811
FXUS21 KWNC 111905
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 11 2023
SYNOPSIS: BUILDING MID-LEVEL HIGH PRESSURE IS EXPECTED TO MAINTAIN AN ELEVATED
RISK OF EXCESSIVE HEAT OVER THE SOUTHERN TIER OF THE CONTIGUOUS U.S. (CONUS)
WITH INCREASED CHANCES FOR EXCESSIVE HEAT EMERGING ACROSS PARTS OF THE CENTRAL
AND EASTERN CONUS LATER IN AUGUST. A POTENTIAL MOISTURE SURGE FROM TROPICAL
ACTIVITY FAVORED IN THE EAST PACIFIC INCREASES THE RISK FOR HEAVY PRECIPITATION
AND FLASH FLOODING ACROSS PARTS OF THE DESERT SOUTHWEST. POTENTIALLY STRONG
SURFACE PRESSURE GRADIENTS ENHANCE THE RISK FOR HIGH WINDS ACROSS PARTS OF THE
SOUTHWESTERN CONUS WHICH MAY WORSEN ONGOING WILDFIRES ACROSS THE REGION. THE
EXCESSIVE HEAT POTENTIAL COUPLED WITH ABNORMALLY DRY CONDITIONS ACROSS THE
SOUTH-CENTRAL CONUS SUPPORTS THE ELEVATED POTENTIAL FOR RAPID ONSET DROUGHT
ACROSS PARTS OF TEXAS AND THE LOWER MISSISSIPPI VALLEY.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT FOR PARTS OF THE SOUTHERN PLAINS, LOWER
MISSISSIPPI VALLEY AND SOUTHEAST, SAT-MON, AUG 19-21.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE CENTRAL AND NORTHERN PLAINS,
MIDDLE MISSISSIPPI, TENNESSEE AND OHIO VALLEYS, AND MID-ATLANTIC, SAT-WED, AUG
19-23.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF SOUTHERN PLAINS, LOWER
MISSISSIPPI, SOUTHEAST, SAT-FRI, AUG 19-25.
SLIGHT RISK OF HIGH WINDS FOR MUCH OF THE DESERT SOUTHWEST AND THE FOUR
CORNERS, SAT-TUE, AUG 19-22.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE DESERT SOUTHWEST, SAT-WED,
AUG 19-23.
FLOODING POSSIBLE FOR PARTS OF THE DESERT SOUTHWEST.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR MONDAY AUGUST 14 - FRIDAY AUGUST 18:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY AUGUST 19 - FRIDAY AUGUST 25: TOWARDS THE END OF NEXT WEEK,
500-HPA HEIGHT FORECASTS FROM THE ECMWF AND GEFS CONTINUE TO ADVERTISE A
PATTERN FLIP OVER THE CONUS. STRONG RIDGING ESTABLISHED OVER THE WESTERN NORTH
AMERICA DURING WEEK-1 IS FAVORED TO RAPIDLY WEAKEN AND TRANSITION INTO MORE OF
A TROUGHING REGIME WHERE RESULTANT NORTHWESTERLY FLOW IS EXPECTED TO HELP
MODERATE TEMPERATURES AND QUELL ANY EXCESSIVE HEAT POTENTIAL OVER THE
NORTHWESTERN CONUS DURING WEEK-2. INTERESTINGLY, MUCH OF THIS PATTERN
TRANSITION IN THE WEST APPEARS TO BE FACILITATED BY THE PHASING OF A LINGERING
CUTOFF CIRCULATION ASSOCIATED WITH THE REMNANTS OF TROPICAL CYCLONE EUGENE THAT
DISSIPATED OVER THE EASTERN PACIFIC EARLIER THIS WEEK. MEANWHILE, MULTI-MODEL
ENSEMBLE GUIDANCE HAS BEEN CONSISTENT REGARDING THE DEVELOPMENT OF ANOMALOUS
MID-LEVEL RIDGING DOWNSTREAM OVER THE CENTRAL AND EASTERN CONUS. WHILE THERE
ARE SOME DISCREPANCIES BETWEEN THE GEFS AND ECMWF IN REGARDS TO THIS FEATURE,
BOTH SUPPORT INCREASED CHANCES FOR ABOVE-NORMAL TEMPERATURES FOR MANY AREAS
EAST OF THE ROCKIES LATER IN AUGUST.
