531
FXUS21 KWNC 111957
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 11 2023
SYNOPSIS: THE WEEK-2 MID-LEVEL HEIGHT PATTERN ACROSS NORTH AMERICA IS FORECAST
TO BE DOMINATED BY MID-LEVEL LOW PRESSURE ACROSS ALASKA AND THE EASTERN
PACIFIC, WITH BROAD MID-LEVEL HIGH PRESSURE DOWNSTREAM OVER MUCH OF THE U.S.
AND CANADA. THIS FAVORS INCREASED HEAVY PRECIPITATION CHANCES ACROSS
SOUTHEASTERN ALASKA, WITH ENHANCED ONSHORE FLOW CONTRIBUTING TO ELEVATED
CHANCES FOR HEAVY PRECIPITATION ACROSS PARTS OF CALIFORNIA EARLY IN THE PERIOD.
ATMOSPHERIC RIVER SIGNALS INCREASE LATER IN WEEK-2 ALONG THE WEST COAST OF THE
CONUS AS MID-LEVEL LOW PRESSURE DIGS FURTHER INTO THE EASTERN PACIFIC. AN
ANOMALOUSLY WARM PATTERN IS FORECAST OVER MUCH OF THE CONUS LEADING UP TO
CHRISTMAS, WITH A POTENTIAL SURFACE LOW NEAR THE EASTERN SEABOARD LATE IN
WEEK-1 AND POSSIBLY LINGERING INTO THE OUTSET OF WEEK-2.
HAZARDS
SLIGHT RISK OF PERIODS OF HEAVY PRECIPITATION FOR PORTIONS OF CALIFORNIA AND
OREGON, TUE-THU, DEC 19-21 AND SAT-MON, DEC 23-25.
SLIGHT RISK OF HIGH WINDS FOR COASTAL AREAS OF THE PACIFIC NORTHWEST AND
NORTHERN AND CENTRAL CALIFORNIA, SAT-MON, DEC 23-25.
SLIGHT RISK OF HEAVY SNOW FOR PORTIONS OF THE CENTRAL SIERRA-NEVADA AND KLAMATH
MOUNTAINS, TUE-THU, DEC 19-21.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEAST ALASKA, TUE-MON,
DEC 19-25.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHEAST ALASKA AND SOUTH-CENTRAL
ALASKA, TUE-MON, DEC 19-25.
DETAILED SUMMARY
FOR THURSDAY DECEMBER 14 - MONDAY DECEMBER 18:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY DECEMBER 19 - MONDAY DECEMBER 25: EARLY IN WEEK-2, SHORTWAVE
MID-LEVEL TROUGHING IS FORECAST OFF THE COAST OF CALIFORNIA LEADING TO
INCREASED CHANCES FOR HEAVY PRECIPITATION ACROSS MUCH OF THE STATE. BOTH THE
GEFS AND ECMWF PROBABILISTIC EXTREMES TOOLS (PETS) INDICATE AT LEAST A 20
PERCENT CHANCE OF 3-DAY PRECIPITATION TOTALS EXCEEDING THE 85TH CLIMATOLOGICAL
PERCENTILE AND 1-INCH, WITH PROBABILITIES EXCEEDING 40 PERCENT IN THE ECMWF PET
ACROSS SOUTHERN CALIFORNIA. THE INTEGRATED VAPOR TRANSPORT (IVT) TOOL DERIVED
FROM THE 0Z GEFS AND ECMWF ENSEMBLES ALSO DEPICT ELEVATED CHANCES (GREATER THAN
50 PERCENT) FOR IVT VALUES TO EXCEED 250 KG/MS, WITH DAILY PRECIPITATION TOTALS
HIGHEST FOR DAYS 8-10 IN THE UNCALIBRATED GUIDANCE WITH A BREAK LIKELY AROUND
THE MIDDLE OF WEEK-2. FOLLOWING THE PREDICTED LULL IN ACTIVITY, THE 0Z GEFS AND
ECMWF ENSEMBLES DEPICT LONGWAVE TROUGHING BUILDING ACROSS THE EASTERN PACIFIC.
THIS FAVORS HEAVY PRECIPITATION CHANCES INCREASING ACROSS CALIFORNIA, POSSIBLY
EXTENDING FURTHER NORTH INTO OREGON BY THE END OF THE PERIOD AS THE IVT-DERIVED
TOOLS SUGGEST POTENTIAL FOR ATMOSPHERIC RIVER ACTIVITY, CONSISTENT WITH THE
PLACEMENT OF THE LONGWAVE TROUGH AND THE EL NINO CONDITIONS CURRENTLY PRESENT.
