568
FXUS21 KWNC 101901
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EST NOVEMBER 10 2023
SYNOPSIS: MODELS ARE CONTINUING TO INDICATE THE POSSIBILITY OF LEE CYCLOGENESIS
EARLY IN WEEK-2 AHEAD OF MID-LEVEL LOW PRESSURE FORECAST OVER THE SOUTHWESTERN
CONTIGUOUS U.S. (CONUS). THIS FAVORS ENHANCED PRECIPITATION CHANCES OVER
PORTIONS OF THE MIDDLE AND LOWER MISSISSIPPI VALLEY, WITH THE POTENTIAL
SHIFTING INTO THE NORTHEASTERN CONUS BY THE MIDDLE OF THE PERIOD, COINCIDING
WITH THE START OF AN EXPECTED BUSY TRAVEL WEEK AHEAD OF THE THANKSGIVING
HOLIDAY. MID-LEVEL LOW PRESSURE INITIALLY FORECAST OVER ALASKA FAVORS STRONG
ONSHORE FLOW ACROSS SOUTHEAST ALASKA, AND EXTENDING SOUTHWARD INTO THE
NORTHWESTERN CONUS. THIS MAY LEAD TO EPISODES OF HIGH WINDS AND HEAVY
PRECIPITATION ACROSS BOTH OF THESE AREAS EARLY IN THE PERIOD, WITH HAZARDS
THRESHOLDS MORE LIKELY TO BE REACHED ACROSS THE PACIFIC NORTHWEST.
HAZARDS
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE PACIFIC NORTHWEST AND
NORTHERN CALIFORNIA, SAT-MON, NOV 18-20.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE PACIFIC NORTHWEST, SAT-MON, NOV
18-20.
SLIGHT RISK OF HEAVY SNOW FOR PORTIONS OF THE CASCADES AND NORTHERN ROCKIES,
SAT-MON, NOV 18-20.
SLIGHT RISK OF HEAVY PRECIPITATION FROM THE MIDDLE AND LOWER MISSISSIPPI VALLEY
EXTENDING THROUGH THE NORTHEAST, SAT-TUE, NOV 18-21.
DETAILED SUMMARY
FOR MONDAY NOVEMBER 13 - FRIDAY NOVEMBER 17:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY NOVEMBER 18 - FRIDAY NOVEMBER 24: THE 0Z GEFS, ECMWF, AND CANADIAN
ENSEMBLES REMAIN IN GOOD AGREEMENT REGARDING 500-HPA TROUGHING ACROSS
SOUTHWESTERN AND SOUTH-CENTRAL CONUS EARLY IN WEEK-2. THE OVERALL PATTERN
DOWNSTREAM OVER THE EAST IS RATHER UNCERTAIN GIVEN THE POTENTIAL DEVELOPMENT OF
A SURFACE LOW ACROSS THE SOUTHEAST/GULF COAST IN WEEK-1 WHICH COULD LEAD TO
ENHANCED PRECIPITATION ALONG THE EASTERN SEABOARD LATE IN WEEK-1 INTO THE
OUTSET OF WEEK-2. TROPICAL CYCLONE (TC) DEVELOPMENT IS POSSIBLE ACROSS THE
CARIBBEAN DURING THE NEXT WEEK (30 PERCENT CHANCE IN THE NEXT 7-DAYS PER THE
NATIONAL HURRICANE CENTER UPDATE 1PM EST 11/10), WITH BOTH THE ECMWF AND GEFS
ENSEMBLES IN BETTER AGREEMENT COMPARED TO YESTERDAY WITH A NORTHEASTWARD TRACK
TOWARD JAMAICA, CUBA, HISPANIOLA, AND THE BAHAMAS. THE EVOLUTION OF THESE
SYSTEMS, AS WELL AS THE ELEVATED POTENTIAL FOR LEE CYCLOGENESIS IN WEEK-2, WILL
PLAY A ROLE IN DETERMINING THE SENSIBLE WEATHER ACROSS THE EAST DURING THE
FORECAST PERIOD.
