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FXUS21 KWNC 291908
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 29 2023
SYNOPSIS: BY NEXT WEEKEND, STRENGTHENING MID-LEVEL LOW PRESSURE FAVORED OVER
EASTERN NORTH AMERICA IS EXPECTED TO PROMOTE MUCH COOLER CONDITIONS, WITH THE
POTENTIAL FOR ELEVATED WINDS, AS WELL AS NEAR OR BELOW FREEZING MINIMUM
TEMPERATURES TO ADVERSELY IMPACT VULNERABLE VEGETATION OVER PARTS OF THE
EASTERN CONTIGUOUS U.S. THERE ARE CONTINUED INDICATIONS OF COASTAL LOW
FORMATION IN THE WESTERN ATLANTIC WHICH MAY BRING HEAVY PRECIPITATION AMOUNTS
TO PORTIONS OF THE MID-ATLANTIC AND NORTHEAST EARLY IN THE PERIOD. OVER ALASKA,
INCREASED ONSHORE FLOW FROM THE GULF OF ALASKA SUPPORTS AN INCREASED RISK OF
HEAVY PRECIPITATION AND HIGH WINDS OVER PARTS OF THE SOUTH-CENTRAL MAINLAND AND
THE SOUTHEAST.
HAZARDS
SLIGHT CHANCE OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE GREAT
PLAINS AND MISSISSIPPI VALLEY, SAT-SUN, OCT 7-8.
SLIGHT CHANCE OF MUCH BELOW-NORMAL TEMPERATURES FOR THE GREAT LAKES, OHIO AND
TENNESSEE VALLEYS EASTWARD TO THE EASTERN SEABOARD, SUN-TUE, OCT 8-10.
SLIGHT RISK OF EPISODES OF HIGH WINDS OVER THE NORTH-CENTRAL AND NORTHEASTERN
CONUS, SUN-WED, OCT 8-11.
SLIGHT RISK OF EPISODES OF HIGH WINDS OVER THE SOUTH-CENTRAL MAINLAND ALASKA
AND THE SOUTHEAST, SAT-WED, OCT 7-11.
SLIGHT CHANCE OF HEAVY PRECIPITATION FOR PARTS OF THE MID-ATLANTIC AND
NORTHEAST, SAT-MON, OCT 7-9.
SLIGHT CHANCE OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTH-CENTRAL MAINLAND
ALASKA AND THE SOUTHEAST, SAT-WED, OCT 7-11.
RAPID ONSET DROUGHT (ROD) RISK OVER PARTS OF THE LOWER MISSISSIPPI AND
TENNESSEE VALLEYS, SOUTHEAST AND SOUTHERN APPALACHIANS.
DETAILED SUMMARY
FOR MONDAY OCTOBER 02 - FRIDAY OCTOBER 06:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY OCTOBER 07 - FRIDAY OCTOBER 13: BETTER RUN-TO-RUN CONTINUITY
EXISTS IN THE DYNAMICAL MODELS THIS MORNING AS THE MAJOR 500-HPA PATTERN CHANGE
THAT WAS ADVERTISED YESTERDAY OVER NORTH AMERICA APPEARS TO BE ON TRACK. BOTH
THE ECMWF AND GEFS MEAN WEEK-2 500-HPA ANOMALY FIELDS FEATURE STRONGLY
AMPLIFIED TROUGHING OVER THE NORTHERN PACIFIC, WITH ANOMALOUS RIDGING
(TROUGHING) DOWNSTREAM OVER THE WESTERN (EASTERN) CONUS. WITH THE ANOMALOUS
RIDGING EXTENDING INTO THE HIGHER LATITUDES OVER THE WEST, THE AMPLIFYING
TROUGH FAVORED OVER THE EASTERN CONUS REMAINS THE MAIN HAZARD FOCUS GIVEN THE
POTENTIAL TO BRING UNSEASONABLY COLD TEMPERATURES INTO THE CENTRAL AND EASTERN
CONUS THROUGH THE MIDDLE OF THE PERIOD. OVER TIME, MUCH OF THE ANOMALOUS
TROUGHING IS FAVORED TO EASE, THOUGH MODELS SHOW THE PROGRESSION OF THE
MID-LEVEL FEATURES UPSTREAM, WHICH WOULD ALLOW FOR INCREASING CHANCES FOR
ENHANCED ONSHORE FLOW OVER THE WEST COAST.
