092
FXUS21 KWNC 051915
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 05 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 THRESHOLDS.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR MUCH OF CALIFORNIA, THU-SUN, DEC 14-17.
SLIGHT RISK OF HEAVY SNOW FOR THE SIERRA-NEVADA AND KLAMATH MOUNTAINS, THU-SUN,
DEC 14-17.
SLIGHT RISK OF HIGH WINDS FOR COASTAL CALIFORNIA, THU-SUN, DEC 14-17.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEAST ALASKA, WED-FRI,
DEC 13-15.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHEAST ALASKA, WED-FRI, DEC 13-15.
DETAILED SUMMARY
FOR FRIDAY DECEMBER 08 - TUESDAY DECEMBER 12:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY DECEMBER 13 - TUESDAY DECEMBER 19: 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.
ACROSS THE GREAT LAKES AND NORTHEAST, SURFACE HIGH PRESSURE AND SOUTHWESTERLY
MID-LEVEL FLOW LOOK LIKELY TO BRING AN END TO ANY THREAT FOR HIGH WINDS AND
LAKE EFFECT SNOW ACROSS THE REGION. THEREFORE, THE WIND AND SNOW HAZARDS HAVE
BEEN ALLOWED TO TIME OFF TODAY. WHILE A PERIOD OF BELOW-NORMAL TEMPERATURES IS
POSSIBLE OVER THE SOUTHEAST IN WEEK-2, IMPACTS TO VEGETATION ARE FORECAST TO BE
MINIMAL, EVEN IF SOME AREAS HAVE TEMPERATURES NEAR OR BELOW 40 DEG F,
PRECLUDING A RELATED HAZARD.
THERE IS GROWING CONFIDENCE IN THE WEEK-2 FORECAST ACROSS THE WEST DUE TO
INCREASING CONVERGENCE AMONG THE MODELS FOR A PERIOD OF HEAVY PRECIPITATION AND
HIGH WIND ACROSS PORTIONS OF CALIFORNIA. THE GEFS, ECMWF, AND CANADIAN
ENSEMBLES DEPICT POSITIVE 500-HPA HEIGHTS ACROSS WESTERN NORTH AMERICA AND
NEGATIVE 500-HPA HEIGHT ANOMALIES UPSTREAM ACROSS THE EASTERN PACIFIC. THE
ECMWF IS MORE AMPLIFIED WITH THIS PATTERN RELATIVE TO THE GEFS AND CANADIAN BUT
ALL ARE CONVERGING UPON THIS SOLUTION. THIS PATTERN WOULD PROMOTE ENHANCED
ONSHORE FLOW INTO PARTS OF CALIFORNIA, PARTICULARLY BY THE MIDDLE OF THE PERIOD
AS THE PATTERN PROGRESSES EASTWARD. 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.
SOME AREAS, PARTICULARLY IN THE GEFS, INDICATE CHANCES OF GREATER THAN 2 INCHES
OF PRECIPITATION IN PARTS OF NORTHERN CALIFORNIA. RAW MODEL TOOLS ALSO INDICATE
SUPPORT FOR THIS OUTCOME. THE INTEGRATED VAPOR TRANSPORT (IVT) TOOL IS NOT AS
ROBUST AS THE PETS BUT HAS BEEN TRENDING IN THE DIRECTION OF A HIGHER WATER
VAPOR TRANSPORT INTO CALIFORNIA DURING THE MIDDLE OF THE PERIOD. THEREFORE, A
SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR MUCH OF CALIFORNIA FOR DAY
9-12, DEC 14-17. MEANWHILE, A SLIGHT RISK OF HEAVY SNOW IS POSTED FOR HIGHER
ELEVATIONS OF THE SIERRA-NEVADA AND KLAMATH MOUNTAINS FOR DEC 14-17. FINALLY,
PETS ALSO SUPPORT THE ADDITION OF A COASTAL HIGH WIND HAZARD ASSOCIATED WITH
THIS SYSTEM THUS A CORRESPONDING HAZARD IS POSTED FOR DEC 14-17 AS WELL.
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 13-15. 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
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