173
FXUS21 KWNC 171923
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 17 2024
SYNOPSIS: STRONGLY ANOMALOUS MID-LEVEL LOW PRESSURE SHIFTING EASTWARD FROM THE
NORTHEASTERN PACIFIC FAVORS AN ELEVATED RISK OF ATMOSPHERIC RIVER (AR) ACTIVITY
TO BRING HEAVY PRECIPITATION, PERIODS OF HIGH WINDS, AND HIGH ELEVATION
SNOWFALL FOR MANY PARTS OF THE WEST COAST AND THE INTERIOR WEST. GIVEN THE
TIMING OF THE AR POTENTIAL, ANY REALIZATION OF THE ASSOCIATED HAZARD RISKS IS
LIKELY TO DISRUPT HOLIDAY TRAVEL AND MAY LEAD TO LOCALIZED FLOODING OVER PARTS
OF THE WEST COAST. SURFACE LOW FORMATION IN THE LEE OF THE ROCKIES MAY LEAD TO
HEAVY PRECIPITATION AMOUNTS OVER PARTS OF THE SOUTH-CENTRAL CONUS EARLY IN THE
PERIOD. PERSISTENT SURFACE LOW PRESSURE IN THE GULF OF ALASKA IS EXPECTED TO
PROMOTE AN INCREASED RISK OF HEAVY PRECIPITATION AND HIGH WINDS FOR SOUTHERN
MAINLAND ALASKA AND THE SOUTHEAST.
HAZARDS
MODERATE RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, THU-FRI, DEC 26-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, WED-SUN, DEC 25-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS, THU-FRI, DEC 26-27.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS WED-SUN, DEC 25-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR PARTS OF NORTHERN INTERMOUNTAIN
AND NORTHERN ROCKIES, WED-SAT, DEC 25-28.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR MANY PARTS OF THE NORTHERN
INTERMOUNTAIN, CENTRAL AND NORTHERN ROCKIES, WED-SUN, DEC 25-29.
MODERATE RISK OF HIGH WINDS FOR THE PACIFIC NORTHWEST, NORTHERN HALF OF
CALIFORNIA, NORTHERN AND CENTRAL INTERMOUNTAIN, AND NORTHERN HIGH PLAINS,
WED-SAT, DEC 25-28.
SLIGHT RISK OF HIGH WINDS FOR THE WEST COAST AND INTERIOR WEST, WED-MON, DEC
25-30.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, LOWER AND
MIDDLE MISSISSIPPI, TENNESSEE VALLEYS, AND THE SOUTHEAST, THU-SAT, DEC 26-28.
SLIGHT RISK OF HIGH WINDS FOR SOUTHERN MAINLAND ALASKA AND THE SOUTHEAST,
WED-TUE, DEC 25-31.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHEASTERN MAINLAND
ALASKA AND THE SOUTHEAST, WED-TUE, DEC 25-31.
POSSIBLE FLOODING FOR PARTS OF CENTRAL AND NORTHERN CALIFORNIA TO SOUTHWESTERN
OREGON.
DETAILED SUMMARY
FOR FRIDAY DECEMBER 20 - TUESDAY DECEMBER 24:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY DECEMBER 25 - TUESDAY DECEMBER 31: THE WEEK-2 HAZARDS PERSPECTIVE
LOOKS TO BE TRACKING WELL RELATIVE TO YESTERDAY. 500-HPA HEIGHT FORECASTS FROM
THE DYNAMICAL MODELS CONTINUE TO FAVOR A STRONGLY AMPLIFIED LONGWAVE TROUGH
OVER THE NORTHEASTERN PACIFIC WITH AN ANOMALOUS RIDGE AXIS EXTENDING OFF THE
WEST COAST AND INTO THE NORTHERN HIGH PLAINS. THIS PATTERN REMAINS CONDUCIVE
FOR ENHANCED ONSHORE FLOW AND POSSIBLE ATMOSPHERIC RIVER (AR) ACTIVITY WHICH
COULD BRING MULTIPLE WEATHER RELATED HAZARDS FOR MANY PARTS OF THE WEST DURING
WEEK-2. BY EARLY NEXT WEEK, ENSEMBLES SHOW A SHORTWAVE FEATURE PROPAGATING INTO
THE WESTERN CONUS, WHICH IS EXPECTED TO BRING A ROUND OF ENHANCED PRECIPITATION
TO THE WEST COAST. MUCH OF THE ENHANCED PRECIPITATION TIED TO THIS FIRST
SHORTWAVE LOOKS TO TIME OFF RIGHT BEFORE THE START OF WEEK-2 (DEC 25) BUT IS
SOON FOLLOWED BY FALLING HEIGHTS OVER THE NORTHWESTERN CONUS, CONSISTENT WITH
ADDITIONAL SHORTWAVE ENERGY RETURNING LATER NEXT WEEK.
