827
FXUS21 KWNC 251812
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 25 2023
SYNOPSIS: A MAJOR HEAT WAVE IS FORECAST TO PERSIST ACROSS THE SOUTHERN
CONTIGUOUS U.S. (CONUS) DURING EARLY AUGUST AS MID-LEVEL HIGH PRESSURE BUILDS
OVER MUCH OF THE WESTERN AND SOUTHERN CONUS BRINGING CONTINUED RISKS FOR
EXCESSIVE HEAT. A MID-LEVEL TROUGH AND SURFACE LOW PRESSURE MAY BRING HEAVY
RAIN BACK TO THE NORTHEAST DURING THE PERIOD. THE COMBINATION OF RECENT DRYNESS
AND PREDICTED ABOVE-NORMAL TEMPERATURES AND BELOW-NORMAL PRECIPITATION DURING
WEEK-2 SUPPORTS RAPID ONSET DROUGHT (ROD) RISK OVER PARTS OF SOUTHERN TEXAS AND
THE INTERIOR PACIFIC NORTHWEST. ROD RISK IS ALSO ELEVATED ACROSS NORTHWESTERN
NORTH DAKOTA DUE TO ANTECEDENT DRY CONDITIONS AND LITTLE PRECIPITATION FORECAST
DURING WEEK-1.
HAZARDS
HIGH RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHERN PLAINS, LOWER
MISSISSIPPI VALLEY AND CENTRAL GULF COAST, WED-THU, AUG 2-3.
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE CENTRAL AND SOUTHERN
PLAINS, MIDDLE AND LOWER MISSISSIPPI, AND TENNESSEE VALLEYS, AND THE SOUTHEAST,
WED-SAT, AUG 2-5.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE SOUTHERN CONUS EXCLUDING PORTIONS
OF THE SOUTHERN CALIFORNIA COAST, WED-TUE, AUG 2-8.
SLIGHT RISK OF EXCESSIVE HEAT PARTS OF THE INTERIOR NORTHWEST, WED-SAT, AUG 2-5.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE NORTHEAST AND MID-ATLANTIC,
WED-FRI, AUG 2-4.
RAPID ONSET DROUGHT RISK ACROSS PORTIONS OF SOUTHERN TEXAS AND INTO LOUISIANA,
INTERIOR PORTIONS OF THE PACIFIC NORTHWEST, AND NORTHWESTERN NORTH DAKOTA.
DETAILED SUMMARY
FOR FRIDAY JULY 28 - TUESDAY AUGUST 01:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY AUGUST 02 - TUESDAY AUGUST 08: THE FIRST WEEK OF AUGUST IS
FORECAST TO BEGIN WITH POSITIVE 500-HPA HEIGHT ANOMALIES FOR MUCH OF THE
SOUTHERN AND WESTERN CONUS, HOWEVER, THE STRENGTH OF THESE ANOMALIES IS
FORECAST TO WEAKEN ACROSS THE SOUTHEAST BY THE MIDDLE TO END OF WEEK-2. THE
POSITIVE 500-HPA HEIGHT ANOMALIES AT THE ONSET OF THE PERIOD ARE ANTICIPATED TO
INDUCE A STRONG TEMPERATURE RESPONSE.THE PROBABILISTIC EXTREMES TOOLS (PETS)
FROM THE GEFS AND ECMWF INDICATE A 50 TO 70% CHANCE FOR TEMPERATURES TO EXCEED
THE 85TH PERCENTILE ACROSS MANY PARTS OF THE SOUTHERN TIER. THIS IS SUPPORTED
BY RAW DYNAMICAL TOOLS SHOWING INCREASED CHANCES FOR APPARENT TEMPERATURES TO
REACH 110 DEG F IN SOME AREAS, SUGGESTIVE OF A VERY HOT AND HUMID AIR MASS.
