597
FXUS21 KWNC 211841
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 21 2023
SYNOPSIS: A MAJOR HEAT WAVE IS FORECAST TO PERSIST ACROSS MUCH OF THE
CONTIGUOUS U.S. (CONUS) DURING LATE JULY INTO EARLY AUGUST AS MID-LEVEL HIGH
PRESSURE BUILDS OVER MUCH OF THE WESTERN AND CENTRAL CONUS. EXCESSIVE HEAT IS
ALSO LIKELY TO SPREAD OVER PARTS OF THE NORTHEAST LATE IN WEEK-1, BEFORE
DECREASING MID-LEVEL HEIGHTS MODERATE TEMPERATURES BY THE MIDDLE OF WEEK-2,
WITH THE HIGHEST EXCESSIVE HEAT RISK BECOMING MORE FOCUSED ACROSS THE
SOUTH-CENTRAL CONUS. 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 CENTRAL AND SOUTHERN PLAINS,
MIDDLE AND LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, AND SOUTHEAST, SAT-MON,
JUL 29-31.
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF SOUTHWEST, SOUTHERN ROCKIES,
GREAT PLAINS, MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS, AND THE SOUTHEAST,
SAT-TUE, JUL 29-AUG 1.
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE MID-ATLANTIC AND OHIO
VALLEY, SAT, JUL 29.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE CONUS EXCLUDING PORTIONS OF NEW
ENGLAND, THE UPPER MISSISSIPPI VALLEY, GREAT LAKES, AND WEST COAST, SAT-FRI,
JUL 29-AUG 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 MONDAY JULY 24 - FRIDAY JULY 28:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY JULY 29 - FRIDAY AUGUST 04: TODAY’S MODELS CONTINUE THE TREND OF
BUILDING MORE HEAT ACROSS THE SOUTH-CENTRAL CONUS, WHILE DECREASING EXCESSIVE
HEAT CHANCES ARE DEPICTED ACROSS THE WESTERN AND NORTHERN TIER OF THE CONUS.
ANOMALOUS MID-LEVEL RIDGING AND 500-HPA HEIGHT ANOMALIES OF GREATER THAN
60-METERS ARE FORECAST TO EXPAND ACROSS MUCH OF THE OHIO VALLEY, GREAT LAKES,
AND NORTHEAST LATE IN WEEK-1. WHILE THESE AREAS HAVE BEEN GENERALLY SPARED FROM
THE STRONGEST HEAT THIS SUMMER, THE OVERALL PATTERN EVOLUTION FAVORS WHAT COULD
BE THE HOTTEST WEATHER SO FAR THIS SEASON, WITH MAXIMUM TEMPERATURES POSSIBLY
WELL INTO 90S DEG F ACROSS PARTS OF THE OHIO VALLEY AND MID-ATLANTIC, WITH
GENERALLY 80S TO LOWER 90S DEG F PREDICTED ACROSS MORE NORTHERN AREAS. THIS
EVOLUTION IS SUPPORTED BY THE GEFS AND ECMWF PROBABILISTIC EXTREMES TOOLS
(PETS), WHICH BOTH INDICATE AT LEAST A 40 PERCENT CHANCE OF MAXIMUM
TEMPERATURES EXCEEDING THE 85TH PERCENTILE ACROSS PORTIONS OF THE OHIO VALLEY
AND MID-ATLANTIC ON DAY-8 (JUL 29) SUPPORTING A MODERATE RISK OF EXCESSIVE HEAT
OVER THESE AREAS. THE HEAT IS FORECAST TO BE SHORT-LIVED AS STRENGTHENING
TROUGHING OVER EASTERN CANADA FAVORS DECREASING MID-LEVEL HEIGHTS ACROSS THE
NORTH-CENTRAL AND NORTHEASTERN CONUS EARLY IN WEEK-2 RESULTING IN A QUICK
MODERATION IN TEMPERATURES, AND A GREATER FOCUS OF EXCESSIVE HEAT ACROSS THE
SOUTH, ALLOWING THE MODERATE RISK TO BE DISCONTINUED AFTER DAY-8 (JUL 29). OF
NOTE IS THAT WHILE FOR SIMPLICITY THE SLIGHT RISK FOR EXCESSIVE HEAT COVERS A
BROAD AREA OF THE CONUS FOR ALL OF WEEK-2, THE NORTHEAST IS UNLIKELY TO HAVE
EXCESSIVE HEAT CONCERNS AFTER DAY-9 (JUL 30) GIVEN THE FORECAST ARRIVAL OF A
RELATIVELY COOLER AIR MASS.
