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FXUS21 KWNC 281828
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 28 2023
SYNOPSIS: TOWARDS THE END OF NEXT WEEK, 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 GREAT PLAINS AND MIDWEST. THERE ARE CONTINUED INDICATIONS OF
COASTAL LOW FORMATION IN THE WESTERN ATLANTIC WHICH MAY BRING HEAVY
PRECIPITATION AMOUNTS TO PORTIONS OF THE CENTRAL EASTERN SEABOARD EARLY IN THE
PERIOD.
HAZARDS
SLIGHT CHANCE OF MUCH BELOW-NORMAL TEMPERATURES FOR PORTIONS OF THE NORTHERN
AND CENTRAL PLAINS, AND THE UPPER AND MIDDLE MISSISSIPPI VALLEY, SAT-TUE, OCT
7-10.
SLIGHT RISK OF EPISODES OF HIGH WINDS OVER THE NORTH-CENTRAL CONUS, SAT-TUE,
OCT 7-10.
SLIGHT CHANCE OF HEAVY PRECIPITATION ALONG THE CENTRAL EASTERN SEABOARD,
FRI-SUN, OCT 6-8.
SLIGHT CHANCE OF HEAVY PRECIPITATION FOR THE SOUTHERN PLAINS, FRI-SUN, OCT 6-8.
RAPID ONSET DROUGHT (ROD) RISK OVER PARTS OF THE LOWER MISSISSIPPI AND
TENNESSEE VALLEYS, SOUTHEAST AND SOUTHERN APPALACHIANS.
DETAILED SUMMARY
FOR SUNDAY OCTOBER 01 - THURSDAY OCTOBER 05:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY OCTOBER 06 - THURSDAY OCTOBER 12: COMPARED TO PREVIOUS GUIDANCE,
DYNAMICAL MODELS ARE ADVERTISING QUITE A SUBSTANTIAL CHANGE IN THE EVOLUTION OF
THE 500-HPA HEIGHT ANOMALIES OVER NORTH AMERICA. BOTH THE ECMWF AND GEFS
ENSEMBLES FAVOR A MORE PROGRESSIVE PATTERN, EACH DEPICTING AN AMPLIFYING TROUGH
OVER THE NORTH PACIFIC TO THE SOUTH OF THE ALEUTIANS. THE DEVELOPMENT OF THIS
MID-LEVEL FEATURE IS NOW EXPECTED TO PRODUCE A RIDGE/TROUGH PATTERN DOWNSTREAM
OVER NORTH AMERICA, AND BRING INCREASINGLY WARMER (COLDER) THAN NORMAL
TEMPERATURES THROUGHOUT THE WESTERN (CENTRAL AND EASTERN) CONUS.
FROM A HAZARDS PERSPECTIVE, THE MARKED SHIFT OF PREVAILING NORTHWESTERLY FLOW
FAVORED OVER THE NORTH-CENTRAL CONUS BY LATE NEXT WEEK INCREASES CONCERNS FOR
CHILLY NIGHT TIME AUTUMN TEMPERATURES, WHERE THE FIRST FROSTS AND/OR FREEZES OF
THE SEASON COULD OCCUR AND IMPACT VULNERABLE VEGETATION THROUGHOUT PARTS OF THE
GREAT PLAINS AND THE MIDWEST. BASED ON PROBABILISTIC EXTREME TOOL (PET)
GUIDANCE FROM THE GEFS AND ECMWF, THERE ARE INCREASED CHANCES (>20%) FOR
MINIMUM TEMPERATURES FALLING BELOW THE 15TH CLIMATOLOGICAL PERCENTILE AND BELOW
FREEZING OVERSPREADING THE NORTH-CENTRAL CONUS BEGINNING ON DAY 9 (OCT 7).
SIMILAR CHANCES FOR MINIMUM TEMPERATURES FALLING BELOW 40 DEGREES F ARE
INDICATED DIPPING AS FAR SOUTH AS THE CENTRAL PLAINS AND MIDDLE MISSISSIPPI
VALLEY. OVER TIME, THE INCREASED COLD SIGNALS IN THE PETS SHIFT EASTWARD INTO
THE GREAT LAKES AND OHIO VALLEY AND BEYOND THROUGH DAYS 12 AND 13 (OCT 10-11).
