070
FXUS21 KWNC 181946
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 18 2025
SYNOPSIS: AN ACTIVE, STORMY PATTERN REMAINS FAVORED THROUGH THE END OF 2025 FOR
MUCH OF THE WESTERN CONTIGUOUS U.S. (CONUS). STRONG MID-LEVEL LOW PRESSURE
PREDICTED 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
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. THE ENHANCED ATMOSPHERIC MOISTURE IS FAVORED TO
OVERSPREAD THE INTERIOR WEST, INCREASING CHANCES FOR HIGH ELEVATION HEAVY SNOW
ALONG WITH EPISODES OF HIGH WINDS FOR MUCH OF THE WESTERN AND CENTRAL CONUS.
FARTHER NORTH, STRONG 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
MODERATE RISK OF HEAVY PRECIPITATION FOR WESTERN NEVADA, AND CALIFORNIA, FRI,
DEC 26.
MODERATE RISK OF HEAVY SNOW FOR THE SIERRA NEVADA MOUNTAINS, FRI, DEC 26.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF CALIFORNIA, WESTERN NEVADA,
AND THE DESERT SOUTHWEST, FRI-SAT, DEC 26-27.
SLIGHT RISK OF HEAVY SNOW FOR PARTS OF THE SIERRA NEVADA MOUNTAINS, FRI-SAT,
DEC 26-27.
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-THU, DEC 26-JAN 1.
MODERATE RISK OF HEAVY SNOW FOR THE CASCADES AND KLAMATH MOUNTAINS, FRI-THU,
DEC 26-JAN 1.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PACIFIC NORTHWEST AND NORTHERN
CALIFORNIA, FRI-THU, DEC 26-JAN 1.
SLIGHT RISK OF HEAVY SNOW FOR THE CASCADES, KLAMATH, AND THE NORTHERN SIERRA
NEVADA MOUNTAINS, FRI-THU, DEC 26- JAN 1.
MODERATE RISK OF HEAVY SNOW FOR THE NORTHERN ROCKIES AND NORTHERN
INTERMOUNTAIN, SAT-THU, DEC 27-JAN 1.
SLIGHT RISK OF HEAVY SNOW FOR THE CENTRAL AND NORTHERN ROCKIES, PARTS OF THE
GREAT BASIN AND NORTHERN INTERMOUNTAIN, FRI-THU, DEC 26-JAN 1.
SLIGHT RISK OF HEAVY SNOW FOR THE FOUR CORNERS REGION, FRI-SAT, DEC 26-27.
MODERATE RISK OF HIGH WINDS FOR MUCH OF THE NORTHWESTERN CONUS, FRI-THU, DEC
26-JAN 1.
MODERATE RISK OF HIGH WINDS FOR MANY PARTS OF THE SOUTHWESTERN CONUS, FRI-SAT,
DEC 26-27.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE CENTRAL AND WESTERN CONUS, FRI-THU,
DEC 26-JAN 1.
MODERATE RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MUCH OF SOUTHERN MAINLAND
AND SOUTHEAST ALASKA, FRI-TUE, DEC 26-30.
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MANY PARTS OF THE SOUTHERN
AND CENTRAL MAINLAND AND SOUTHEAST ALASKA, FRI-THU, DEC 26-JAN 1.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF SOUTHERN ALASKA, INCLUDING THE ALASKA
RANGE, FRI-THU, DEC 26-JAN 1.
FLOODING POSSIBLE FOR MUCH OF THE WEST COAST.
