151
FXUS21 KWNC 041754
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 04 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 GREAT PLAINS AND MISSISSIPPI VALLEY 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 SOUTHWEST, SOUTHERN PLAINS,
AND LOWER MISSISSIPPI VALLEY, TUE-WED, SEP 12-13.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE CENTRAL PLAINS AND
MIDDLE MISSISSIPPI VALLEY, TUE-THU, SEP 12-14.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE WESTERN CONUS, TUE-MON, SEP 12-18.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEASTERN MAINLAND AND
SOUTHEAST ALASKA, TUE-THU, SEP 12-14.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR THURSDAY SEPTEMBER 07 - MONDAY SEPTEMBER 11:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY SEPTEMBER 12 - MONDAY SEPTEMBER 18: THE MID-LEVEL HEIGHT PATTERN
ACROSS THE CONUS DURING THE PERIOD IS FAVORED TO BE MOSTLY ZONAL WITH
NEAR-NORMAL HEIGHTS DURING THE WEEK-2 PERIOD. EXCEPTIONS TO THIS PATTERN ARE
EARLY IN THE PERIOD WITH SLIGHT NEGATIVE 500-HPA HEIGHT ANOMALIES IN PARTS OF
THE NORTHEAST AND SLIGHTLY POSITIVE 500-HPA HEIGHT ANOMALIES IN THE SOUTHWEST.
IN ALASKA, A MORE AMPLIFIED PATTERN IS FAVORED WITH NEGATIVE HEIGHT ANOMALIES
FAVORED IN THE GULF OF ALASKA, PARTICULARLY EARLY IN THE PERIOD, LEADING TO A
MORE ACTIVE PATTERN IN SOUTHERN ALASKA.
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 95TH CLIMATOLOGICAL PERCENTILE ACROSS
PORTIONS OF THE SOUTHERN PLAINS AND LOWER-MISSISSIPPI VALLEY AND 95 DEG F IN
THE LOWER MISSISSIPPI VALLEY AND 100 DEG F IN THE SOUTHERN PLAINS. SIMILARLY,
IN THE DESERT SOUTHWEST, THERE ARE SIGNALS FOR TEMPERATURES TO EXCEED 105 DEG
F. IN BOTH CASES, THE SIGNALS CONTINUE TO SUBSIDE BY THE MIDDLE OF THE PERIOD.
THEREFORE, A SLIGHT RISK OF EXCESSIVE HEAT IS POSTED FOR SEP 12 AND 13 FOR
PORTIONS OF THE SOUTHWEST, SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY.
THE PETS AND RAW MODEL GUIDANCE CONTINUE TO INDICATE INCREASED CHANCES OF HEAVY
PRECIPITATION ACROSS PORTIONS OF THE GREAT PLAINS AND MISSISSIPPI VALLEY. THE
ECMWF PET IN PARTICULAR IS HIGHLIGHTING A LARGE AREA OF GREATER THAN 40% CHANCE
OF 3 DAY PRECIPITATION AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1 INCH. TODAY,
THE GEFS HAS COME INTO BETTER AGREEMENT WITH THIS SOLUTION ALTHOUGH WITH LOWER
CHANCES (>20%). THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION IS POSTED FOR
PARTS OF THE CENTRAL PLAINS AND MIDDLE MISSISSIPPI VALLEY FOR SEP 12-14.
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.
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 DURING THE NEXT TWO
WEEKS, AND HIGH EVAPOTRANSPIRATION RATES ASSOCIATED WITH PERSISTENT
ABOVE-NORMAL TEMPERATURES. RECENT INDICATIONS OF HEAVY PRECIPITATION NEARING
THE ROD RISK AREA MAY NECESSITATE MODIFYING THE HAZARD IN THE NEXT FEW DAYS.
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. THUS, A SLIGHT RISK OF HEAVY PRECIPITATION
IS POSTED FOR PORTIONS OF THE SOUTHERN MAINLAND AND MUCH OF SOUTHEAST ALASKA
FOR SEP 12-14. 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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