741
FXUS21 KWNC 161954
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 16 2024
SYNOPSIS: STRONGLY ANOMALOUS MID-LEVEL LOW PRESSURE SHIFTING EASTWARD FROM THE
NORTHEASTERN PACIFIC FAVORS AN ELEVATED RISK OF ATMOSPHERIC RIVER 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. A DEPARTING AREA OF MID-LEVEL LOW PRESSURE OVER THE EASTERN
CONTIGUOUS U.S. (CONUS) MAY LEAD TO MUCH BELOW NORMAL MINIMUM TEMPERATURES TO
PORTIONS OF THE SOUTHEAST BEFORE TEMPERATURES ARE EXPECTED TO MODERATE LATER IN
WEEK-2. 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, TUE, DEC 24, AND THU-FRI, DEC 26-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE PACIFIC NORTHWEST AND PORTIONS OF
CALIFORNIA, TUE-SUN, DEC 24-29.
MODERATE RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS, TUE, DEC 24, AND THU-FRI, DEC 26-27.
SLIGHT RISK OF HIGH ELEVATION SNOWFALL FOR THE CASCADES, KLAMATH, AND SIERRA
NEVADA MOUNTAINS TUE-SUN, DEC 24-29.
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, TUE-SUN, DEC 24-29.
MODERATE RISK OF HIGH WINDS FOR THE PACIFIC NORTHWEST, NORTHERN INTERMOUNTAIN,
AND NORTHERN HIGH PLAINS, TUE-SAT, DEC 24-28.
SLIGHT RISK OF HIGH WINDS FOR THE WEST COAST AND INTERIOR WEST, TUE-MON, DEC
24-30.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS AND LOWER
MISSISSIPPI VALLEY, TUE-FRI, DEC 24-27.
SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES FOR THE SOUTHEAST, TUE, DEC 24.
SLIGHT RISK OF HIGH WINDS FOR SOUTHERN MAINLAND ALASKA AND THE SOUTHEAST,
TUE-SAT, DEC 24-28.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHEASTERN MAINLAND
ALASKA AND THE SOUTHEAST, TUE-SAT, DEC 24-28.
POSSIBLE FLOODING FOR PARTS OF CENTRAL AND NORTHERN CALIFORNIA TO SOUTHWESTERN
OREGON.
DETAILED SUMMARY
FOR THURSDAY DECEMBER 19 - MONDAY DECEMBER 23:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY DECEMBER 24 - MONDAY DECEMBER 30: LATE IN WEEK-1, DYNAMICAL MODELS
REMAIN IN GOOD AGREEMENT FEATURING AN AMPLIFIED TROUGH LIFTING OUT OVER EASTERN
NORTH AMERICA FOLLOWED BY A FAIRLY RAPID BUILDING OF 500-HPA HEIGHTS AND A
MODERATION OF TEMPERATURES FOR MANY AREAS EAST OF THE MISSISSIPPI BY THE START
OF WEEK-2. MUCH OF THE ANOMALOUSLY COLD AIR LOOKS TO PEAK IN STRENGTH NEXT
WEEKEND AND INTO EARLY NEXT WEEK BEFORE THE STRONG MEAN SURFACE HIGH BEGINS TO
WEAKEN OVER THE EASTERN CONUS, THOUGH THE GEFS AND ECWMF ENSEMBLES ARE SOMEWHAT
SPLIT ON ITS LOCATION AND HOW FAST THIS TRANSITION OCCURS. BASED ON RAW AND
CALIBRATED TEMPERATURE TOOLS, THE ECMWF FAVORS A MORE PERSISTENT SOLUTION WHERE
THERE REMAINS 30-40% CHANCES FOR MINIMUM TEMPERATURES FALLING BELOW THE 15TH
PERCENTILE OVER THE SOUTHEAST. DUE MINIMUM TEMPERATURE THRESHOLDS BEING LESS
STRINGENT FOR THIS PART OF THE COUNTRY, AND TO CAPTURE ANY LINGERING HAZARDOUS
COLD POTENTIAL ASSOCIATED WITH THE EXITING TROUGH, A SLIGHT RISK OF MUCH BELOW
NORMAL TEMPERATURES IS POSTED FOR DAY 8 (DEC 24).