IN THE WAKE OF A LARGE FRONTAL BOUNDARY FAVORED OVER CENTRAL CONUS LATE IN
WEEK-1, THE GEFS AND ECMWF ENSEMBLES SHOW STRONG MEAN SURFACE HIGH PRESSURE
DESCENDING SOUTHWARD FROM THE CANADIAN PRAIRIES. THE ADVECTION OF ANOMALOUSLY
COLD AIR IS WELL REFLECTED IN PROBABILISTIC EXTREMES TOOLS (PETS), WHICH DEPICT
A BROAD AREA WITH INCREASED CHANCES (20-40%) FOR MINIMUM TEMPERATURES FALLING
BELOW THE 15TH CLIMATOLOGICAL PERCENTILE THAT EXPANDS EASTWARD AND INTO THE
SOUTHERN TIER OF THE U.S. DURING THE FIRST SEVERAL DAYS OF THE PERIOD. THESE
TOOLS SHOW 20% CHANCES FOR SUBFREEZING TEMPERATURES OVER THE UPPER MISSISSIPPI
VALLEY, GREAT LAKES, AND THE NORTHEAST, AND DIPPING BELOW 40 DEGREES F ACROSS
THE MIDWEST AND FURTHER SOUTH INTO PARTS OF THE LOWER MISSISSIPPI AND TENNESSEE
VALLEYS AND CAROLINAS. GIVEN THE ELEVATED POTENTIAL FOR THE FIRST FROSTS AND/OR
FREEZES OF THE SEASON TO IMPACT VULNERABLE VEGETATION OVER MANY PARTS OF
NORTHEASTERN CONUS, A PAIR OF SLIGHT RISK AREAS FOR MUCH BELOW NORMAL
TEMPERATURES ARE ISSUED TO CAPTURE THE EVOLUTION OF THE COLD SIGNALS IN THE
PETS. THE FIRST IS HIGHLIGHTED MAINLY OVER THE GREAT PLAINS AND MISSISSIPPI
VALLEY AND IS VALID FOR OCT 7-8, AND THE SECOND IS HIGHLIGHTED FROM THE OHIO
AND TENNESSEE VALLEYS EASTWARD TO THE EASTERN SEABOARD, VALID FOR OCT 8-10.
BASED ON THE PREVAILING MID-LEVEL HEIGHT PATTERN AND THE ECMWF PET, A MODERATE
RISK FOR MUCH BELOW NORMAL TEMPERATURES WAS CONSIDERED FROM THE TENNESSEE
VALLEY TO THE MID-ATLANTIC, HOWEVER THERE IS LESS SUPPORT FOR THIS DESIGNATION
IN THE RAW TEMPERATURE TOOLS WHICH DEPICT MORE MODEST NEGATIVE TEMPERATURE
DEPARTURES DURING THE PREDICTED COLD SNAP. ACCOMPANYING THE COLD RISK, A SLIGHT
RISK OF HIGH WINDS IS ALSO ISSUED (OCT 8-11) OVER THE NORTH-CENTRAL AND
NORTHEASTERN CONUS WHERE THE BASE OF THE TROUGH IS FAVORED TO AMPLIFY ALOFT.
THIS IS SUPPORTED BY POTENTIALLY STRENGTHENING SURFACE PRESSURE GRADIENTS IN
THE DYNAMICAL MODEL ENSEMBLES AND PET GUIDANCE.
AS THE AFOREMENTIONED FRONTAL SYSTEM TRACKS ACROSS THE EASTERN U.S. NEXT
WEEKEND, THERE ARE CONTINUED INDICATIONS IN THE GUIDANCE FOR SURFACE TROUGHING
OFFSHORE ACROSS THE WESTERN ATLANTIC. BOTH THE ECMWF AND GEFS ENSEMBLES FAVOR
AN AREA OF DEEPENING MEAN LOW PRESSURE ASSOCIATED WITH THESE SYNOPTIC FEATURES
EARLY IN WEEK-2, WITH ENHANCED PRECIPITATION AMOUNTS OVERSPREADING THE
NORTHEASTERN CONUS. A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED OVER THE
MID-ATLANTIC AND NORTHEAST WHERE THERE IS GOOD AGREEMENT IN THE GEFS AND ECMWF
PETS INDICATING AT LEAST A 20% CHANCE OF 3-DAY AMOUNTS EXCEEDING THE 85TH
PERCENTILE AND 1 INCH. GIVEN ANTECEDENT WETNESS OVER THE NORTHEASTERN CONUS
WHERE MANY AREAS ARE CURRENTLY REGISTERING SOIL MOISTURE AND STREAMFLOWS IN THE
HIGHEST PERCENTILES, ANY HEAVY PRECIPITATION MAY LEAD TO LOCALIZED FLOODING IN
OVER PARTS OF THE MID-ATLANTIC AND NORTHEAST EARLY IN WEEK-2.