DETAILED SUMMARY
FOR SUNDAY DECEMBER 21 - THURSDAY DECEMBER 25:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY DECEMBER 26 - THURSDAY JANUARY 01: SINCE EARLIER THIS WEEK, THE
WEEK-2 HAZARDS PERSPECTIVE HAS BEEN TRACKING QUITE WELL, WHERE THERE CONTINUES
TO BE GOOD MODEL SUPPORT AND RUN-TO-RUN CONSISTENCY FAVORING CONTINUED
ATMOSPHERIC RIVER (AR) ACTIVITY AFFECTING THE WEST COAST, WITH THE RETURN OF
VERY COLD ARCTIC AIR OVER MUCH OF ALASKA. DOWNSTREAM OF A HIGHLY AMPLIFIED
RIDGE OVER THE NORTH-CENTRAL PACIFIC, BOTH THE GEFS AND ECMWF ENSEMBLES AGREE
IN FEATURING A STRONGLY ANOMALOUS TROUGH CENTER OFFSHORE OF CALIFORNIA,
BRINGING AN ELEVATED RISK OF MULTIPLE AR-RELATED HAZARDS OVER THE LOWER HALF OF
THE WEST COAST LATER NEXT WEEK. AS THIS MID-LEVEL FEATURE WEAKENS HEADING INTO
WEEK-2, THESE RISKS ARE EXPECTED TO AMELIORATE, HOWEVER, ANOTHER 500-HPA TROUGH
IN THE NORTHEASTERN PACIFIC IS FAVORED TO BECOME MORE AMPLIFIED, RESULTING IN
THE RETURN OF ENHANCED ONSHORE FLOW WITH SEVERAL AR-RELATED HAZARDS FOCUSED
FURTHER NORTH OVER THE NORTHWESTERN CONUS EARLY IN WEEK-2. THIS RENEWED AR
THREAT IS PARTICULARLY CONCERNING IN LIGHT OF THE COPIOUS PRECIPITATION
RECEIVED (OVER 20 INCHES IN SOME LOCATIONS OVER A 10-DAY SPAN) THAT HAS
TRIGGERED RECENT FLOODING OVER PARTS OF WASHINGTON, AS WELL AS A VERY WET
WEEK-1 FORECAST TO MAINTAIN SATURATED GROUND CONDITIONS IN THIS PART OF THE
COUNTRY. LATER IN WEEK-2, ENSEMBLES SHOW LITTLE INDICATION OF ANY PROGRESSION
OF THE LONGWAVE FEATURES OVER NORTH AMERICA, AS SEVERAL EXTENDED RANGE 500-HPA
HEIGHT SOLUTIONS APPEAR TO LOCK INTO 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, INTEGRATED VAPOR TRANSPORT (IVT) TOOLS
DEPICT A SOUTHWARD SURGE OF THE AR ACTIVITY ASSOCIATED WITH THE TROUGHING
INITIALLY FAVORED OFFSHORE OF CALIFORNIA, WITH THE CORE OF ENHANCED
PROBABILITIES FOR IVT VALUES EXCEEDING 250 KG/M/S BETWEEN 30N-35N. BASED ON
DAILY MEAN PRECIPITATION AMOUNTS FROM THE ECMWF, GEFS AND CANADIAN ENSEMBLES,
THE HIGHEST DAILY PRECIPITATION AMOUNTS OVER CALIFORNIA (FROM THE BAY AREA
SOUTHWARD) ARE TIMING OFF INTO WEEK-1. SINCE YESTERDAY, BOTH RAW AND CALIBRATED
PROBABILISTIC EXTREME TOOLS (PETS) TOOLS SHOW WEAKER PRECIPITATION SIGNALS, BUT
THESE TOOLS MAINTAIN ELEVATED (>40%) CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE
85TH PERCENTILE EARLY IN THE PERIOD. THEREFORE, THE HIGH RISKS OF HEAVY
PRECIPITATION AND HIGH ELEVATION SNOW ARE DISCONTINUED, BUT THE CORRESPONDING
MODERATE RISK AREAS REMAIN POSTED FOR DEC 26 BEFORE ANOMALOUS TROUGHING ALOFT
DEAMPLIFIES. DUE TO THE ECMWF AND CANADIAN FAVORING MORE MID-LEVEL RIDGING TO
DEVELOP AND BRING DRIER CONDITIONS THROUGHOUT MUCH OF THE SOUTHWESTERN CONUS BY
DAY 10 (DEC 28), SLIGHT RISK AREAS OF HEAVY PRECIPITATION AND HIGH ELEVATION
HEAVY SNOW ARE ISSUED OVER CALIFORNIA AND ADJACENT AREAS OF THE WEST, VALID FOR
DEC 26-27. DESPITE THE LOWER RISK DESIGNATIONS IN THE UPDATED OUTLOOK, ANY
LINGERING AR ACTIVITY INTO EARLY WEEK-2 OVER THE MORE HEAVILY POPULATED AREAS
OF CALIFORNIA IS LIKELY TO CAUSE MAJOR DISRUPTIONS TO TRAVEL OVER THE HOLIDAY
SEASON.