EVEN WITH AN EVOLVING HEIGHT PATTERN THROUGHOUT THE CONUS, THE SOUTHERN TIER IS
NOT OUT OF THE WOODS JUST YET IN REGARDS TO THE EXCESSIVE HEAT POTENTIAL.
RELATIVE TO YESTERDAY, 500-HPA HEIGHTS ANOMALIES ARE SLIGHTLY STRONGER OVER
THIS PART OF THE COUNTRY NEXT WEEKEND, AND PROBABILISTIC EXTREMES TOOLS (PETS)
FROM THE GEFS AND ECMWF FEATURE HIGHER HEAT SIGNALS OVER THE SOUTHERN PLAINS,
LOWER MISSISSIPPI VALLEY AND THE SOUTHEAST, WITH AT LEAST 40-50% CHANCES FOR
MAXIMUM TEMPERATURES EXCEEDING THE 85TH PERCENTILE. GIVEN ADDED SUPPORT FROM
THE NATIONAL BLEND OF MODELS (NBM) FAVORING DAYTIME TEMPERATURES APPROACHING
DAILY RECORDS EARLY IN WEEK-2 OVER PARTS OF TEXAS AND LOUISIANA, A MODERATE
RISK OF EXCESSIVE HEAT REMAINS ISSUED (NOW VALID THROUGH AUG 21) AND IS
EXTENDED EASTWARD INTO PARTS OF GEORGIA AND FLORIDA TO ACCOUNT FOR INCREASED
HEAT SIGNALS IN THE CALIBRATED GUIDANCE.
AS 500-HPA RIDGING IS FAVORED TO AMPLIFY OVER THE CENTRAL CONUS THROUGH THE
MIDDLE OF WEEK-2, PETS INDICATE INCREASED CHANCES FOR TEMPERATURES EXCEEDING
THE 85TH CLIMATOLOGICAL PERCENTILE THROUGH MUCH OF THE GREAT PLAINS AND
MIDWEST. HOWEVER, THERE IS ADDED UNCERTAINTY IN REGARDS TO THE NORTHERN EXTENT
OF THIS EXCESSIVE HEAT POTENTIAL GIVEN THAT THE GEFS HAS CONSISTENTLY
MAINTAINED A CONSIDERABLE WARM BIAS OVER THE MIDWEST THIS SUMMER, WITH MAXIMUM
TEMPERATURES ALSO BECOMING INCREASINGLY INFLUENCED BY DECREASING SUN ANGLES
OVER THE HIGHER LATITUDES OF THE CONUS LATER IN AUGUST. A PAIR OF SLIGHT RISK
AREAS OF EXCESSIVE HEAT ARE POSTED, PRIMARILY BASED ON THE ECMWF PET (DUE TO
BETTER SKILL RECENTLY OBSERVED OVER THIS PART OF THE COUNTRY) AND THE FAVORED
EVOLUTION OF THE MID-LEVEL PATTERN. THE FIRST SLIGHT RISK COVERS PARTS OF THE
CENTRAL AND NORTHERN PLAINS, MIDDLE MISSISSIPPI, OHIO AND TENNESSEE VALLEYS,
AND MID-ATLANTIC, VALID THROUGH AUG 23 BEFORE THE MID-LEVEL RIDGING EASES, AND
THE SECOND IS POSTED FOR THE SOUTH-CENTRAL AND SOUTHEASTERN CONUS WHERE BOTH
PETS MAINTAIN INCREASED CHANCES FOR TEMPERATURES EXCEEDING THE 85TH PERCENTILE
THROUGHOUT THE ENTIRETY OF WEEK-2.
