413
FXUS21 KWNC 162004
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 16 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 MORE NORTHERLY FLOW AND ANOMALOUSLY COLD
TEMPERATURES FOR MUCH OF SOUTHERN ALASKA DURING THE PERIOD.
HAZARDS
HIGH RISK OF HEAVY PRECIPITATION FOR SOUTHWESTERN OREGON AND COASTAL CALIFORNIA
(EXCLUDING AREAS SOUTH OF LOS ANGELES, CA), WED-THU, DEC 24-25.
HIGH RISK OF HEAVY SNOW FOR THE SIERRA NEVADA AND KLAMATH MOUNTAIN RANGES,
WED-FRI, DEC 24-26.
MODERATE RISK OF HEAVY PRECIPITATION FOR SOUTHWESTERN OREGON, WESTERN NEVADA,
AND CALIFORNIA, WED-FRI, DEC 24-26.
MODERATE RISK OF HEAVY SNOW FOR THE KLAMATH RANGE, AND SIERRA NEVADA, WED-FRI,
DEC 24-26.
MODERATE RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST, FRI-TUE, DEC
26-30.
MODERATE RISK OF HEAVY SNOW FOR THE CASCADES, FRI-TUE, DEC 26-30.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE WEST COAST AND PORTIONS OF THE
DESERT SOUTHWEST, WED-TUE, DEC 24-30.
SLIGHT RISK OF HEAVY SNOW FOR THE CASCADES, KLAMATH, AND SIERRA NEVADA,
WED-TUE, DEC 24-30.
MODERATE RISK OF HEAVY SNOW FOR THE NORTHERN ROCKIES AND NORTHERN
INTERMOUNTAIN, WED-TUE, DEC 24-30.
SLIGHT RISK OF HEAVY SNOW FOR THE CENTRAL AND NORTHERN ROCKIES, PARTS OF THE
GREAT BASIN AND NORTHERN INTERMOUNTAIN, WED-TUE, DEC 24-30.
SLIGHT RISK OF HEAVY SNOW FOR THE LOWER FOUR CORNERS, WED-FRI, DEC 24-26.
MODERATE RISK OF HIGH WINDS FOR MUCH OF THE NORTHWESTERN CONUS, WED-TUE, DEC
24-30.
MODERATE RISK OF HIGH WINDS FOR MANY PARTS OF THE SOUTHWESTERN CONUS, WED-SAT,
DEC 24-27.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE CENTRAL AND WESTERN CONUS, WED-TUE,
DEC 24-30.
SLIGHT RISK OF MUCH BELOW-NORMAL TEMPERATURES FOR MUCH OF SOUTHERN MAINLAND AND
SOUTHEAST ALASKA, WED-TUE, DEC 24-30.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF SOUTHERN ALASKA, INCLUDING THE ALASKA
RANGE, WED-TUE, DEC 24-30.
FLOODING POSSIBLE FOR MUCH OF THE WEST COAST.
DETAILED SUMMARY
FOR FRIDAY DECEMBER 19 - TUESDAY DECEMBER 23:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY DECEMBER 24 - TUESDAY DECEMBER 30: THE WEEK-2 HAZARDS PERSPECTIVE
REMAINS ON TRACK SINCE YESTERDAY. DYNAMICAL MODELS CONTINUE TO FEATURE A HIGHLY
AMPLIFIED 500-HPA RIDGE OVER NORTH-CENTRAL PACIFIC, WITH STRONG TROUGHING
DOWNSTREAM OVER THE EASTERN PACIFIC HEADING INTO THE PERIOD. ANALYSIS OF THE
MEAN 6-10 DAY 500-HPA HEIGHT PATTERN FROM THE GEFS AND ECMWF ENSEMBLES REVEALS
TWO MINIMA IN THE NEGATIVE HEIGHT ANOMALY FIELDS, WITH BOTH OF THESE FEATURES
INDICATING THE EVOLUTION OF THE POTENTIAL ATMOSPHERIC RIVER (AR) ACTIVITY
FAVORED OVER THE WEST COAST DURING THE PERIOD. THE FIRST FEATURE IS A DEEP
TROUGH THAT PEAKS IN STRENGTH BY THE OUTSET OF WEEK-2, PROMOTING A SOUTHWARD
SURGE OF ENHANCED TROPOSPHERIC MOISTURE INTO THE LOWER HALF OF THE WEST COAST.
AS THIS INITIAL MID-LEVEL FEATURE GRADUALLY DEAMPLIFIES, THE SECOND FEATURE IS
A REDEVELOPING 500-HPA TROUGH FURTHER NORTH OFFSHORE OF BRITISH COLUMBIA. WHILE
THIS IS EXPECTED TO BRING DRIER CONDITIONS ACROSS CALIFORNIA (MAINLY SOUTH OF
THE BAY AREA), ENHANCED ONSHORE FLOW LOOKS TO MIGRATE NORTHWARD TO MANY PARTS
OF THE PACIFIC NORTHWEST TOWARDS THE MIDDLE OF THE PERIOD. ANY RENEWED AR
ACTIVITY OVER THE NORTHWESTERN CONUS MAY BE PARTICULARLY IMPACTFUL IN LIGHT OF
THE RECENT FLOODING OVER MANY PORTIONS OF WASHINGTON, ALONG WITH A VERY WET
WEEK-1 FORECAST. LATER IN WEEK-2, THE 500-HPA HEIGHT PATTERN LOOKS TO REMAIN
PRETTY STABLE, WHICH UNFORTUNATELY IMPLIES THE PERSISTENCE OF POTENTIALLY
HAZARDOUS WEATHER OVER THE WESTERN CONUS THROUGH THE END OF DECEMBER.