THE EVOLUTION OF THE HEIGHT PATTERN AND THE INCREASES IN MID-TROPSOPHERIC
MOISTURE ARE WELL REPRESENTED IN THE LATEST INTEGRATED VAPOR TRANSPORT (IVT)
TOOLS, WHICH CONTINUES TO SIGNAL ADDITIONAL AR ACTIVITY WITH >60% (40%) CHANCES
FOR IVT VALUES EXCEEDING 250 (500) KG/M/S ON DAYS 9 AND 10 (DEC 26-27) INLINE
WITH PREVIOUS HAZARDS OUTLOOK. GIVEN THIS, AND GOOD SUPPORT IN THE
PROBABILISTIC EXTREME TOOLS (PETS), A MODERATE RISK OF HEAVY PRECIPITATION
REMAINS POSTED OVER THE WEST COAST FOR DEC 26-27, AS WELL AS A BROADER
CORRESPONDING SLIGHT RISK AREA VALID THROUGH DEC 29 BEFORE THE PATTERN IS
FAVORED TO DEAMPLIFY. BASED ON GOOD RUN TO RUN CONTINUITY AND AGREEMENT AMONG
THE MODELS, A HIGH RISK FOR HEAVY PRECIPITATION WAS CONSIDERED FOR INCLUSION,
HOWEVER THIS HIGHER RISK DESIGNATION IS IMPEDED BY THE PETS WHICH CONTINUE TO
FAVOR, AT THEIR MAXIMUM, ONLY 40% CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE 85TH
PERCENTILE. AN ACCOMPANYING MODERATE AND SLIGHT RISK HAZARD FOR HIGH ELEVATION
HEAVY SNOWFALL ALSO REMAINS POSTED FOR THE WEST COAST, VALID FOR THE SAME
PERIODS AS THE HEAVY PRECIPITATION RISKS. MUCH OF THE ENHANCED MOISTURE FROM
LATE WEEK-1 AND EARLY WEEK-2 IS EXPECTED TO WORK ITS WAY INTO THE INTERIOR
WEST. BASED ON CONTINUED SUPPORT IN THE PETS AND UNCALIBRATED SNOW GUIDANCE, A
MODERATE RISK OF HEAVY SNOW REMAINS POSTED OVER THE NORTHERN INTERMOUNTAIN AND
NORTHERN ROCKIES (DEC 25-28) , WITH A BROADER SLIGHT RISK OF HEAVY SNOW
EXTENDING FURTHER SOUTH INTO THE FOUR CORNERS (DEC 25-29).
