397
FXUS21 KWNC 291803
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 29 2023
SYNOPSIS: AN INCREASED LIKELIHOOD FOR EXCESSIVE HEAT IS PREDICTED INITIALLY FOR
THE MID-ATLANTIC AND SHIFT WESTWARD WITH TIME. EXCESSIVE HEAT CONTINUES TO BE
FORECAST ACROSS THE DESERT SOUTHWEST, GREAT PLAINS, MISSISSIPPI, TENNESSEE, AND
OHIO VALLEYS, AND THE SOUTHEAST THROUGHOUT WEEK-2. 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 CENTRAL AND SOUTHERN GREAT
PLAINS AND MISSISSIPPI VALLEY, WED-SAT, SEP 6-9.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE DESERT SOUTHWEST, GREAT
PLAINS, MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS, AND THE SOUTHEAST, WED-TUE,
SEP 6-12.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE MID-ATLANTIC, WED-THU, SEP
6-7.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE SOUTHWEST, FRI-TUE, SEP 8-12.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR FRIDAY SEPTEMBER 01 - TUESDAY SEPTEMBER 05:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY SEPTEMBER 06 - TUESDAY SEPTEMBER 12: COMPARED TO YESTERDAY,
MULTIPLE MODEL ENSEMBLE MEANS INDICATE MID-LEVEL RIDGING BEING SLIGHTLY WEAKER
ACROSS THE MIDWEST AND SHIFTED WESTWARD TOWARD THE GREAT PLAINS AND
SOUTHWESTWARD TO THE SOUTHWEST. THIS TRANSLATES TO A WESTWARD SHIFT IN HEAT
OVER TIME. MID-LEVEL RIDGING IS PREDICTED OVER THE NORTHEASTERN CONUS AT THE
END OF WEEK-1, CONTINUING INTO THE ONSET OF WEEK-2 ALBEIT WEAKER.
A SLIGHT RISK OF EXCESSIVE HEAT IS DESIGNATED FOR PARTS OF THE MID-ATLANTIC,
SEP 6-7, WHERE HEAT IS ANTICIPATED TO LINGER FROM WEEK-1. THE ECMWF
PROBABILISTIC EXTREMES TOOL (PET) SHOWS THE GREATEST CHANCES OF EXCESSIVE HEAT,
INDICATING AT LEAST A 50% CHANCE OF MAXIMUM TEMPERATURES EXCEEDING THE 90TH
PERCENTILE AND 90 DEG F, WITH HEAT INDEX VALUES POTENTIALLY BEING HIGHER. THE
NATIONAL BLEND OF MODELS (NBM) DOES NOT INDICATE WIDESPREAD NEAR RECORD
TEMPERATURES IN THIS REGION BY DAY 8, BUT DOES SHOW A NUMBER OF LOCATIONS
POTENTIALLY REACHING NEAR RECORD MAXIMUM TEMPERATURES AT THE END OF WEEK-1,
WHICH COULD LINGER INTO THE START OF WEEK-2. AS THE MID-LEVEL RIDGE SHIFTS
WESTWARD BY THE MIDDLE OF THE PERIOD, A BROAD SLIGHT RISK AREA IS HIGHLIGHTED
FOR EXCESSIVE HEAT, EXTENDING FROM THE GREAT PLAINS, MISSISSIPPI, TENNESSEE,
AND OHIO VALLEYS, AND THE SOUTHEAST TO THE DESERT SOUTHWEST THROUGHOUT WEEK-2.
THE DESERT SOUTHWEST HAS BEEN ADDED IN THE SLIGHT RISK OF EXCESSIVE HEAT TODAY
GIVEN MULTIPLE MODEL ENSEMBLE MEANS FAVORING POSITIVE MID-LEVEL HEIGHT
DEPARTURES EXTENDING FURTHER INTO THE SOUTHWEST COMPARED TO YESTERDAY. AN
EMBEDDED MODERATE RISK AREA FOR EXCESSIVE HEAT REMAINS ISSUED, AND IS EXPANDED
SLIGHTLY FURTHER WESTWARD BUT WITH LESS NORTHERN EXTENT WHILE ALSO EXTENDING
FURTHER IN TIME (SEP 6-9) COMPARED TO YESTERDAY. WITHIN THE HIGHLIGHTED
MODERATE RISK AREA, THE PETS SHOW AT LEAST A 40% CHANCE OF MAXIMUM TEMPERATURES
EXCEEDING THE 90TH PERCENTILE AND ENHANCED PROBABILITIES IN THE CALIBRATED HEAT
TOOLS AND EXPERIMENTAL HEAT RISK TOOL. THE ECMWF PET HAS SIGNIFICANTLY LESS
COVERAGE OF THIS THRESHOLD COMPARED TO THE GEFS COUNTERPART, DUE TO THE RIDGE
BEING SLIGHTLY MORE AMPLIFIED IN ADDITION TO POTENTIAL HEAT BIASES IN THE GEFS
ACROSS THE MIDWEST. THE NBM SHOWS THE POTENTIAL FOR NEAR RECORD HIGH
TEMPERATURES ACROSS TEXAS AND THE LOWER MISSISSIPPI VALLEY (EXCEEDING TRIPLE
DIGITS) THROUGH THE MIDDLE OF THE PERIOD.
IN THE SOUTHWEST, SURFACE HIGH PRESSURE OVER THE COLORADO PLATEAU AND SURFACE
LOW PRESSURE IN NORTHERN GULF OF CALIFORNIA SUPPORTS INCREASED CHANCES FOR HIGH
WINDS DURING WEEK-2. A SLIGHT RISK FOR HIGH WINDS IS POSTED FOR THE SOUTHWEST
FOR DAYS 10-14 (SEP 8-12) BASED ON WHEN MULTIPLE MODELS INDICATE STRENGTHENING
OF THE THERMAL LOW OVER BAJA CALIFORNIA AND PRESSURE GRADIENTS WOULD BE
STRONGEST.
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 CONTINUE TO BE FORECAST SOUTH OF
THE ALEUTIAN ISLANDS. NORTH OF THIS RIDGE, ABOVE-NORMAL PRECIPITATION IS
FAVORED ACROSS MUCH OF THE MAINLAND, HOWEVER, THIS IS UNLIKELY TO REACH
HAZARDOUS THRESHOLDS. THERE IS ALSO A THREAT OF A RECURVING TROPICAL CYCLONE
THAT COULD APPROACH WESTERN ALASKA DURING THE WEEK-2 PERIOD, BRINGING UNSETTLED
WEATHER TO THIS AREA IN ADDITION TO PARTS OF SOUTHERN COASTAL REGIONS OF THE
STATE. HOWEVER, UNCERTAINTY REMAINS VERY HIGH THUS NO CORRESPONDING HAZARD IS
POSTED.
FORECASTER: MELISSA OU
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