813
FXUS21 KWNC 081910
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 08 2023
SYNOPSIS: A TRANSIENT MID-LEVEL FLOW PATTERN IS FORECAST ACROSS MUCH OF NORTH
AMERICA DURING WEEK-2 WITH MID-LEVEL HIGH PRESSURE SHIFTING EASTWARD ACROSS THE
CONTIGUOUS U.S. (CONUS), AND MID-LEVEL LOW PRESSURE OVER ALASKA AND DIGGING
TOWARD THE WEST COAST OF THE CONUS EARLY IN THE PERIOD. MULTIPLE MODELS
MAINTAIN ELEVATED CHANCES FOR HEAVY PRECIPITATION (HIGH ELEVATION SNOW) ACROSS
PORTIONS OF CALIFORNIA EARLY IN THE PERIOD. HOWEVER, SIGNALS ARE WEAKER
REGARDING THE POTENTIAL FOR PRECIPITATION TOTALS REACHING HAZARDS THRESHOLDS
ACROSS THE PACIFIC NORTHWEST WHERE THE PRECIPITATION CLIMATOLOGY IS HIGHER.
ABOVE-NORMAL TEMPERATURES ARE FORECAST ACROSS MOST OF THE CONUS DURING WEEK-2,
WITH MODELS CONTINUING TO INDICATE POTENTIAL SURFACE LOW DEVELOPMENT NEAR OR
OFFSHORE THE EAST COAST, ALTHOUGH THERE IS LITTLE CONSISTENCY IN TERMS OF
TIMING OR TRACKING.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF CALIFORNIA AND SOUTHERN
OREGON, SAT-SUN, DEC 16-17.
SLIGHT RISK OF HEAVY SNOW FOR PORTIONS OF THE SIERRA-NEVADA AND KLAMATH
MOUNTAINS, SAT-SUN, DEC 16-17.
SLIGHT RISK OF HIGH WINDS FOR COASTAL PORTIONS OF CALIFORNIA AND SOUTHERN
OREGON, SAT-SUN, DEC 16-17.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEAST ALASKA, SAT-FRI,
DEC 16-22.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHEAST ALASKA AND SOUTH-CENTRAL
ALASKA, SAT-FRI, DEC 16-22.
DETAILED SUMMARY
FOR MONDAY DECEMBER 11 - FRIDAY DECEMBER 15:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY DECEMBER 16 - FRIDAY DECEMBER 22: MID-LEVEL TROUGHING IS FORECAST
TO AMPLIFY ACROSS THE EASTERN PACIFIC AND NEAR THE WEST COAST OF THE CONUS
AROUND THE OUTSET OF WEEK-2. BOTH THE UNCALIBRATED 0Z GEFS AND ECMWF ENSEMBLES
INDICATE INCREASED MOISTURE ADVECTION OVER CALIFORNIA ON DAY-8 (12/16), WITH
BOTH OF THE CORRESPONDING PROBABILISTIC EXTREMES TOOLS (PETS) DEPICTING AT
LEAST A 20 PERCENT CHANCE OF PRECIPITATION TOTALS EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE AND 1-INCH OVER SOME AREAS. A COMBINATION OF THE
TROUGHING QUICKLY MOVING INLAND ACROSS THE CONUS AND RISING MID-LEVEL HEIGHTS
FAVOR A QUICK END TO THE ELEVATED PRECIPITATION CHANCES ACROSS CALIFORNIA, WITH
THE ASSOCIATED SLIGHT RISKS FOR HEAVY PRECIPITATION, HIGH ELEVATION HEAVY SNOW,
AND COASTAL HIGH WINDS VALID FOR DEC 16-17 ONLY. COMPARED TO YESTERDAY, THE
GEFS PET HAS DIMINISHED THE SIGNAL FOR HAZARDOUS PRECIPITATION ACROSS THE
PACIFIC NORTHWEST, ALTHOUGH OCCASIONAL EPISODES OF HEAVY PRECIPITATION CANNOT
BE RULED OUT, ESPECIALLY LATER IN THE PERIOD ACROSS PARTS OF COASTAL WASHINGTON
AS ENHANCED ONSHORE FLOW SPREADS SOUTHWARD WITH TIME FROM SOUTHEAST ALASKA AND
BRITISH COLUMBIA. WHILE THE GEFS AND ECMWF PETS DEPICT AT LEAST A 20 PERCENT
CHANCE OF 3-DAY PRECIPITATION TOTALS EXCEEDING 1.5 INCHES OVER COASTAL PARTS OF
THE PACIFIC NORTHWEST, THE HIGHER PRECIPITATION CLIMATOLOGY RESULTS IN
PROBABILITIES FOR EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE OF LESS THAN 20
PERCENT. GIVEN THIS TREND, THE ASSOCIATED HAZARDS ARE REMOVED FROM THE REGION
AND ONLY INCLUDE PORTIONS OF NORTHERN AND CENTRAL CALIFORNIA AND SOUTHERN
OREGON.
