566
FXUS21 KWNC 061930
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST DECEMBER 06 2024
SYNOPSIS: MID-LEVEL LOW PRESSURE IS FORECAST ACROSS THE WEST COAST OF THE
CONTIGUOUS U.S. (CONUS) EARLY IN WEEK-2 FAVORING AN ACTIVE PATTERN IN THE WEST,
WITH INCREASED CHANCES FOR HEAVY COASTAL RAIN AND HIGH ELEVATION HEAVY SNOW.
THERE ARE INCREASED SIGNALS IN THE MODEL GUIDANCE FOR A SURFACE LOW EJECTING
OUT OF THE ROCKIES RESULTING IN ENHANCED CHANCES FOR HEAVY PRECIPITATION ACROSS
PARTS OF THE EAST-CENTRAL CONUS, WITH HEAVY SNOW POSSIBLE ACROSS THE UPPER
MIDWEST. A TREND TOWARD MID-LEVEL HIGH PRESSURE AND WARMER TEMPERATURES IS
ANTICIPATED FOR MUCH OF THE EAST.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION ACROSS PORTIONS OF THE PACIFIC NORTHWEST AND
THE NORTHERN HALF OF CALIFORNIA, SAT-MON, DEC 14-16.
SLIGHT RISK OF HEAVY PRECIPITATION ACROSS PORTIONS OF THE SOUTHERN PLAINS,
MIDDLE AND LOWER MISSISSIPPI VALLEY, AND TENNESSEE VALLEY, SUN-TUE, DEC 15-17.
SLIGHT RISK OF HEAVY SNOW ACROSS PORTIONS OF THE CASCADES, KLAMATH, AND SIERRA
NEVADA, SAT-MON, DEC 14-16.
SLIGHT RISK OF HEAVY SNOW ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND
GREAT LAKES, SUN-TUE, DEC 15-17.
SLIGHT RISK OF HEAVY SNOW ACROSS PORTIONS OF THE NORTHERN AND CENTRAL ROCKIES,
SAT-MON, DEC 14-16.
SLIGHT RISK OF HIGH WINDS ACROSS MUCH OF THE NORTHWESTERN QUADRANT OF THE CONUS
EXTENDING INTO THE NORTHERN AND CENTRAL PLAINS, SAT-MON, DEC 14-16.
DETAILED SUMMARY
FOR MONDAY DECEMBER 09 - FRIDAY DECEMBER 13:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY DECEMBER 14 - FRIDAY DECEMBER 20: THE 0Z ECENS AND GEFS ARE IN
GOOD AGREEMENT REGARDING TROUGHING ALONG THE WEST COAST OF THE CONUS DURING
WEEK-2, WHICH IS SUPPORTIVE OF INCREASED ONSHORE FLOW ACROSS THE WEST COAST.
HOWEVER, THE INTEGRATED VAPOR TRANSPORT (IVT) TOOLS AND THE PROBABILITIES FOR
PRECIPITATION AMOUNTS DEEMED HAZARDOUS IN THE PROBABILISTIC EXTREMES TOOLS
(PETS) ARE MARGINAL AT BEST, IN ADDITION TO MODELS DEPICTING A RETURN TO
RIDGING OVER THE WESTERN CONUS LATER IN WEEK-2. THE GEFS PET CONTINUES TO
DEPICT PARTS OF THE WEST COAST HAVING AT LEAST A 20 PERCENT CHANCE OF 3-DAY
PRECIPITATION TOTALS EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE AND 1-INCH
THROUGH THE MIDDLE OF THE PERIOD, WITH THE ECENS PET ONLY INDICATING THESE
ENHANCED PROBABILITIES ACROSS PARTS OF NORTHERN CALIFORNIA DURING THE EARLY
PART OF WEEK-2. TO MAINTAIN CONTINUITY, A SLIGHT RISK OF HEAVY PRECIPITATION
REMAINS POSTED ACROSS PORTIONS OF THE PACIFIC NORTHWEST EXTENDING SOUTHWARD
INTO THE NORTHERN HALF OF CALIFORNIA, DEC 14-16. AN ACCOMPANYING SLIGHT RISK OF
HEAVY SNOW IS ALSO POSTED ACROSS THE CASCADES, KLAMATH, SIERRA NEVADA, AND THE
NORTHERN AND CENTRAL ROCKIES, SUPPORTED BY THE GEFS SNOW WATER EQUIVALENT (SWE)
PET WHICH DEPICTS AT LEAST A 20 PERCENT CHANCE OF SWE EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE AND 1-INCH, AND THE UNCALIBRATED ECENS PROBABILITIES
INDICATING ELEVATED CHANCES FOR DAILY SNOWFALL TOTALS GREATER THAN 6 INCHES
OVER PARTS OF THE HIGHLIGHTED AREAS.