THE STRONGEST SIGNALS FOR HAZARDOUS WEATHER ACROSS THE WESTERN CONUS RESIDE IN
THE WIND PETS, WHERE THE ECMWF FEATURES 70-90% CHANCES FOR WIND SPEEDS
EXCEEDING THE 85TH AND 90TH PERCENTILE LEVELS TIED TO THE AR POTENTIAL OVER THE
NORTHWESTERN CONUS. DESPITE THE GEFS BEING CONSIDERABLY MORE LIMITED IN
COVERAGE, A FAIRLY BROAD MODERATE RISK AREA FOR HIGH WINDS IS POSTED FOR DEC
25-28 GIVEN THE ECMWF, ALONG WITH A BROAD RISK FOR HIGH WINDS ISSUED FOR THE
WESTERN THIRD OF THE CONUS VALID THROUGH DAY 13 (DEC 30). ALTHOUGH
ABOVE-NORMAL PRECIPITATION IS FAVORED TO QUELL FIRE WEATHER POTENTIAL FOR MUCH
OF THE WEST COAST, THE PATTERN ALSO SUPPORTS THE POTENTIAL FOR SANTA ANA WIND
RISKS FURTHER SOUTH WHERE CONDITIONS ARE FAVORED TO BE NEAR TO BELOW-NORMAL
DURING WEEK-2. THE EXPECTED TIMING OF THE AR RISK IS LIKELY TO DISRUPT HOLIDAY
TRAVEL AND MAY LEAD TO LOCALIZED FLOODING OVER PARTS OF CENTRAL AND NORTHERN
CALIFORNIA WHERE A POSSIBLE FLOODING HAZARD IS POSTED. THIS HIGHLIGHTED REGION
IS EXPECTED TO RECEIVE POTENTIALLY HEAVY PRECIPITATION DURING WEEK-1 AND
STREAMFLOWS AND SOIL MOISTURE ARE ALREADY REGISTERING IN THE UPPER PERCENTILES
BASED ON ABOVE-NORMAL ANTECEDENT PRECIPITATION DURING THE PAST 30 DAYS.
UNDERNEATH THE STRONGEST POSITIVE 500-HPA HEIGHT DEPARTURES FAVORED OVER THE
MIDWEST, THERE IS BETTER AGREEMENT IN THE ENSEMBLES FOR THE AFOREMENTIONED
SHORTWAVE ENERGY PROPAGATING OVER THE INTERIOR WEST WITH SURFACE LOW FORMATION
IN THE LEE OF THE ROCKIES EARLY IN WEEK-2. WHILE THE DETAILS OF THE STORM
TRACK STILL REMAIN UNCLEAR, INCREASED PRECIPITATION AMOUNTS ARE PREDOMINATELY
FAVORED IN THE DAILY MEAN ENSEMBLES ON DAYS 9-11 (DEC 26-28) DUE TO GOOD GULF
OF MEXICO RETURN FLOW WITH THE SURFACE HIGH ESTABLISHED OVER THE WESTERN
ATLANTIC. BOTH THE GEFS AND ECMWF PETS CONTINUE TO SUPPORT A SLIGHT RISK OF
HEAVY PRECIPITATION FOR THE CORRESPONDING PERIOD, THOUGH THE HIGHEST WET
SIGNALS ARE SHIFTED A BIT TO THE EAST RELATIVE TO PREVIOUS GUIDANCE AND THIS
ADJUSTMENT IS REFLECTED IN THE UPDATED OUTLOOK.
MUCH OF THE AMPLIFIED MID-LEVEL TROUGHING FAVORED OVER THE NORTHEASTERN PACIFIC
IS EXPECTED TO BRING AN INCREASED RISK OF HEAVY PRECIPITATION AND HIGH WINDS TO
PORTIONS OF SOUTHERN ALASKA. BASED ON PETS AND ONE OR MORE SURFACE LOW
PASSAGES IN THE GULF OF ALASKA, A SLIGHT RISK OF HIGH WINDS IS POSTED FOR THE
SOUTHCENTRAL MAINLAND THROUGH THE SOUTHEAST, ALONG WITH A SMALLER SLIGHT RISK
OF HEAVY PRECIPITATION ISSUED WITH ITS WESTERN COVERAGE LIMITED TO PRINCE
WILLIAM SOUND. RELATIVE TO YESTERDAY, THESE HAZARDS ARE NOW VALID FOR ALL OF
WEEK-2 GIVEN THE PERSISTENCE OF THE TROUGHING ANOMALIES FAVORED THROUGH THE END
OF THE CALENDAR YEAR.
FORECASTER: NICK NOVELLA
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