DURING WEEK-2 THE GREATEST EXCESSIVE HEAT RISK IS FORECAST TO BE FOCUSED ACROSS
THE CENTRAL CONUS. THE GEFS PET DEPICTS VERY HIGH PROBABILITIES (70-80 PERCENT)
FOR MAXIMUM TEMPERATURES EXCEEDING THE 85TH CLIMATOLOGICAL PERCENTILE ACROSS
THE MIDDLE MISSISSIPPI VALLEY EARLY IN WEEK-2, ALTHOUGH THE TEMPERATURE
DEPICTION IS LIKELY BIASED TOO HIGH BASED ON RECENT SKILL ASSESSMENT. HOWEVER,
EVEN THE ECMWF PET DEPICTS PROBABILITIES IN THE 30-50 PERCENT RANGE FOR MAXIMUM
TEMPERATURES REACHING THE 85TH PERCENTILE, INCREASING TO OVER 60 PERCENT ON
DAY-10 (JUL 31) OVER PARTS OF EASTERN OKLAHOMA AND ADJACENT AREAS OF WESTERN
ARKANSAS AND SOUTHWESTERN MISSOURI. ACTUAL TEMPERATURES ARE FORECAST TO BE IN
THE UPPER 90S TO LOW 100S DEG F, WITH HEAT GUIDANCE FROM THE ECMWF ENSEMBLE
INDICATING AT LEAST A 60 PERCENT CHANCE HEAT INDICES EXCEED 105 DEG F OVER
PARTS OF THE CENTRAL PLAINS, MIDDLE MISSISSIPPI VALLEY, AND TENNESSEE VALLEY,
AND 110 DEG F ACROSS PARTS OF THE SOUTHERN PLAINS, LOWER MISSISSIPPI VALLEY,
AND SOUTHEAST. ADDITIONALLY, THE NATIONAL BLEND OF MODELS INDICATES NEAR DAILY
RECORD HIGH TEMPERATURES ACROSS PARTS OF TEXAS AND THE CENTRAL GULF COAST BY
THE MIDDLE OF WEEK-2. GIVEN THESE STRONG SIGNALS IN THE GUIDANCE, A HIGH RISK
FOR EXCESSIVE HEAT IS POSTED ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN
PLAINS, MIDDLE AND LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, AND SOUTHEAST
FROM JUL 29-31. A BROADER MODERATE RISK FOR EXCESSIVE HEAT SURROUNDS THE HIGH
RISK AND IS EXTENDED THROUGH AUG 1. THE MODERATE RISK INCLUDES MORE OF THE
SOUTHEAST INCLUDING FLORIDA, AND ALSO EXTENDS INTO THE DESERT SOUTHWEST. WHILE
PROBABILITIES FOR REACHING EXCESSIVE HEAT CRITERIA OVER THE DESERT SOUTHWEST NO
LONGER NECESSITATE A HIGH RISK, BOTH THE GEFS AND ECWF PETS INDICATE AT LEAST A
40 PERCENT CHANCE MAXIMUM TEMPERATURES EXCEED THE 85TH PERCENTILE EARLY IN THE
PERIOD SUPPORTING THE MODERATE RISK. THE INCREASE IN MONSOON MOISTURE FAVORS AN
OVERALL DECREASE IN TEMPERATURES ACROSS THE SOUTHWEST AND FOUR CORNERS HEADING
INTO EARLY AUGUST. PETS GENERALLY MAINTAIN SUPPORT FOR THE SLIGHT RISK ACROSS
MUCH OF THE INTERIOR WESTERN CONUS THROUGH THE CANADIAN BORDER WHERE EXCESSIVE
HEAT THRESHOLDS ARE LOWER IN MOUNTAINOUS REGIONS.
FOR PRECIPITATION, SIGNALS FOR ABOVE-NORMAL RAINFALL CONTINUE TO BE QUITE
TENUOUS THROUGHOUT MUCH OF THE U.S. DURING THE PERIOD, ALTHOUGH OCCASIONAL
FRONTAL DISTURBANCES ARE LIKELY TO BRING ELEVATED CHANCES FOR HEAVY
PRECIPITATION AND THUNDERSTORMS INTO PARTS OF THE NORTH-CENTRAL AND
NORTHEASTERN CONUS, ESPECIALLY AS MID-LEVEL HEIGHTS DECREASE EARLY IN THE
PERIOD. FOR THE NORTH AMERICAN MONSOON REGION, THE ECMWF CONTINUES TO FAVOR A
SLIGHTLY SUPPRESSED RIDGE AXIS, TRANSLATING INTO LOWER PRECIPITATION AMOUNTS IN
THE UNCALIBRATED GUIDANCE COMPARED TO THE GEFS. WHILE LOCALLY HEAVY AMOUNTS ARE
POSSIBLE DURING WEEK-2, THERE IS INSUFFICIENT SUPPORT IN THE TOOLS TO INCLUDE A
CORRESPONDING HEAVY PRECIPITATION OR FLOODING HAZARD AT THIS TIME.
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. NEAR- TO ABOVE-NORMAL
TEMPERATURES ARE PREDICTED ACROSS MUCH OF THE MAINLAND AND SOUTHEAST, WITH
BELOW-NORMAL TEMPERATURES SLIGHTLY FAVORED ACROSS PARTS OF THE ALEUTIANS AND
EXTREME WESTERN MAINLAND. ABOVE-NORMAL PRECIPITATION IS FORECAST ACROSS THE
NORTHERN TWO-THIRDS OF THE MAINLAND, WITH NEAR- TO BELOW-NORMAL PRECIPITATION
FORECAST ACROSS SOUTHERN ALASKA, INCLUDING THE ALEUTIANS.
FORECASTER: THOMAS COLLOW
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