ACROSS THE WESTERN CONUS, THERE ARE GROWING SIGNS FOR ENHANCED ONSHORE FLOW
TIED TO THE ANOMALOUS TROUGHING OVER THE NORTHEASTERN PACIFIC SHIFTING EASTWARD
LATER IN WEEK-2 WHICH IS SUPPORTED IN THE GEFS BASED INTEGRATED VAPOR TRANSPORT
(IVT) TOOL. FOR DAYS 10-14 (OCT 9-13) BOTH PETS DEPICT A BROAD AREA WITH
INCREASED CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE 85TH PERCENTILE FROM THE
EASTERN PACIFIC EXTENDING INTO THE WEST COAST SUGGESTIVE OF POSSIBLE
ATMOSPHERIC RIVER ACTIVITY. WHILE 3-DAY PRECIPITATION AMOUNTS EXCEEDING AN INCH
ARE FAVORED OVER THE PACIFIC NORTHWEST, PRECIPITATION AMOUNTS ARE NOT EXPECTED
TO EXCEED HAZARDS CRITERIA AND NO CORRESPONDING HAZARDS ARE ISSUED AT THIS
TIME. THE HEAVY PRECIPITATION POTENTIAL OVER THE WEST COAST WILL CONTINUE TO BE
MONITORED IN UPCOMING HAZARD OUTLOOKS.
IN THE TROPICS, THE NATIONAL HURRICANE CENTER (NHC) IS MONITORING AN AREA OF
POTENTIAL TROPICAL CYCLONE (TC) FORMATION TO THE SOUTH OF MEXICO WITH 20%
CHANCES OF DEVELOPING DURING THE NEXT SEVEN DAYS. PROBABILISTIC TC GENESIS
TOOLS FROM THE GEFS AND ECMWF ARE FAIRLY SUPPORTIVE OF FORMATION WITH A MEAN
ENSEMBLE TRACK NEARING THE GULF OF CALIFORNIA. TIED TO THIS POTENTIAL TC, PETS
SHOW INCREASED WET SIGNALS OVER WESTERN MEXICO AND EXTENDING INTO THE SONORAN
DESERT HOWEVER, ACTUAL AMOUNTS ARE WELL BELOW THRESHOLDS AND NO CORRESPONDING
HAZARDS ARE ISSUED.
THE PERSISTENCE OF ABOVE (BELOW)-NORMAL TEMPERATURES (PRECIPITATION) HAVE LED
TO STRONG MOISTURE DEFICITS ACROSS MANY PARTS OF THE SOUTHEASTERN CONUS. A
RAPID ONSET DROUGHT (ROD) RISK REMAINS ISSUED OVER PARTS OF TENNESSEE, WESTERN
NORTH CAROLINA, NORTHWESTERN GEORGIA, MISSISSIPPI AND EASTERN ARKANSAS IN THE
UPDATED OUTLOOK. MANY OF THESE AREAS HAVE RECEIVED LESS THAN 50% OF THEIR
NORMAL RAINFALL ACCUMULATION SINCE LATE AUGUST, AND NEAR TO BELOW NORMAL
PRECIPITATION IS FORECAST DURING THE NEXT TWO WEEKS.
GIVEN BETTER CONTINUITY IN THE ENSEMBLES FAVORING ANOMALOUS TROUGHING TO THE
SOUTH OF THE ALEUTIANS AND RIDGING OVER WESTERN CANADA TO INDUCE ENHANCED
ONSHORE FLOW OVER ALASKA, A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED (OCT
7-11) FOR PARTS OF THE SOUTH-CENTRAL MAINLAND AND SOUTHEAST. WHILE THE
HIGHLIGHTED AREA IS CLIMATOLOGICALLY VERY WET DURING THIS TIME OF THE YEAR,
THERE IS GOOD AGREEMENT IN THE GEFS AND ECMWF PETS INDICATING AT LEAST A 20%
CHANCE OF PRECIPITATION AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 2 INCHES
THROUGH THE MIDDLE OF THE PERIOD. THE ENHANCED ONSHORE FLOW IS ALSO EXPECTED TO
BRING PERIODS OF HIGH WINDS, AND A CORRESPONDING SLIGHT RISK AREA IS POSTED
(OCT 7-11) BASED ON SUPPORT FROM THE ECMWF PET.
FORECASTER: NICK NOVELLA
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