TIED TO THE RELOADING 500-HPA TROUGH IN THE NORTHEASTERN PACIFIC, THERE IS GOOD
MODEL AGREEMENT SHOWING THE RETURN OF INCREASING PRECIPITATION AMOUNTS OVER THE
NORTHWESTERN CONUS THROUGH THE MIDDLE OF WEEK-2, WHICH IS ALSO REFLECTED IN THE
IVT TOOLS. WHILE THERE ARE SOME MODEL VARIATIONS IN REGARDS TO THE STRENGTH AND
SHAPE OF THE AMPLIFYING TROUGH, THE PRECIPITATION RESPONSES APPEAR TO BE ROBUST
AMONG THE GEFS, ECMWF, AND CANADIAN ENSEMBLES WITH THE HIGHEST AMOUNTS
REMAINING FAVORED ON DAYS 9 AND 10 (DEC 27-28) OVER THE PACIFIC NORTHWEST IN
THE UPDATED GUIDANCE. WITH THE ECMWF FAVORING THE STRONGEST TROUGH, ITS
UNCALIBRATED TOOL SHOWS PROBABILITIES IN EXCESS OF 80% FOR 3-DAY AMOUNTS
EXCEEDING 2 INCHES OVER WESTERN WASHINGTON, WITH 50-60% CHANCES OF EXCEEDING 2
INCHES STRETCHING FURTHER SOUTH INTO WESTERN OREGON. ALTHOUGH THE CALIBRATED
GEFS AND ECMWF 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 HIGHER AMOUNTS DEPICTED IN THE
RAW TOOLS OVER ALREADY SATURATED, AND FLOODING SUSCEPTIBLE AREAS OF THE PACIFIC
NORTHWEST SUPPORTS THE CONTINUATION OF A HIGH RISK OF HEAVY PRECIPITATION AND
HIGH ELEVATION SNOW FOR WESTERN WASHINGTON FOR DEC 27-28. BROADER MODERATE AND
SLIGHT RISKS OF HEAVY PRECIPITATION AND HIGH ELEVATION SNOWFALL REMAIN ISSUED
OVER THE PACIFIC NORTHWEST WITH COVERAGE EXTENDING INTO NORTHERN CALIFORNIA,
WHERE THERE IS INCREASED TOOL SUPPORT FOR THE HEAVY PRECIPITATION AND SNOW
RISKS AGAIN SHIFTING SOUTHWARD WITH TIME, ALL VALID THROUGH THE END OF WEEK-2.
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 26-27) 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 (DEC 26-27) 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 REMAINING AMPLIFIED AND
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, ANOMALOUSLY COLD TEMPERATURES ARE EXPECTED TO DEVELOP OVER MANY
PARTS OF CENTRAL AND SOUTHERN MAINLAND. OVER THE PAST FEW DAYS, BOTH RAW AND
CALIBRATED TEMPERATURE TOOLS HAVE TRENDED COLDER, WHERE THERE ARE HIGHER
CHANCES FOR NEGATIVE TEMPERATURE DEPARTURES EXCEEDING MORE THAN 30 DEGREES F.
BASED ON A MORE EXPANDED COVERAGE OF PROBABILITIES IN EXCESS OF 40-50% FOR
MINIMUM TEMPERATURES FALLING BELOW THE 15TH PERCENTILE, A MODERATE RISK OF MUCH
BELOW NORMAL TEMPERATURES REMAINS ISSUED AND NOW INCLUDES MORE OF THE MAINLAND,
ALEUTIANS AND THE SOUTHEAST, AND IS NOW VALID THROUGH DEC 30. SHOULD THIS COLD
TREND CONTINUE, A HIGH RISK DESIGNATION WILL BE CONSIDERED IN UPCOMING
OUTLOOKS. 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.
OVER THE CENTRAL PACIFIC, MID-LEVEL TROUGHING IS FAVORED UNDERNEATH THE STRONG
RIDGE CENTER OVER THE NORTH-CENTRAL PACIFIC, WHICH MAY LEAD TO KONA LOW
DEVELOPMENT TO THE WEST OF THE HAWAIIAN ISLANDS. PETS INDICATE ELEVATED (>40%)
CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE 85TH PERCENTILE WITH AMOUNTS EXCEEDING
AN INCH, WHICH MAY BRING LOCALLY HEAVY PRECIPITATION OVER THE WESTERN HAWAII
ISLANDS DURING WEEK-2.
FORECASTER: NICK NOVELLA
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