056
FXUS21 KWNC 071857
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 07 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 ACROSS ALASKA AND DIGGING
TOWARD THE WEST COAST OF THE CONUS EARLY IN THE PERIOD. ALTHOUGH MID-LEVEL
HEIGHTS ARE FORECAST TO INCREASE BY THE MIDDLE OF THE PERIOD ALONG THE WEST
COAST, A TIGHT PRESSURE GRADIENT FAVORS PERSISTENCE OF THE ENHANCED ONSHORE
FLOW PATTERN ACROSS SOUTHEAST ALASKA AND THE WESTERN CONUS, RESULTING IN
ELEVATED CHANCES OF HEAVY PRECIPITATION. THERE ARE SOME SIGNALS FOR SURFACE LOW
PRESSURE DEVELOPING NEAR THE EAST COAST IN THE MIDDLE OF THE PERIOD, ALTHOUGH
THE TRANSIENT PATTERN AND LACK OF MODEL AGREEMENT REDUCES CONFIDENCE IN A
SIGNIFICANT EVENT AT THIS TIME.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR MUCH OF THE PACIFIC NORTHWEST AND THE
NORTHERN HALF OF CALIFORNIA, SAT-TUE, DEC 16-19.
SLIGHT RISK OF HEAVY SNOW FOR PORTIONS OF THE SIERRA-NEVADA AND KLAMATH
MOUNTAINS, CASCADES, AND NORTHERN ROCKIES, SAT-TUE, DEC 16-19.
SLIGHT RISK OF HIGH WINDS FOR COASTAL PORTIONS OF NORTHERN CALIFORNIA AND THE
PACIFIC NORTHWEST, SAT-TUE, DEC 16-19.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEAST ALASKA, FRI-THU,
DEC 15-21.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHEAST ALASKA AND SOUTH-CENTRAL
ALASKA, FRI-THU, DEC 15-21.
DETAILED SUMMARY
FOR SUNDAY DECEMBER 10 - THURSDAY DECEMBER 14:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY DECEMBER 15 - THURSDAY DECEMBER 21: EARLY IN WEEK-2, TROUGHING IS
FORECAST TO AMPLIFY ACROSS THE EASTERN PACIFIC AND NEAR THE WEST COAST OF THE
CONUS. THIS FAVORS A PERIOD OF ENHANCED ONSHORE FLOW ALONG THE WEST COAST. BY
THE MIDDLE OF THE PERIOD, A TREND TOWARD MORE RIDGING AND RISING MID-LEVEL
HEIGHTS IS FAVORED OVER THE REGION. HOWEVER, A TIGHT MID-LEVEL PRESSURE
GRADIENT IS FORECAST TO REMAIN IN PLACE DUE TO THE TROUGHING NEAR ALASKA
MAINTAINING THE ENHANCED ONSHORE FLOW PATTERN ALONG THE WEST COAST, WITH THE 0Z
GEFS INDICATING A STRONGER PRECIPITATION SIGNAL OVER THE NORTHWESTERN U.S.