THEREFORE, A HIGH RISK OF EXCESSIVE HEAT IS POSTED FOR PORTIONS OF THE SOUTHERN
PLAINS, LOWER-MISSISSIPPI VALLEY AND CENTRAL GULF COAST FOR AUG 2-3. ACROSS
OTHER PORTIONS OF THE SOUTHEAST AND SOUTHERN AND CENTRAL PLAINS, THE ECMWF AND
GEFS PETS CONTINUE TO SHOW ENHANCED CHANCES (30-50%) FOR TEMPERATURES TO EXCEED
THE 85TH PERCENTILE DURING THIS PERIOD. ABOVE-NORMAL SEA SURFACE TEMPERATURES
ARE ALSO FORECAST TO PERSIST IN THE GULF OF MEXICO AND WESTERN ATLANTIC OCEAN
THAT MAY HELP TO INCREASE ATMOSPHERIC MOISTURE LEADING TO HIGHER APPARENT
TEMPERATURES. THEREFORE A BROADER MODERATE RISK OF EXCESSIVE HEAT IS POSTED FOR
AUG 2-5 FOR MUCH OF THE SOUTHEASTERN CONUS. IN THE SOUTHWEST, PROBABILITIES FOR
EXCESSIVE HEAT ARE GENERALLY REDUCED RELATIVE TO PREVIOUS FORECASTS, RESULTING
IN ONLY A SLIGHT RISK OF EXCESSIVE HEAT BEING POSTED FOR THIS REGION. THE
SLIGHT RISK EXTENDS TO MUCH OF THE REST OF THE SOUTHERN CONUS, TO ENCOMPASS THE
MODERATE AND HIGH RISK HAZARDS, FOR ALL OF WEEK-2. A SLIGHT NORTHWARD EXPANSION
OF THE SLIGHT AND MODERATE RISKS WAS ADDED INTO PARTS OF THE MIDDLE-MISSISSIPPI
VALLEY AND CENTRAL PLAINS WHERE THE ECMWF PET CONTINUES SHOWS INCREASED CHANCES
(>40%), RELATIVE TO YESTERDAY, OF TEMPERATURES EXCEEDING THE 85TH
CLIMATOLOGICAL PERCENTILE IN THIS REGION. FINALLY, A SLIGHT RISK OF EXCESSIVE
HEAT IS POSTED FOR INTERIOR PORTIONS OF THE PACIFIC NORTHWEST WHERE PETS SHOW
ENHANCED CHANCES FOR TEMPERATURES TO REACH THE 85TH PERCENTILE AND 100 DEGF FOR
AUG 2-5 UNDER POSITIVE 500-HPA HEIGHT ANOMALIES.
IN THE NORTHEAST, NEGATIVE 500-HPA HEIGHT ANOMALIES HAVE BECOME REESTABLISHED
AND ARE FAVORED TO REMAIN THROUGHOUT WEEK-2 BRINGING REDUCED CHANCES FOR
EXCESSIVE HEAT INTO THE NORTHEAST AND NORTH-CENTRAL CONUS WITH NORTHERLY FLOW
BECOMING MORE ESTABLISHED. THE TROUGHING FEATURE IN THE NORTHEAST MAY BRING
RENEWED CONCERNS FOR HEAVY PRECIPITATION TO THE NORTHEAST WHERE THERE HAS BEEN
REPEATED FLOODING IN RECENT WEEKS. THE ECMWF PET IS SHOWING INCREASED CHANCES
FOR PRECIPITATION EXCEEDING THE 85TH PERCENTILE AND 3 DAY PRECIPITATION
EXCEEDING HALF AN INCH. UNCALIBRATED RAW MODEL GUIDANCE FROM THE GEFS AND ECMWF
INDICATE FAIR CHANCES OF HEAVY PRECIPITATION EARLY IN THE PERIOD. THEREFORE, A
SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR AUG 2-4.
IN THE SOUTHWEST, MID-LEVEL HIGH PRESSURE AND A SURFACE LOW NEAR THE NORTHERN
GULF OF CALIFORNIA MAY HELP TO BRING ENHANCED PRECIPITATION TO PARTS OF THE
AREA. HOWEVER, PRECIPITATION PETS FROM THE GEFS, ECMWF, AND CANADIAN DO NOT
HAVE A PARTICULARLY STRONG SIGNAL FOR HEAVY PRECIPITATION IN THIS REGION AS OF
YET, WITH ONLY SMALL AREAS SEEING CHANCES FOR GREATER THAN 0.5 INCHES OF
PRECIPITATION DURING A THREE DAY PERIOD. THEREFORE, NO CORRESPONDING HAZARD IS
POSTED FOR PRECIPITATION.
DUE TO LITTLE TO NO RAINFALL FORECAST AND THE ONGOING EXCESSIVE HEAT CONDITIONS
FAVORED TO PERSIST ACROSS THE SOUTHERN TIER, THERE CONTINUES TO BE AN ELEVATED
RISK OF RAPID ONSET DROUGHT (ROD) DEVELOPMENT FOR PARTS OF THE SOUTHERN PLAINS
AND LOWER MISSISSIPPI VALLEY GIVEN THE INCREASED EVAPOTRANSPIRATION RATES.
ADDITIONAL ROD RISK AREAS ARE INDICATED ACROSS INTERIOR PORTIONS OF THE PACIFIC
NORTHWEST AND NORTHWESTERN NORTH DAKOTA DUE TO SOIL MOISTURES IN BOTH OF THESE
REGIONS BEING BELOW THE 30TH CLIMATOLOGICAL PERCENTILE AND WITH LITTLE
ADDITIONAL RAINFALL FORECAST DURING WEEK-1.
NO HAZARDS ARE POSTED FOR ALASKA DURING WEEK-2. ABOVE-NORMAL TEMPERATURES ARE
PREDICTED ACROSS AND NEAR-NORMAL PRECIPITATION IS FORECAST ACROSS MUCH OF THE
STATE.
FORECASTER: RYAN BOLT
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