN ADDITION, A SEPARATE SLIGHT RISK AREA OF HEAVY SNOW IS POSTED OVER THE LOWER
FOUR CORNERS (DEC 25-27) ASSOCIATED WITH THE DEEP TROUGHING FAVORED INITIALLY
IN THE PERIOD, AS LOCALLY HEAVY SNOWFALL IS POSSIBLE OVER THE HIGHER ELEVATIONS
OF ARIZONA AND NEW MEXICO.
ACCOMPANYING THE RISKS OF HEAVY PRECIPITATION AND HEAVY SNOW, THE FAVORED AR
ACTIVITY ALSO SUPPORTS PERIODS OF HIGH WINDS REACHING HAZARD CRITERIA
THROUGHOUT THE PERIOD. AT THE BASE AND AHEAD OF THE MEAN TROUGHING FAVORED
EARLY IN THE PERIOD, A MODERATE RISK IS POSTED FOR MUCH OF THE SOUTHWESTERN
CONUS WHERE PETS SHOW ELEVATED CHANCES FOR WINDS EXCEEDING THE 85TH PERCENTILE
EARLY IN WEEK-2. A SEPARATE MODERATE RISK AREA FOR HIGH WINDS IS ISSUED FOR ALL
OF WEEK-2 OVER THE NORTHWESTERN CONUS WHERE ANOMALOUS TROUGHING REMAINS
PREDOMINATELY FAVORED. FARTHER EAST, INDIVIDUAL SHORTWAVES ARE ANTICIPATED TO
PERIODICALLY BE EJECTED EASTWARD, INDUCING ONE OR MORE SURFACE LOWS DEVELOPING
OVER THE GREAT PLAINS. SURFACE HIGH PRESSURE SHOULD BE FAIRLY PERSISTENT
DOWNSTREAM FROM THE AXIS OF THE MID-LEVEL RIDGE COVERING THE CONUS. BETWEEN
THESE SYSTEMS, A TIGHT PRESSURE GRADIENT MAY PERIODICALLY BE ESTABLISHED ACROSS
THE CENTRAL CONUS, INCREASING THE ODDS FOR EPISODES OF HIGH WINDS ACROSS THE
CENTRAL CONUS. WITH THE PATTERN CHANGING LITTLE OVER THE COURSE OF WEEK-2, A
SLIGHT RISK OF HIGH WINDS IS SUPPORTED FOR THE ENTIRE PERIOD.
FOR ALASKA, ANOMALOUSLY POSITIVE 500-HPA HEIGHTS OVER THE NORTH-CENTRAL PACIFIC
REMAIN FAVORED TO BUILD NORTHWARD IN THE BERING STRAIT, ALLOWING FOR THE
DEVELOPMENT OF TROUGHING DOWNSTREAM OVER THE MAINLAND. AS A RESULT OF THIS
PATTERN SHIFT, TEMPERATURES ARE EXPECTED TO BRIEFLY MODERATE OVER THE SOUTHEAST
LATE IN WEEK-1 BEFORE ANOMALOUSLY COLD AIR BEGINS TO REDEVELOP AND EXPAND
WESTWARD INTO THE CENTRAL AND WESTERN PORTIONS OF THE MAINLAND. SINCE
YESTERDAY, BOTH RAW AND CALIBRATED TEMPERATURE TOOLS HAVE TRENDED STRONGER WITH
THE COLD SIGNALS, LIKELY TIED TO THE AFOREMENTIONED STRONGER TROUGHING
DEVELOPING OVER THE NORTHEASTERN PACIFIC TO ESTABLISH MORE OF A NORTHERLY FLOW
COMPONENT OVER THE STATE. THEREFORE, A MODERATE RISK OF MUCH BELOW NORMAL
TEMPERATURES IS POSTED FROM THE NORTHERN PART OF THE ALASKAN PENINSULA EASTWARD
THROUGH YAKATAT IN THE SOUTHEAST, WHERE THE ENSEMBLES SHOW ELEVATED CHANCES
(30-40%) FOR NEGATIVE TEMPERATURE DEPARTURES EXCEEDING MORE THAN 30 DEGREES F.
A BROADER SLIGHT AREA OF MUCH BELOW NORMAL TEMPERATURES REMAINS ISSUED FOR MUCH
OF THE MAINLAND AND SOUTHEAST, VALID FOR ALL OF WEEK-2. HIGH WINDS ALSO REMAIN
POSSIBLE IN GAPS AND INLETS, AND CLOSE TO THE LOWEST SURFACE PRESSURES. DESPITE
DAY TO DAY VARIATION REGARDING THE PRECISE LOCATIONS OF THE HIGHEST WINDS, A
SLIGHT RISK OF HIGH WINDS IS POSTED FOR MOST OF SOUTHERN AND SOUTHEASTERN
ALASKA FOR ALL OF WEEK-2. THE PROBABILISTIC EXTREMES TOOLS (PETS) SHOW
ENHANCED CHANCES OF WIND SPEEDS EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE IN
THESE AREAS. THE VERY COLD WEATHER AND HIGH WINDS ALONG THE SOUTHERN TIER OF
THE STATE INCREASES THE THREAT FOR HEAVY FREEZING SPRAY AT TIMES NEAR LARGER
BODIES OF WATER.
FORECASTER: NICK NOVELLA
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