A SLIGHT RISK OF HEAVY PRECIPITATION IS HIGHLIGHTED ACROSS MUCH OF CALIFORNIA
AND WESTERN OREGON FOR 12/19-12/21 RELATED TO THE INITIAL SHORTWAVE TROUGH, AND
THEN AGAIN ON 12/23-12/25 FOR THE POTENTIAL ATMOSPHERIC RIVER EVENT. HEAVY SNOW
IS POSSIBLE ACROSS THE HIGHER ELEVATIONS OF CALIFORNIA, ALTHOUGH ABOVE-NORMAL
TEMPERATURES FAVOR HIGH SNOW LEVELS REDUCING THE LIKELIHOOD OF WIDESPREAD SNOW
IMPACTS. THEREFORE, THE SLIGHT RISK FOR HEAVY SNOW IS SCOPED TO THE KLAMATH
MOUNTAINS OF NORTHERN CALIFORNIA AND THE CENTRAL SIERRA NEVADAS FOR
12/19-12/21. THE ECMWF PET SUPPORTS INCLUDING A SLIGHT RISK OF HIGH WINDS
ACROSS COASTAL PORTIONS OF THE PACIFIC NORTHWEST AND NORTHERN AND CENTRAL
CALIFORNIA FOR 12/23-12/25, WITH PROBABILITIES EXCEEDING 20 PERCENT FOR WIND
SPEEDS GREATER THAN THE 85TH CLIMATOLOGICAL PERCENTILE AND 25-MPH, ALSO
CONSISTENT WITH THE PREDICTED INCREASE IN ONSHORE FLOW DURING THIS TIMEFRAME.
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 SOUTH-CENTRAL
AND SOUTHEASTERN ALASKA, ALONG WITH THE POTENTIAL FOR WIND SPEEDS EXCEEDING
40-MPH. THEREFORE, SLIGHT RISKS OF HEAVY PRECIPITATION AND HIGH WINDS CONTINUE
ACROSS THESE AREAS FOR DEC 19-25 (ENTIRETY OF WEEK-2). PROBABILITIES FOR 3-DAY
PRECIPITATION EXCEEDING THE 85TH PERCENTILE INCREASE TO 40 TO 60 PERCENT IN THE
ECMWF PET OVER PARTS OF SOUTHEAST ALASKA BY THE SECOND HALF OF THE PERIOD, WITH
THE GEFS BEING MORE TONED DOWN. TRENDS WILL BE MONITORED, AND IF AGREEMENT
IMPROVES BETWEEN THE GEFS AND ECMWF, A MODERATE RISK MAY BE CONSIDERED IN
FUTURE OUTLOOKS. THE ENHANCED SOUTHERLY FLOW ALSO FAVORS NEAR- TO ABOVE-NORMAL
TEMPERATURES ACROSS MUCH OF ALASKA.
A TRANQUIL PATTERN IS FAVORED ACROSS MUCH OF THE CENTRAL AND EASTERN CONUS WITH
RIDGING GENERALLY LEADING TO ABOVE-NORMAL TEMPERATURES OVER MOST AREAS. THE
LARGEST POSITIVE WEEK-2 MEAN TEMPERATURE ANOMALIES (+10 DEG F OR GREATER) ARE
DEPICTED ACROSS THE NORTHERN PLAINS AND UPPER MISSISSIPPI VALLEY IN THE
GEFS/ECMWF CONSOLIDATED REFORECAST TOOL. RECENT GFS AND ECMWF DETERMINISTIC AND
ENSEMBLE GUIDANCE DEPICTS SURFACE LOW PRESSURE ALONG THE EASTERN SEABOARD LATE
IN WEEK-1 BRINGING INCREASED CHANCES OF PRECIPITATION, POSSIBLY CONTINUING INTO
THE OUTSET OF WEEK-2, WITH MOST AREAS FAVORED TO HAVE RAIN DUE TO THE LACK OF
COLD AIR. DEPENDING ON THE STRENGTH AND TRACK OF THE SYSTEM, GUSTY NORTHERLY
FLOW AND SOME LAKE EFFECT SNOW MAY OCCUR ON THE BACKSIDE OF THE SYSTEM EARLY IN
WEEK-2, ALTHOUGH CONFIDENCE IS TOO LOW TO INCLUDE ANY RELATED HAZARDS.
FORECASTER: THOMAS COLLOW
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page