IN THE TROPICS, THE MADDEN-JULIAN OSCILLATION (MJO) IS FAVORED TO REMAIN
DISORGANIZED THROUGH LATE AUGUST. HOWEVER, UPPER-LEVEL VELOCITY POTENTIAL
ANOMALY FORECASTS SHOW AN ENVELOPE OF ENHANCED DIVERGENCE ALOFT CENTERED OVER
THE WESTERN HEMISPHERE, WITH EQUATORIAL KELVIN AND ROSSBY WAVE ACTIVITY
SUPPORTIVE OF TROPICAL CYCLONE DEVELOPMENT IN THE EASTERN PACIFIC. THIS IS
REFLECTED IN THE ENSEMBLE AND DETERMINISTIC GUIDANCE WHICH REMAIN BULLISH ON AN
AREA OF DEEPENING LOW PRESSURE TO THE SOUTH OF MEXICO LATE IN WEEK-1. SEVERAL
MODEL SOLUTIONS TRACK THE LOW NORTHWESTWARD WHICH IS LIKELY TO TRIGGER A GULF
OF CALIFORNIA MOISTURE SURGE, BRINGING ENHANCED LOWER-LEVEL MOISTURE AND
POTENTIALLY HEAVY RAINFALL INTO THE DESERT SOUTHWEST. THERE IS AGREEMENT IN THE
GEFS AND ECMWF PETS SHOWING INCREASED CHANCES FOR 3-DAY PRECIPITATION AMOUNTS
EXCEEDING THE 85TH PERCENTILE ACROSS THIS AREA, SUPPORTING A SLIGHT RISK OF
HEAVY PRECIPITATION REMAINING POSTED THROUGH AUG 23. HEAVY PRECIPITATION COULD
LEAD TO FLOODING AND/OR FLASH FLOODING ACROSS AREAS IN THE SOUTHWESTERN CONUS
DURING WEEK-2. IT IS WORTH NOTING THAT FLASH FLOODING CAN OCCUR EVEN IN AREAS
THAT DO NOT DIRECTLY RECEIVE RAINFALL DUE TO RUNOFF FROM PRECIPITATION
UPSTREAM.
THE PREDICTED THERMAL LOW OVER THE DESERT SOUTHWEST AND SURFACE HIGH PRESSURE
OVER THE COLORADO PLATEAU FAVOR AN ENHANCED PRESSURE GRADIENT ACROSS PARTS OF
THE DESERT SOUTHWEST AND FOUR CORNERS. THEREFORE, A SLIGHT RISK OF HIGH WINDS
IS POSTED AUG 19-22 WHERE PETS CONTINUE TO DEPICT INCREASED CHANCES OF WIND
SPEEDS EXCEEDING THE 85TH PERCENTILE. A RAPID ONSET DROUGHT (ROD) RISK MAINS
POSTED ACROSS PARTS OF THE LOWER MISSISSIPPI VALLEY AND TEXAS DUE TO THE
COMBINATION OF INCREASED 30-DAY PRECIPITATION DEFICITS, LITTLE TO NO RAINFALL
FAVORED IN THE NEXT TWO WEEKS, AND HIGH EVAPOTRANSPIRATION RATES ASSOCIATED
WITH THE ABOVE-NORMAL TEMPERATURES.
OVER ALASKA, NO HAZARDS ARE POSTED. ANOMALOUS TROUGHING FAVORED OVER THE BERING
SEA IS EXPECTED TO BRING ABOVE-NORMAL PRECIPITATION FOR MUCH OF NORTHERN
MAINLAND, HOWEVER TOTALS ARE NOT EXPECTED TO REACH HAZARD THRESHOLDS.
FORECASTER: NICK NOVELLA
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