800
FXUS21 KWNC 051841
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 05 2023
SYNOPSIS: MID-LEVEL ZONAL FLOW FORECAST ACROSS THE CONTIGUOUS U.S. (CONUS) IS
EXPECTED TO REDUCE CHANCES FOR EXCESSIVE HEAT ACROSS THE SOUTHERN TIER BY THE
WEEK-2 PERIOD. SIGNALS HAVE INCREASED FOR CHANCES OF HEAVY PRECIPITATION ACROSS
PORTIONS OF THE SOUTHERN AND CENTRAL PLAINS ALONG WITH ANOTHER AREA ALONG THE
EASTERN SEABOARD EARLY IN THE WEEK-2 PERIOD. STRONG SURFACE PRESSURE GRADIENTS
FAVORED IN THE SOUTHWEST MAY BRING ENHANCED CHANCES FOR EPISODES OF HIGH WINDS,
AND COULD WORSEN ONGOING WILDFIRES THROUGHOUT THE FOUR CORNERS. LOW PRESSURE IN
THE GULF OF ALASKA MAY BRING HEAVY PRECIPITATION TO SOUTHEAST ALASKA AND THE
SOUTHERN ALASKAN MAINLAND.
HAZARDS
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE GULF COAST AND LOWER
MISSISSIPPI VALLEY, WED, SEP 13.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE CENTRAL AND SOUTHERN
PLAINS, WED-FRI, SEP 13-15
SLIGHT RISK OF HEAVY PRECIPITATION FOR MUCH OF THE EASTERN SEABOARD, WED-SAT,
SEP 13-16.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE WESTERN CONUS, WED-TUE, SEP 13-19.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEASTERN MAINLAND AND
SOUTHEAST ALASKA, WED-SUN, SEP 13-17.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE LOWER MISSISSIPPI VALLEY.
DETAILED SUMMARY
FOR FRIDAY SEPTEMBER 08 - TUESDAY SEPTEMBER 12:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY SEPTEMBER 13 - TUESDAY SEPTEMBER 19: THE MID-LEVEL HEIGHT PATTERN
ACROSS THE CONUS DURING THE PERIOD IS FAVORED TO BE MOSTLY ZONAL BUT IS MORE
AMPLIFIED RELATIVE TO YESTERDAY. NEGATIVE 500-HPA HEIGHT ANOMALIES AND
TROUGHING IS FORECAST ACROSS THE EASTERN CONUS. POSITIVE 500-HPA HEIGHT
ANOMALIES AND SLIGHT RIDGING IS FAVORED ACROSS THE WEST. IN ALASKA, A STRONG
TROUGH AND NEGATIVE 500-HPA HEIGHTS ARE FAVORED ACROSS THE WESTERN PARTS OF THE
STATE, LEADING TO STRONG ONSHORE FLOW.
AS THE FORECAST MOVES INTO THE SECOND HALF OF SEPTEMBER AND WITH A ZONAL FLOW
PATTERN BEING FAVORED ACROSS MUCH OF THE CONUS CHANCES FOR EXCESSIVE HEAT ARE
MUCH REDUCED IN THE PROBABILISTIC EXTREMES TOOLS (PET) FROM THE GEFS AND ECMWF
RELATIVE TO PREVIOUS WEEKS. EARLY IN THE PERIOD THERE REMAINS A GREATER THAN
20% CHANCE OF TEMPERATURES EXCEEDING THE 90TH CLIMATOLOGICAL PERCENTILE ACROSS
PORTIONS OF THE WESTERN GULF COAST AND 95 DEG F. HOWEVER, THESE SIGNALS
CONTINUE TO REDUCE RELATIVE TO PREVIOUS GUIDANCE AND THEREFORE, A SLIGHT RISK
OF EXCESSIVE HEAT IS POSTED FOR ONLY SEP 13. IN THE DESERT SOUTHWEST, THE PET
TOOLS HAVE FURTHER REDUCED THE CHANCES OF TEMPERATURE EXCEEDING THE 90TH
PERCENTILE RELATIVE TO YESTERDAY AND THE RAW GUIDANCE INDICATES TEMPERATURES TO
BE AROUND 100 DEG F WHICH WOULD BE UNLIKELY TO REACH HAZARDOUS CRITERIA.