COMPARED TO THE 0Z ECMWF ENSEMBLE. THE GEFS-BASED INTEGRATED VAPOR TRANSPORT
(IVT) TOOL INDICATES ELEVATED PROBABILITIES FOR IVT TO EXCEED 250 KG/MS FOR
MUCH OF THE PERIOD WHILE A MORE INTERMITTENT SIGNAL IS DEPICTED IN THE
ECMWF-BASED IVT GUIDANCE. THE ECMWF PROBABILISTIC EXTREMES TOOL (PET) CONTINUES
TO INDICATE AT LEAST A 20 PERCENT CHANCE FOR 3-DAY PRECIPITATION TOTALS TO
EXCEED THE 85TH CLIMATOLOGICAL PERCENTILE AND 1-INCH ACROSS PARTS OF NORTHERN
AND CENTRAL CALIFORNIA, WITH THE GEFS PET DEPICTING THESE ELEVATED
PROBABILITIES FURTHER NORTH ACROSS THE PACIFIC NORTHWEST. CLIMATOLOGY FAVORS
HIGHER PRECIPITATION TOTALS TO THE NORTH, AND THE ECMWF PET DEPICTS SOME AREAS
HAVING AT LEAST A 20 PERCENT CHANCE 3-DAY PRECIPITATION TOTALS EXCEED 1.5 OR
EVEN 2 INCHES ACROSS PARTS OF THE PACIFIC NORTHWEST, DESPITE THE WEAKER SIGNALS
IN PERCENTILE SPACE. AS A RESULT, THE SLIGHT RISK FOR HEAVY PRECIPITATION IS
EXTENDED NORTHWARD COMPARED TO YESTERDAY, AND INCLUDES THE NORTHERN HALF OF
CALIFORNIA AND THE PACIFIC NORTHWEST, AND IS VALID DEC 16-19 BASED ON FORECAST
DAILY PRECIPITATION TOTALS IN THE UNCALIBRATED 0Z GEFS AND ECMWF ENSEMBLES. THE
ACCOMPANYING SLIGHT RISKS OF HIGH WINDS AND HEAVY SNOW ARE ALSO EXPANDED TO
INCLUDE PARTS OF THE COASTAL PACIFIC NORTHWEST AND CASCADES RESPECTIVELY, WITH
A SLIGHT RISK OF HEAVY SNOW NOW POSTED ACROSS THE NORTHERN ROCKIES FOR THE SAME
TIME FRAME.
A TRANQUIL PATTERN IS FAVORED ACROSS MUCH OF THE CENTRAL AND EASTERN CONUS WITH
RIDGING GENERALLY FAVORING ABOVE-NORMAL TEMPERATURES OVER MOST AREAS. AROUND
THE MIDDLE OF THE PERIOD, THERE ARE SOME INDICATIONS, PARTICULARLY IN THE 0Z
ECMWF ENSEMBLE, OF SHORTWAVE TROUGHING DEVELOPING ACROSS THE EAST WHICH COULD
RESULT IN SURFACE LOW PRESSURE FORMING NEAR THE EASTERN SEABOARD. THE ECMWF PET
DEPICTS SOME AREAS ALONG THE EAST COAST HAVING AT LEAST A 20 PERCENT CHANCE
3-DAY PRECIPITATION EXCEEDS THE 85TH CLIMATOLOGICAL PERCENTILE AND 1-INCH IN
THE DAY 10-12 TIMEFRAME. WINTRY WEATHER MAY BECOME A CONCERN OVER SOME AREAS
GIVEN THE TIME OF YEAR, ALTHOUGH A SIGNIFICANT EVENT SEEMS UNLIKELY DUE TO THE
LACK OF A PROMINENT COLD AIR MASS, ALONG WITH THE NORTH ATLANTIC OSCILLATION
FORECAST TO BE IN A NEUTRAL TO POSITIVE PHASE. DUE TO THE UNCERTAINTY AND
LITTLE SIGNAL IN THE GEFS PET, NO RELATED HAZARDS ARE ISSUED ACROSS THE EAST,
BUT THE POTENTIAL WILL CONTINUE TO BE MONITORED.
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. BOTH TOOLS NOW
PROLONG THESE ELEVATED PROBABILITIES INTO THE DAY 12-14 INTERVAL AS WELL.
THEREFORE, SLIGHT RISKS OF HEAVY PRECIPITATION AND HIGH WINDS ARE POSTED ACROSS
SOUTHEAST ALASKA FOR DEC 15-21 (ENTIRETY OF WEEK-2). RIDGING FORECAST ACROSS
THE BERING SEA AND INCREASED NORTHERLY FLOW OVER ALASKA FAVORS BELOW-NORMAL
TEMPERATURES OVER MUCH OF NORTHERN AND WESTERN ALASKA DURING WEEK-2, WITH THE
LARGEST NEGATIVE TEMPERATURE ANOMALIES PREDICTED OVER THE SOUTHWESTERN
MAINLAND, CONSISTENT WITH THE REFORECAST CONSOLIDATED 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.
FORECASTER: THOMAS COLLOW
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