FROM A HAZARDS PERSPECTIVE, THE MORE CONCERNING HEIGHT PATTERN LIES UPSTREAM AS
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 CONDUCIVE FOR ENHANCED ONSHORE FLOW AND
POSSIBLE ATMOSPHERIC RIVER (AR) ACTIVITY. ONE OR MORE SHORTWAVE FEATURES ARE
EVIDENT IN THE ENSEMBLES PROPAGATING INTO THE NORTHWESTERN CONUS WHICH IS
CONSISTENT WITH MULTIPLE INCREASES IN THE GEFS AND ECMWF INTEGRATED VAPOR
TRANSPORT (IVT) TOOLS ON AND AROUND DAY 8 (DEC 24) AND DAYS 10-12 (DEC 26-DEC
28). ELEVATED CHANCES (30-40%) OF 3-DAY PRECIPITATION AMOUNTS EXCEEDING THE
85TH PERCENTILE AND 2 INCHES (1 INCH) ARE DEPICTED IN THE GEFS AND ECWMF PETS
OVER THE PACIFIC NORTHWEST (PARTS OF CALIFORNIA) THROUGH THE MIDDLE OF THE
PERIOD. ALTHOUGH THE GEFS PET BECOMES WEAKER WITH WET SIGNALS THAN THE ECMWF
WITH TIME, THERE IS A CLEAR TROPICAL LINKAGE IN PERCENTILE SPACE IN BOTH PETS
REFLECTIVE OF A CONTINUED AR RISK, WITH MORE ROBUST PROBABILITIES IN THE
UNCALIBRATED ECWMF (40-70%) FOR AMOUNTS EXCEEDING 2 INCHES OVER THE WEST COAST
DURING EARLY AND MIDDLE PORTIONS OF THE PERIOD. THEREFORE, A MODERATE RISK OF
HEAVY PRECIPITATION IS POSTED OVER THE WEST COAST, WITH VALID PERIODS
CONCURRENT WITH THE MOISTURE INCREASES IN THE IVT TOOLS, AS WELL AS A BROADER
CORRESPONDING SLIGHT RISK AREA FOR DEC 24-29. THE INCREASE IN MID-TROPOSPHERIC
MOISTURE ALSO SUPPORTS AN ACCOMPANYING MODERATE AND SLIGHT RISK HAZARD FOR HIGH
ELEVATION HEAVY SNOWFALL OVER THE WEST COAST, VALID FOR THE SAME PERIODS AS THE
HEAVY PRECIPITATION RISKS. AS MUCH OF THE MOISTURE WORKS ITS WAY FURTHER
INLAND, AN ADDITIONAL MODERATE (DEC 25-28) AND SLIGHT RISK (DEC 24-29) OF HEAVY
SNOW IS POSTED OVER HIGHER ELEVATIONS OF THE INTERIOR WEST WHICH IS ALSO
SUPPORTED BY THE PETS AND UNCALIBRATED SNOWFALL GUIDANCE.
MORE NOTABLE THAN THE CALIBRATED PRECIPITATION AND SNOW WATER EQUIVALENT (SWE)
GUIDANCE ARE SOME OF THE SIGNALS 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 24-28 GIVEN THE ECMWF, ALONG WITH A BROAD RISK FOR
HIGH WINDS ISSUED FOR THE WESTERN THIRD OF THE CONUS FOR ALL OF WEEK-2. AND
WHILE ABOVE-NORMAL PRECIPITATION IS FAVORED FOR MUCH OF THE WEST COAST, THE
PATTERN ALSO SUPPORTS THE POTENTIAL FOR SANTA ANA WIND RISK 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 ARE SOME INDICATIONS IN THE ENSEMBLES FOR SHORTWAVE TROUGHING TO
THE EAST OF THE FOUR CORNERS THROUGH THE MIDDLE OF WEEK-2, WITH SOME OF THIS
SHORTWAVE ENERGY POSSIBLY TIED TO THE AFOREMENTIONED FIRST DISTURBANCE
ENCROACHING THE WEST COAST. THERE IS NOT QUITE ENOUGH SUPPORT FOR SURFACE LOW
FORMATION IN THE LEE OF THE ROCKIES, HOWEVER ENSEMBLES DO SHOW MORE RETURN FLOW
FROM THE GULF OF MEXICO WITH THE COLD SURFACE HIGH SLIDING OFFSHORE INTO THE
ATLANTIC, WITH INCREASED PRECIPITATION AMOUNTS OVER THE SOUTH-CENTRAL CONUS IN
BOTH THE RAW AND CALIBRATED GUIDANCE. AS A RESULT, A SLIGHT RISK OF HEAVY
PRECIPITATION IS POSTED OVER THE ARKLATEX REGION FOR DEC 24-27 WHERE PETS
DEPICT INCREASED CHANCES FOR AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1 INCH.
WHILE PERHAPS NOT THE SAME HAZARD POTENTIAL CALIBER AS THE WEST COAST, 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, BOTH VALID FOR DEC 24-28.
FORECASTER: NICK NOVELLA
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