THE LARGE-SCALE PATTERN ALSO SUPPORTS A BROAD SLIGHT RISK OF HIGH WINDS, WHICH
IS HIGHLIGHTED DEC 14-16 ACROSS MUCH OF THE NORTHWESTERN QUADRANT OF THE CONUS
EXTENDING INTO THE GREAT PLAINS, WITH THE STRONGEST SIGNALS FOR WIND SPEEDS
GREATER THAN 25-MPH INDICATED FOR THE IMMEDIATE WEST COAST AND ALSO ACROSS THE
HIGH PLAINS TIED TO INCREASED DOWNSLOPE FLOW. PROBABILITIES FOR WIND SPEEDS
EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE ARE ELEVATED IN THE GEFS AND ECENS
PETS ACROSS THE INTERMOUNTAIN REGION, SUPPORTING AN INCREASED HIGH WIND RISK IN
THESE AREAS AS WELL.
A WARMING TREND IS LIKELY ACROSS THE EAST DURING WEEK-2, WHICH COULD RAISE
FLOODING CONCERNS ALONG PARTS OF THE GREAT LAKES, PARTICULARLY ERIE AND
ONTARIO, DUE TO EXPECTED SNOWMELT. RETURN FLOW IS ALSO FORECAST TO INCREASE
ACROSS THE EAST-CENTRAL CONUS ON THE BACKSIDE OF SURFACE HIGH PRESSURE. THE
ECENS AND GEFS PETS ARE IN BETTER AGREEMENT TODAY, BOTH DEPICTING PARTS OF THE
REGION HAVING AT LEAST A 20 PERCENT CHANCE OF 3-DAY PRECIPITATION TOTALS
EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE AND 1-INCH. THE DETERMINISTIC RUNS
OF THE 6Z GFS AND 0Z ECMWF ALONG WITH SEVERAL ENSEMBLE MEMBERS INDICATE A
SURFACE LOW EJECTING OUT OF THE ROCKIES INTO THE GREAT PLAINS AND TRACKING
TOWARD THE GREAT LAKES AROUND THE MIDDLE OF WEEK-2. THESE ENHANCED
PRECIPITATION SIGNALS SUPPORT A SLIGHT RISK FOR HEAVY PRECIPITATION ACROSS THE
SOUTHERN PLAINS, MIDDLE AND LOWER MISSISSIPPI VALLEY, AND TENNESSEE VALLEY, DEC
15-17. THIS MAY RESULT IN SOME LOCALIZED FLOODING ACROSS THE LOWER MISSISSIPPI
VALLEY GIVEN ANTECEDENT HEAVY RAINFALL FORECAST DURING WEEK-1, BUT UNCERTAINTY
REGARDING EXACT TIMING AND LOCATION OF THE HEAVIEST PRECIPITATION IN WEEK-2
PRECLUDES A RELATED FLOODING POSSIBLE HAZARD. THE SURFACE LOW TRACK COULD ALSO
FAVOR AN AREA OF HEAVY SNOW ON ITS BACKSIDE ACROSS THE UPPER MIDWEST OR GREAT
LAKES, AND THERE IS SOME MODEST SUPPORT IN THE GUIDANCE. ECENS PROBABILITIES
EXCEED 20 PERCENT FOR 3-DAY SNOWFALL GREATER THAN 4-INCHES OVER SOME AREAS, AND
THE GEFS PET DEPICTS A SMALL AREA WITH 3-DAY SWE EXCEEDING THE 85TH PERCENTILE
AND A HALF-INCH. AS A RESULT, A SLIGHT RISK FOR HEAVY SNOW IS ALSO POSTED
ACROSS PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND GREAT LAKES, DEC 15-17.
A VARIABLE MID-LEVEL HEIGHT PATTERN IS FORECAST ACROSS ALASKA DURING WEEK-2.
FOLLOWING SOME MID-LEVEL RIDGING EARLY IN THE PERIOD, AN AMPLIFIED TROUGH IS
FORECAST TO MOVE CLOSER TO THE ALEUTIANS BY THE SECOND HALF OF THE PERIOD,
RESULTING IN DECREASING MID-LEVEL HEIGHTS ACROSS THE STATE. THIS FEATURE ALSO
FAVORS INCREASING SOUTHERLY FLOW AND RENEWED HEAVY PRECIPITATION CHANCES ALONG
SOUTHERN AND SOUTHEASTERN COASTAL AREAS. THE GEFS AND ECENS PETS DEPICT AT
LEAST A 20 PERCENT CHANCE OF 3-DAY PRECIPITATION EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE AND 2-INCHES OVER SOME OF THESE AREAS BEGINNING ON
DAY-10 (DEC 16). FOR NOW, NO RELATED HAZARD IS ISSUED BUT TRENDS WILL CONTINUE
TO BE MONITORED. RENEWED ONSHORE FLOW IS ALSO POSSIBLE ACROSS THE NORTHWESTERN
CONUS BY THE END OF WEEK-2 DEPENDING ON THE POSITIONING OF THE DOWNSTREAM RIDGE
AXIS.
FORECASTER: THOMAS COLLOW
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