622
FXUS21 KWNC 281825
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 28 2023
SYNOPSIS: THE SEEMINGLY ENDLESS SUMMER HEAT THAT IS FORECAST IN THE GREAT
PLAINS AND MIDWEST DURING WEEK-1 IS FORECAST TO CONTINUE ACROSS MUCH OF THE
CENTRAL CONTIGUOUS UNITED STATES (CONUS) DURING WEEK-2 WITH THE GREATEST
POTENTIAL FOR EXCESSIVE HEAT FAVORED EARLY IN THE PERIOD. HOWEVER, A SURFACE
HIGH PRESSURE OVER THE SOUTHERN APPALACHIANS MAY LIMIT EXCESSIVE HEAT CHANCES
ACROSS THE SOUTHERN ATLANTIC STATES. STRONG SURFACE PRESSURE GRADIENTS FAVORED
IN THE SOUTHWEST MAY BRING CHANCES FOR HIGH WINDS TO THE REGION.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE GREAT PLAINS, MISSISSIPPI
VALLEY AND GREAT LAKES, TUE-THU, SEP 5-7.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF NEW MEXICO, THE GREAT PLAINS, THE
MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS, AND THE SOUTHEAST, TUE-MON, SEP 5-11.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE SOUTHWEST, TUE-MON, SEP 5-11.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR THURSDAY AUGUST 31 - MONDAY SEPTEMBER 04:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY SEPTEMBER 05 - MONDAY SEPTEMBER 11: MID-LEVEL RIDING IS FORECAST TO
REDEVELOP ACROSS THE GREAT PLAINS DURING WEEK-1 AND IS THEN FORECAST TO PERSIST
THROUGH MUCH OF THE WEEK-2 PERIOD. THE HIGHEST 500-HPA HEIGHT ANOMALIES LOOK TO
BE CENTERED ACROSS THE GREAT LAKES BUT MAY RETROGRADE BACK INTO THE GREAT
PLAINS DURING THE PERIOD. AS HAS BEEN THE CASE FOR MUCH OF THE SUMMER, THE
RIDGING FORECAST IS EXPECTED TO BRING MUCH ABOVE-NORMAL TEMPERATURES WITH
ELEVATED CHANCES FOR EXCESSIVE HEAT. THE PROBABILISTIC EXTREMES TOOLS (PET)
FROM THE GEFS AND ECMWF BOTH SHOW ENHANCED CHANCES (>50%) OF TEMPERATURES
REACHING THE 90TH CLIMATOLOGICAL PERCENTILE ACROSS PORTIONS OF THE GREAT LAKES
AND UPPER-MISSISSIPPI VALLEY AND TEMPERATURES NEAR 90 DEG F. FURTHER SOUTH
ACROSS THE CENTRAL AND SOUTHERN PLAINS, ELEVATED CHANCES (>40%) OF TEMPERATURES
EXCEEDING THE 90TH CLIMATOLOGICAL PERCENTILE AND 100 DEG F. ALONG THE GULF
COAST, THE ECMWF PET HAS WEAKER CHANCES OF EXCEEDING THE 90TH CLIMATOLOGICAL
PERCENTILE COMPARED TO PREVIOUS FORECASTS AND RELATIVE TO THE GEFS PET.
HOWEVER, THE NATIONAL BLEND OF MODELS (NBM) ALSO INDICATES A WIDE AREA OF NEAR
RECORD AND RECORD BREAKING TEMPERATURES ON DAYS 9 AND 10 IN AREAS ALONG THE
GULF COAST. THE CALIBRATED HEAT INDEX TOOLS ALSO INDICATE HEAT INDICES ABOVE
110 DEG F IN THESE AREAS CONSISTENT WITH THE WEATHER PREDICTION CENTER’S (WPC)
DAY 7 HEAT INDEX FORECAST. THEREFORE, A MODERATE RISK OF EXCESSIVE HEAT IS
POSTED FOR PORTIONS OF THE GULF COAST, CENTRAL AND SOUTHERN PLAINS, ALONG WITH
THE MIDDLE AND LOWER MISSISSIPPI VALLEY FOR SEP 5-7. MEANWHILE, A BROADER
SLIGHT RISK OF EXCESSIVE HEAT COVERS PORTIONS OF THE SOUTHEAST, GULF COAST,
TENNESSEE AND OHIO RIVER VALLEYS AND GREAT PLAINS WHERE THE PETS FROM THE GEFS
AND ECMWF CONTINUE TO SHOW AT LEAST A 20% CHANCE OF TEMPERATURES EXCEEDING THE
90TH PERCENTILE FOR MUCH OF THE WEEK-2 PERIOD AND APPROACHING HAZARDS CRITERIA.
IN THE SOUTHWEST, SURFACE HIGH PRESSURE OVER THE COLORADO PLATEAU AND SURFACE
LOW PRESSURE IN NORTHERN GULF OF CALIFORNIA LEAD TO INCREASED CHANCES FOR HIGH
WINDS. THE PETS FROM THE ECMWF AND GEFS SUPPORT THIS REGION AND THE OVERALL
PATTERN LOOKS UNCHANGED THROUGHOUT MUCH OF THE WEEK-2 PERIOD. THEREFORE, A
SLIGHT RISK OF HIGH WINDS IS POSTED FOR THE ENTIRE WEEK-2 PERIOD.
ALONG THE PACIFIC NORTHWEST, MID-LEVEL TROUGHING IS FAVORED AND PETS FROM THE
GEFS AND ECMWF INDICATE SLIGHT (>20%) CHANCES OF 3-DAY PRECIPITATION AMOUNTS
EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE AND HALF AN INCH OF PRECIPITATION.
HOWEVER, THESE AMOUNTS WOULD NOT REACH HAZARDOUS THRESHOLDS IN THE REGION AND
RAW PRECIPITATION GUIDANCE INDICATES LOWER AMOUNTS AS WELL. THEREFORE, NO
CORRESPONDING PRECIPITATION HAZARD IS POSTED.
A RAPID ONSET DROUGHT (ROD) RISK REMAINS POSTED ACROSS PARTS OF THE SOUTHERN
PLAINS AND LOWER MISSISSIPPI VALLEY DUE TO THE COMBINATION OF INCREASED 30-DAY
PRECIPITATION DEFICITS, LITTLE TO NO RAINFALL FAVORED IN THE NEXT WEEK, AND
HIGH EVAPOTRANSPIRATION RATES ASSOCIATED WITH THE ABOVE-NORMAL TEMPERATURES.
IN ALASKA, POSITIVE 500-HPA HEIGHT ANOMALIES ARE FORECAST SOUTH OF THE ALEUTIAN
ISLANDS. NORTH OF THIS RIDGE ABOVE-NORMAL PRECIPITATION IS FAVORED ACROSS
PORTIONS OF WESTERN ALASKA, HOWEVER, THIS IS UNLIKELY TO REACH HAZARDOUS
THRESHOLDS. THERE IS ALSO A THREAT OF A RECURVING TROPICAL CYCLONE TO IMPACT
WESTERN ALASKA DURING THE WEEK-2 PERIOD. HOWEVER, UNCERTAINTY REMAINS VERY HIGH
AND NO CORRESPONDING HAZARD IS POSTED. IN SOUTHEAST ALASKA, BELOW-NORMAL
PRECIPITATION IS FAVORED AND UNLIKELY TO REACH THE 85TH CLIMATOLOGICAL
PERCENTILE WITH NORTHWEST FLOW ALOFT.
FORECASTER: RYAN BOLT
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