438
FXUS21 KWNC 061924
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 06 2023
SYNOPSIS: MID-LEVEL HIGH PRESSURE ACROSS WESTERN NORTH AMERICA AT THE ONSET OF
WEEK-2 IS FORECAST TO PROGRESS EASTWARD WHILE AMPLIFIED MID-LEVEL LOW PRESSURE
DEVELOPS OFF THE WEST COAST OF THE CONTIGUOUS U.S. (CONUS). THIS PATTERN FAVORS
INCREASED CHANCES FOR HEAVY RAIN, SNOW, AND HIGH WINDS TO PARTS OF CALIFORNIA
DURING WEEK-2. AN AREA OF SURFACE LOW-PRESSURE IS ANTICIPATED IN THE GULF OF
ALASKA THAT COULD BRING HEAVY RAIN AND GUSTY WINDS TO PORTIONS OF SOUTHEAST
ALASKA AND SOUTHERN MAINLAND ALASKA. ABOVE-NORMAL PRECIPITATION IS FAVORED IN
SOUTHERN FLORIDA BUT IS UNLIKELY TO REACH HAZARDOUS THRESHOLD.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR MUCH OF NORTHERN AND CENTRAL CALIFORNIA,
SAT-MON, DEC 16-18.
SLIGHT RISK OF HEAVY SNOW FOR THE SIERRA-NEVADA AND KLAMATH MOUNTAINS, SAT-MON,
DEC 16-18.
SLIGHT RISK OF HIGH WINDS FOR NORTHERN AND CENTRAL COASTAL CALIFORNIA, SAT-MON,
DEC 16-18.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEAST ALASKA, THU-SAT,
DEC 14-16.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHEAST ALASKA AND SOUTH-CENTRAL
ALASKA, THU-SAT, DEC 14-16.
DETAILED SUMMARY
FOR SATURDAY DECEMBER 09 - WEDNESDAY DECEMBER 13:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY DECEMBER 14 - WEDNESDAY DECEMBER 20: A FRONTAL SYSTEM IS PREDICTED
TO BRING PRECIPITATION TO PARTS OF FLORIDA AND THE SOUTHEAST EARLY IN WEEK-2.
THE ECMWF PROBABILISTIC EXTREMES TOOL (PET) DEPICTS ELEVATED PROBABILITIES (AT
LEAST 40 PERCENT) FOR 3-DAY PRECIPITATION TOTALS GREATER THAN THE 85TH
CLIMATOLOGICAL PERCENTILE ACROSS SOUTH FLORIDA PERSISTING FROM THE END OF
WEEK-1 INTO EARLY WEEK-2. HOWEVER, PRECIPITATION CLIMATOLOGIES DECREASE ACROSS
THE REGION DURING DECEMBER, AND PROBABILITIES FOR 3-DAY PRECIPITATION TOTALS
EXCEEDING 1-INCH ARE LESS THAN 20 PERCENT IN THE GEFS PET. THEREFORE, NO
RELATED HEAVY PRECIPITATION HAZARD IS POSTED ACROSS FLORIDA, ALTHOUGH LINGERING
ROUNDS OF HEAVY PRECIPITATION CANNOT BE RULED OUT, ESPECIALLY IF THE FRONT IS
SLOW TO MOVE OUT OF FLORIDA AS DEPICTED IN THE ECMWF GUIDANCE.
IN WESTERN NORTH AMERICA AN AREA OF STRONG POSITIVE HEIGHT ANOMALIES IS
FORECAST TO DEVELOP LATE IN WEEK-1 AND INTO WEEK-2. UPSTREAM OF THIS IS AN AREA
OF NEGATIVE 500-HPA HEIGHT ANOMALIES ACROSS THE EASTERN PACIFIC. THE MODELS
HAVE COME INTO BETTER AGREEMENT WITH A STRONGER MID-LEVEL TROUGH TODAY RELATIVE
TO YESTERDAY. AS THE PATTERN PROGRESSES EASTWARD THE THROUGH NEARS THE WEST
COAST OF THE CONUS BY DAY 9 OR 10 AND PRECIPITATION IS FORECAST TO BEGIN ACROSS
PARTS OF CALIFORNIA AT THIS TIME. THE OVERALL SIGNAL AMONG THE TOOLS INDICATES
A SLIGHTLY WEAKER SYSTEM ACROSS CALIFORNIA RELATIVE TO YESTERDAY BUT THE GEFS
AND ECMWF PETS INDICATE AT LEAST A 20% CHANCE OF 3-DAY PRECIPITATION EXCEEDING
THE 85TH PERCENTILE AND 1 INCH OF LIQUID EQUIVALENT FOR MUCH OF CALIFORNIA
DURING THE MIDDLE THREE DAY PERIOD. RAW MODEL TOOLS ALSO CONTINUE TO INDICATE
SUPPORT FOR THIS OUTCOME. THE INTEGRATED VAPOR TRANSPORT (IVT) TOOL CONTINUES
TO SHOW LOWER CHANCES FOR A SIGNIFICANT ATMOSPHERIC RIVER EVENT BUT SUPPORTS
CONTINUED ADVECTION OF MOISTURE INTO THE WESTERN CONUS DURING THE MIDDLE OF
WEEK-2. THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR MUCH OF
NORTHERN AND CENTRAL CALIFORNIA FOR DEC 16-18. AN ASSOCIATED RISK OF HEAVY SNOW
IS POSTED FOR THE SIERRA-NEVADA AND KLAMATH MOUNTAINS FOR THE SAME TIME PERIOD.
THE PETS CONTINUE TO SUPPORT A SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE
NORTHERN AND CENTRAL CALIFORNIA COAST LINE BUT NOT AS FAR SOUTH RELATIVE TO
YESTERDAY. THEREFORE, THE CORRESPONDING RISK OF HIGH WIND HAZARD’S SOUTHERN
EXTENT HAS BEEN TRIMMED RELATIVE TO YESTERDAY AND IS POSTED FOR DEC 16-18.
SURFACE LOW PRESSURE FORECAST OVER THE GULF OF ALASKA FAVORS INCREASED ONSHORE
FLOW OVER SOUTHEASTERN ALASKA FOR THE FIRST THREE DAYS OF THE PERIOD, WITH BOTH
THE GEFS AND ECMWF PETS INDICATING AT LEAST A 20 PERCENT CHANCE OF 3-DAY
PRECIPITATION TOTALS EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE AND 2-INCHES
ACROSS THE REGION, ALONG WITH THE POTENTIAL FOR WIND SPEEDS EXCEEDING 40-MPH.
THEREFORE, SLIGHT RISKS OF HEAVY PRECIPITATION AND HIGH WINDS ARE POSTED ACROSS
SOUTHEAST ALASKA FOR DEC 14-16. RIDGING FORECAST ACROSS THE BERING SEA AND
INCREASED NORTHERLY FLOW OVER ALASKA FAVORS BELOW-NORMAL TEMPERATURES OVER MUCH
OF THE STATE DURING WEEK-2, WITH THE LARGEST NEGATIVE TEMPERATURE ANOMALIES
PREDICTED OVER THE WESTERN MAINLAND. ALTHOUGH MINIMUM TEMPERATURES ARE NOT
EXPECTED TO REACH HAZARDS CRITERIA, THE ARRIVAL OF COLDER TEMPERATURES
FOLLOWING A VERY WARM NOVEMBER IS LIKELY TO AID IN RIVER FREEZES.
FORECASTER: RYAN BOLT
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page