HOWEVER, THERE IS INCREASED UNCERTAINTY IN REGARDS TO THE EASTERN EXTENT OF THE
COLD RISK AT THIS LEAD GIVEN LESS SUPPORT FROM THE UNCALIBRATED TEMPERATURE
TOOLS FOR ANOMALOUS COLD PUSHING EAST OF THE MISSISSIPPI RIVER, AS WELL AS
DIFFERENCES IN THE STRENGTH OF THE TROUGHING ALOFT BETWEEN THE ECMWF AND GEFS.
AS A RESULT, A SLIGHT RISK OF MUCH BELOW NORMAL TEMPERATURES IS ISSUED OVER THE
GREAT PLAINS AND MISSISSIPPI VALLEY FOR OCT 7-10. ACCOMPANYING THE COLD RISK, A
SLIGHT RISK OF HIGH WINDS IS ALSO ISSUED (OCT 7-10) OVER A BROAD AREA AT THE
BASE OF THE AMPLIFYING TROUGH FAVORED ALOFT, WHICH IS SUPPORTED BY POTENTIALLY
STRENGTHENING SURFACE PRESSURE GRADIENTS IN THE ENSEMBLES AND PET GUIDANCE.
FOR PRECIPITATION, BOTH THE GEFS AND ECMWF ENSEMBLES FEATURE CYCLONIC SURFACE
FLOW OVER THE CENTRAL EASTERN SEABOARD SUGGESTIVE OF COASTAL LOW FORMATION
EARLY IN THE PERIOD. THE HIGHEST DAILY AMOUNTS ARE EXPECTED TO REMAIN OFFSHORE,
THOUGH THERE IS CONTINUED SUPPORT IN THE RAW AND CALIBRATED TOOLS FOR
POTENTIALLY HEAVY PRECIPITATION AMOUNTS MAKING THEIR WAY ONSHORE AND A SLIGHT
RISK OF HEAVY PRECIPITATION IS HIGHLIGHTED FOR OCT 6-8. RELATIVE TO YESTERDAY,
LARGER WET SIGNALS ARE EVIDENT IN THE PET GUIDANCE OVER COASTAL MID-ATLANTIC
AND THE NORTHEAST RESULTING IN THE NORTHWARD EXPANSION OF THE HEAVY
PRECIPITATION RISK IN THE UPDATED OUTLOOK. ACROSS THE SOUTHERN PLAINS, FRONTAL
ACTIVITY AND MOIST FLOW FROM THE GULF OF MEXICO REMAINS FAVORED TO SUPPORT A
SLIGHT RISK OF HEAVY PRECIPITATION MAINLY OVER TEXAS. CONSISTENT WITH PREVIOUS
GUIDANCE, THE GEFS CONTINUES TO BE TENUOUS WITH THIS POTENTIAL, HOWEVER THE
ECMWF PET FEATURES AT LEAST 20-40% CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE 85TH
PERCENTILE AND 1 INCH, WITH SIMILAR CHANCES IN THE UNCALIBRATED PROBABILITIES.
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 AND IS EXPANDED 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.
LATE IN WEEK-1, STRONG MID-LEVEL RIDGING IS EXPECTED TO BE ESTABLISHED OVER THE
BERING SEA, WHERE RESULTANT NORTHERLY FLOW IS LIKELY TO BRING BELOW-NORMAL
TEMPERATURES OVER MUCH OF THE MAINLAND AND SOUTHEAST. DESPITE SOME SUPPORT IN
THE PET GUIDANCE FOR TEMPERATURES FALLING BELOW THE 15TH PERCENTILE EARLY IN
THE PERIOD, NO TEMPERATURE HAZARDS ARE ISSUED GIVEN ADDED UNCERTAINTY WITH THE
PATTERN CHANGE OVER THE HIGHER LATITUDES AND POTENTIAL FOR WARMING TEMPERATURES
WITH ENHANCED ONSHORE FLOW DURING WEEK-2. WITH THIS PATTERN CHANGE, THE
POTENTIAL FOR HEAVY PRECIPITATION WILL ALSO CONTINUE TO BE MONITORED DURING THE
UPCOMING HAZARD OUTLOOKS.
FORECASTER: NICK NOVELLA
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