SURFACE LOW PRESSURE FORECAST OVER THE GULF OF ALASKA FAVORS INCREASED ONSHORE
FLOW OVER SOUTHEASTERN ALASKA THROUGHOUT WEEK-2. BOTH THE GEFS AND ECMWF PETS
INDICATE AT LEAST A 20 PERCENT CHANCE OF 3-DAY PRECIPITATION TOTALS EXCEEDING
THE 85TH CLIMATOLOGICAL PERCENTILE AND 2-INCHES ACROSS PARTS OF THE REGION,
ALONG WITH THE POTENTIAL FOR WIND SPEEDS EXCEEDING 40-MPH. THEREFORE, SLIGHT
RISKS OF HEAVY PRECIPITATION AND HIGH WINDS CONTINUE ACROSS SOUTHEAST ALASKA
FOR DEC 16-22 (ENTIRETY OF WEEK-2). THE PETS SUPPORT A WESTWARD EXPANSION OF
THE HEAVY PRECIPITATION HAZARD TO INCLUDE MORE OF COASTAL SOUTHEASTERN MAINLAND
ALASKA, THE PRINCE WILLIAM SOUND REGION, AND THE EASTERN KENAI PENINSULA. THE
ENHANCED SOUTHERLY FLOW ALSO FAVORS ABOVE-NORMAL TEMPERATURES ACROSS
SOUTH-CENTRAL AND SOUTHEASTERN ALASKA. CONVERSELY, INCREASED NORTHERLY FLOW
OVER WESTERN ALASKA FAVORS BELOW-NORMAL TEMPERATURES OVER MUCH OF THE REGION
DURING WEEK-2, WITH THE LARGEST NEGATIVE TEMPERATURE ANOMALIES PREDICTED OVER
THE SOUTHWESTERN MAINLAND, CONSISTENT WITH THE REFORECAST CONSOLIDATED WEEK-2
MEAN TEMPERATURE FORECAST TOOL. ALTHOUGH MINIMUM TEMPERATURES ARE NOT EXPECTED
TO REACH HAZARDS CRITERIA, THE RELATIVELY COLDER TEMPERATURES FOLLOWING A VERY
WARM NOVEMBER IS LIKELY TO AID IN RIVER FREEZES.
A TRANQUIL PATTERN IS FAVORED ACROSS MUCH OF THE CENTRAL AND EASTERN CONUS WITH
RIDGING GENERALLY LEADING TO ABOVE-NORMAL TEMPERATURES OVER MOST AREAS. HEAVY
PRECIPITATION COMBINED WITH THESE RELATIVELY WARMER TEMPERATURES OVER PORTIONS
OF NEW ENGLAND EARLY IN WEEK-1 MAY RESULT IN DECREASING SNOW COVER AND
LOCALIZED FLOODING THAT MAY PERSIST INTO WEEK-2. AS THE TROUGHING IN THE WEST
SHIFTS FURTHER EASTWARD, THERE ARE SOME INDICATIONS OF A POTENTIALLY MORE
ACTIVE PATTERN DEVELOPING ACROSS THE EAST, WITH SOME EVIDENCE OF HIGH LATITUDE
BLOCKING BEGINNING TO EMERGE BY THE END OF THE PERIOD. THE LAST SEVERAL RUNS OF
THE GFS AND ECMWF DETERMINISTIC AND ENSEMBLE GUIDANCE DEPICT AN ELEVATED
POTENTIAL FOR SURFACE LOW PRESSURE DEVELOPING NEAR OR OFFSHORE THE EASTERN U.S.
AROUND DAY-10 (DEC 18), AND THEN AGAIN TOWARD THE END OF THE PERIOD. HOWEVER,
THERE IS LITTLE RUN TO RUN CONSISTENCY REGARDING TRACK, STRENGTH, AND TIMING OF
SUCH SYSTEMS PRECLUDING ANY RELATED HAZARDS AT THIS TIME. THIS PATTERN
EVOLUTION WILL CONTINUE TO BE MONITORED, ESPECIALLY CONSIDERING THE APPROACHING
BUSIER HOLIDAY TRAVEL PERIOD IN LATE-DECEMBER.
FORECASTER: THOMAS COLLOW
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page