THEREFORE, NO EXCESSIVE HEAT RISK IS POSTED FOR THIS AREA BUT TEMPERATURES
SHOULD CONTINUE TO BE MONITORED.
IN THE EASTERN CONUS, A BROAD TROUGH IS FAVORED TO BRING BELOW-NORMAL
TEMPERATURES TO PARTS OF THE REGION. ACROSS PARTS OF THE UPPER MIDWEST, THE
PETS INDICATE THE POTENTIAL FOR TEMPERATURES TO FALL BELOW 40 DEG F INTRODUCING
CHANCES FOR AN EARLY FROST TO THE REGION. THE GREAT LAKES SURFACE WATER
TEMPERATURES REMAIN NEAR AVERAGE FOR THIS TIME OF YEAR AND COULD MODERATE
SURFACE TEMPERATURES ENOUGH TO LIMIT A WIDESPREAD FROST. AS SUCH, THERE IS NO
CORRESPONDING MUCH BELOW NORMAL TEMPERATURE HAZARD POSTED BUT VULNERABLE AREAS
SHOULD REMAIN COGNIZANT OF THIS RISK.
THE PETS AND RAW MODEL GUIDANCE CONTINUE TO INDICATE INCREASED CHANCES OF HEAVY
PRECIPITATION ACROSS PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS. THE AREA OF
HIGHEST CONCERN HAS SHIFTED FURTHER SOUTH RELATIVE TO YESTERDAY WITH THE ECMWF
PET IN PARTICULAR HIGHLIGHTING A LARGE AREA OF GREATER THAN 30% CHANCE OF 3 DAY
PRECIPITATION AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1 INCH. THE GEFS IS IN
ROUGH AGREEMENT WITH THE ECMWF, HOWEVER, WITH LOWER PRECIPITATION AMOUNTS. RAW
MODEL GUIDANCE IS ALSO IN GOOD AGREEMENT WITH THE STRONGEST SIGNAL FURTHER
SOUTH ACROSS PORTIONS OF TEXAS AND OKLAHOMA. THUS, A SLIGHT RISK OF HEAVY
PRECIPITATION IS POSTED FOR PARTS OF THE CENTRAL AND SOUTHERN PLAINS FOR SEP
13-15.
IN THE EASTERN CONUS AHEAD OF A MID-LEVEL TROUGH OVER THE GREAT LAKES THERE IS
A RISK OF HEAVY PRECIPITATION FOCUSED ALONG THE EASTERN SEABOARD. THE ECMWF PET
IS SUPPORTIVE OF A RISK FOR 3-DAY PRECIPITATION AMOUNTS TO EXCEED THE 85TH
PERCENTILE AND 1 INCH. WHILE THE GEFS PET IS NOT AS SUPPORTIVE WITH A LESS THAN
20% CHANCE OF PRECIPITATION AMOUNTS GREATER THAN 85TH PERCENTILE THE GUIDANCE
INDICATES AT LEAST A 20% CHANCE OF OVER 3/4 AN INCH. RAW MODEL GUIDANCE FROM
THE GEFS AND ECMWF ARE IN GOOD AGREEMENT WITH THEIR RESPECTIVE PETS AS WELL.
THE DIFFERENCES MAY RELY ON THE STRENGTH OF THE NEGATIVE 500-HPA HEIGHT
ANOMALIES WITH A STRONG FEATURE IN THE GEFS THAT MAY PUSH THE HEAVIEST
PRECIPITATION OFF THE EAST COAST. THE ECMWF HAS A WEAKER TROUGH, IN LINE WITH
THE CANADIAN, THAT WOULD ALLOW THE PRECIPITATION TO DEVELOP NEARER TO THE
COAST. A SECOND CONCERN FOR THIS AREA IS POTENTIAL IMPACTS FROM A TROPICAL
CYCLONE, TROPICAL DEPRESSION 13, THAT MAY COME NEAR TO THE EAST COAST BEFORE IT
RECURVES THAT MAY ADD ADDITIONAL MOISTURE TO THIS BOUNDARY. PLEASE REFER TO THE
NATIONAL HURRICANE CENTER FOR MORE INFORMATION ON TROPICAL DEPRESSION 13.
THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR SEP 13-16.
ACROSS THE SOUTHWESTERN CONUS, SURFACE HIGH PRESSURE FAVORED OVER THE COLORADO
PLATEAU AND A POTENTIALLY DEEPENING THERMAL LOW OVER THE SONORAN DESERT INDUCED
BY WARMER THAN NORMAL TEMPERATURES IN THE REGION SUPPORT THE POTENTIAL FOR
STRENGTHENING PRESSURE GRADIENTS AND EPISODES OF HIGH WINDS. A SLIGHT RISK OF
HIGH WINDS IS ISSUED FOR THE ENTIRETY OF WEEK-2 AND COVERS MUCH OF THE
SOUTHWESTERN CONUS WHERE THERE IS GREATER COVERAGE OF WIND SPEEDS EXCEEDING THE
85TH PERCENTILE BASED ON THE ECMWF PET. THERE ARE CURRENTLY MULTIPLE ACTIVE
WILDFIRES THROUGHOUT THE FOUR CORNERS, AND THE COMBINATION OF ANOMALOUS WARMTH,
DRYNESS, AND ELEVATED WIND POTENTIAL DURING WEEK-2 WOULD LIKELY MAKE THESE
ONGOING INCIDENTS DIFFICULT TO CONTAIN.
AS PREVIOUSLY MENTIONED, THERE IS AN INCREASED RISK OF HEAVY PRECIPITATION
ACROSS THE CENTRAL AND SOUTHERN PLAINS, OVERLAPPING WITH PREVIOUS RAPID ONSET
DROUGHT (ROD) RISKS AREAS. THESE AREAS ARE NOW FAVORED TO SEE AT LEAST SOME
PRECIPITATION EARLY IN THE WEEK-2 PERIOD. THEREFORE, THE ROD RISK HAS BEEN
REDUCED TO AREAS FURTHER EAST IN THE LOWER MISSISSIPPI VALLEY WHERE LESS
RAINFALL IS FAVORED DURING THE NEXT TWO WEEKS AND INCREASED 30 DAY
PRECIPITATION DEFICITS AND HIGH EVAPOTRANSPIRATION RATES ASSOCIATED WITH
ABOVE-NORMAL TEMPERATURES.
IN ALASKA, AN AREA OR MULTIPLE AREAS OF SURFACE LOW PRESSURE APPEAR LIKELY TO
DEVELOP IN THE GULF OF ALASKA THAT COULD BRING INCREASED CHANCES FOR HEAVY
PRECIPITATION AND HIGH WINDS TO PARTS OF THE SOUTHERN MAINLAND AND SOUTHEAST.
THERE IS GOOD AGREEMENT BETWEEN THE GEFS AND ECMWF PETS ALONG WITH THE RAW
DYNAMICAL TOOLS INDICATING 3-DAY PRECIPITATION AMOUNTS EXCEEDING THE 85TH
PERCENTILE AND APPROACHING 3 INCHES FOR BOTH DAYS 8-10 AND 10-12. THUS, A
SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR PORTIONS OF THE SOUTHERN
MAINLAND AND MUCH OF SOUTHEAST ALASKA FOR SEP 13-17. ALSO, THE ECMWF PET AND TO
SOME EXTENT THE GEFS HAVE ELEVATED CHANCES FOR HIGH WINDS INTO MUCH OF THE GULF
OF ALASKA. THESE WINDS ARE LIKELY TO BE MOSTLY A MARITIME HAZARD SO NO
CORRESPONDING WIND HAZARD IS POSTED.
FORECASTER: RYAN BOLT
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