IN TERMS OF LEE CYCLOGENESIS, THE DETERMINISTIC ECMWF AND GFS SOLUTIONS HAVE
NOT BEEN CONSISTENT, BUT THERE IS BETTER AGREEMENT IN THE ENSEMBLE MEAN
SOLUTIONS FROM BOTH MODELS. THE ECMWF AND GEFS PROBABILISTIC EXTREMES TOOLS
(PETS) DEPICT AT LEAST A 20 PERCENT CHANCE 3-DAY PRECIPITATION EXCEEDS THE 85TH
CLIMATOLOGICAL PERCENTILE AND 1-INCH ACROSS PARTS OF THE MIDDLE AND LOWER
MISSISSIPPI AND TENNESSEE VALLEYS, AND SOUTHEAST. BY THE MIDDLE OF WEEK-2, THE
ECMWF PET SHIFTS THESE ELEVATED PROBABILITIES OVER PARTS OF THE NORTHEAST.
THERE IS ALSO CONSIDERABLE SUPPORT FROM THE UNCALIBRATED 0Z ECMWF ENSEMBLE AS
WELL AS THE 0Z AND 6Z GEFS ENSEMBLES, ALL INDICATING ENHANCED PRECIPITATION
TOTALS OVER THE NORTHEAST. THEREFORE, A BROAD SLIGHT RISK OF HEAVY
PRECIPITATION EXTENDS FROM THE MIDDLE AND LOWER MISSISSIPPI VALLEY THROUGH THE
NORTHEAST, VALID NOV 18-21. ANY IMPACTS RELATED TO THIS POTENTIAL SURFACE LOW
COULD BE EXACERBATED GIVEN THE TIME PERIOD ALIGNS WITH THE START OF AN EXPECTED
BUSY TRAVEL PERIOD AHEAD OF THANKSGIVING.
RIDGING AND ASSOCIATED SURFACE HIGH PRESSURE ARE FORECAST TO BUILD ACROSS THE
EASTERN PACIFIC IN THE WAKE OF THE DEPARTING TROUGH. AT THE SAME TIME, STRONG
TROUGHING IS PREDICTED ACROSS ALASKA EARLY IN THE PERIOD, WITH SURFACE LOW
PRESSURE FORECAST ACROSS THE GULF OF ALASKA. THIS SET-UP FAVORS A TIGHT
PRESSURE GRADIENT AND SUBSEQUENT ENHANCED ONSHORE FLOW PATTERN ACROSS
SOUTHEASTERN ALASKA AND EXTENDING INTO THE NORTHWESTERN CONUS. ALTHOUGH ROUNDS
OF HEAVY PRECIPITATION AND HIGH WINDS REMAIN POSSIBLE ACROSS SOUTHEAST ALASKA
EARLY IN THE PERIOD, DAILY PRECIPITATION TOTALS IN THE UNCALIBRATED 0Z ECMWF
AND GEFS ENSEMBLES ARE HIGHEST ON DAY-7, AND THE ASSOCIATED SLIGHT RISKS OF
HEAVY PRECIPITATION AND HIGH WINDS ARE DISCONTINUED ACROSS ALASKA GIVEN
DECREASING CHANCES OF REACHING HAZARDS THRESHOLDS AFTER THE START OF WEEK-2.
COMPARED TO YESTERDAY, UNCALIBRATED TOOLS AND THE PETS ARE MORE ROBUST WITH THE
HEAVY PRECIPITATION SIGNAL ACROSS THE NORTHWESTERN CONUS, AND THE SLIGHT RISK
FOR HEAVY PRECIPITATION HAS BEEN EXTENDED SOUTHWARD THROUGH NORTHERN CALIFORNIA
FOR NOV 18-20. THE SLIGHT RISK OF HEAVY SNOW HAS ALSO BEEN EXTENDED SOUTHWARD
THROUGH THE CASCADES, WITH ANOTHER SLIGHT RISK OF HEAVY SNOW POSTED ACROSS THE
NORTHERN ROCKIES, BOTH VALID NOV 18-20, WITH THE GEFS SNOW WATER EQUIVALENT
(SWE) PET INDICATING AT LAST A 20 PERCENT CHANCE THAT 3-DAY SWE EXCEEDS THE
85TH CLIMATOLOGICAL PERCENTILE AND A HALF-INCH ACROSS PARTS OF THESE AREAS.
HEAVY SNOW MAY ALSO WORK ITS WAY SOUTHWARD INTO THE NORTHERN SIERRA NEVADAS BY
THE MIDDLE OF THE PERIOD BASED ON THE 0Z ECMWF ENSEMBLE, BUT THERE IS LESS
MODEL AGREEMENT REGARDING THIS POTENTIAL AND NO SIGNAL IN THE PET, SO THE
HAZARD IS NOT EXTENDED FURTHER SOUTH AT THIS TIME.
FORECASTER: THOMAS COLLOW
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