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
THOUGH BEFORE RETREATING NORTHWARD. BASED ON CONTINUED SUPPORT IN THE RAW AND
CALIBRATED PROBABILISTIC EXTREME TOOL (PET) GUIDANCE, A HIGH RISK OF HEAVY
PRECIPITATION REMAINS POSTED FOR COASTAL CALIFORNIA AND INTO SOUTHWESTERN
OREGON WHERE THE GEFS AND ECMWF DEPICT 50-70% CHANCES FOR 3-DAY AMOUNTS
EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE, VALID THROUGH DEC 25. WITHIN THE
HIGHLIGHTED AREA, THERE IS GOOD AGREEMENT AMONG UNCALIBRATED TOOLS SHOWING
GREATER THAN 50% CHANCES OF 3-DAY AMOUNTS EXCEEDING 1 INCH, WITH POCKETS OF
GREATER THAN 30% OF AMOUNTS EXCEEDING 2 INCHES EARLY IN THE PERIOD. BASED ON
RAW TOOLS INDICATING 50-60% CHANCES FOR 3-DAY SNOWFALL AMOUNTS EXCEEDING A FOOT
THROUGH THE MIDDLE OF WEEK-2, AN ACCOMPANYING HIGH RISK OF HEAVY SNOW REMAINS
POSTED OVER THE HIGHER ELEVATIONS OF THE SIERRA NEVADA AND KLAMATH RANGES
THOUGH DEC 26. SURROUNDING THESE HIGH RISK AREAS, MODERATE RISKS OF HEAVY
PRECIPITATION AND HIGH ELEVATION SNOW ARE POSTED OVER CALIFORNIA AND SOUTHERN
OREGON THROUGH DEC 26 BEFORE THE ENHANCED ONSHORE FLOW BEGINS TO EASE OVER THE
LOWER HALF OF THE WEST COAST. 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, ENSEMBLES REMAIN
RELATIVELY CONSISTENT WITH THE MAGNITUDE AND COVERAGE OF THE PRECIPITATION
RESPONSES OVER THE PACIFIC NORTHWEST SINCE YESTERDAY. THE GEFS REMAINS
COMPARABLY THE DRIEST AMONG THE MODEL MEAN SOLUTIONS, BUT ITS DAILY TOOLS DO
NONETHELESS POINT TO THE RETURN OF ABOVE-NORMAL AND POTENTIALLY HEAVY
PRECIPITATION FURTHER NORTH FROM DAY 10 (DEC 26) AND BEYOND. BASED ON THE GOOD
AGREEMENT BETWEEN RAW ECMWF AND CANADIAN ENSEMBLES INDICATING 40-50% CHANCES OF
DAILY AMOUNTS EXCEEDING AN INCH OVER PORTIONS OF WESTERN WASHINGTON AND OREGON
(WITH SIMILAR CHANCES FOR 3-DAY AMOUNTS EXCEEDING 2 INCHES IN THE ECMWF), THE
MODERATE RISK AREAS OF HEAVY PRECIPITATION AND HIGH ELEVATION HEAVY SNOW REMAIN
POSTED OVER THE PACIFIC NORTHWEST, BOTH VALID BEGINNING DEC 26. WHILE THE GEFS
IS QUICKER TO DRY OUT OVER THE HIGHLIGHTED AREAS, ALL ENSEMBLES FAVOR THE
PERSISTENCE OF ANOMALOUS MEAN TROUGHING THROUGH THE END OF DECEMBER, SUPPORTIVE
OF THE CORRESPONDING HAZARDS REMAINING VALID THROUGH THE REMAINDER OF WEEK-2.
GIVEN THE MERIDIONAL NATURE 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 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 MOISTURE ASSOCIATED WITH THIS ENHANCED JET 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 WHERE SIGNALS FOR AMOUNTS EXCEEDING A FOOT ARE THE HIGHEST,
BOTH VALID FOR ALL OF WEEK-2. IN ADDITION, A SEPARATE SLIGHT RISK AREA OF
HEAVY SNOW IS POSTED OVER THE LOWER FOUR CORNERS (DEC 24-26) 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 ELEVATED
POTENTIAL FOR 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, MUCH OF THE ANOMALOUSLY POSITIVE 500-HPA HEIGHTS OVER THE
NORTH-CENTRAL PACIFIC ARE FAVORED TO BUILD NORTHWARD ALONG THE BERING STRAIT,
ALLOWING FOR THE DEVELOPMENT OF TROUGHING DOWNSTREAM OVER THE MAINLAND. AS A
RESULT OF THIS SHIFT, TEMPERATURES ARE EXPECTED TO BRIEFLY MODERATE OVER THE
SOUTHEAST BEFORE ANOMALOUSLY COLD AIR BEGINS TO REDEVELOP AND EXPAND WESTWARD
INTO THE CENTRAL AND WESTERN PORTIONS OF THE MAINLAND. BASED ON THE PETS, THE
STRONGEST COLD SIGNALS ARE FAVORED TO BE OFFSHORE IN THE GULF OF ALASKA AND ARE
NOT AS POTENT AS THE COLD AIR FAVORED OVER THE SOUTHEAST LATE IN WEEK-1. AS A
RESULT, A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES REMAINS POSTED, AND IS
EXPANDED NORTHWARD IN THE MAINLAND WHERE THERE IS BETTER SUPPORT FOR MINIMUM
TEMPERATURES FALLING BELOW THE 15TH PERCENTILE. 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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