391
FXUS21 KWNC 011804
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SEPTEMBER 01 2023
SYNOPSIS: FORECAST MID-LEVEL HIGH PRESSURE FAVORED OVER MUCH OF THE CONTIGUOUS
U.S. (CONUS) IS EXPECTED TO BRING NEARLY COAST TO COAST ABOVE-NORMAL
TEMPERATURES, WITH AN ELEVATED RISK OF EXCESSIVE HEAT MAINLY OVER THE
SOUTH-CENTRAL CONUS DURING THE PERIOD. MORE COHERENT SIGNS FOR MEAN SURFACE LOW
PRESSURE ALONG AND OFF THE EASTERN SEABOARD SUPPORTS AN INCREASED RISK FOR
HEAVY PRECIPITATION FROM THE MID-ATLANTIC TO THE NORTHEAST. 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 AND
HIGH WINDS TO SOUTHEAST ALASKA AND THE SOUTHERN ALASKAN MAINLAND.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHWEST, CENTRAL AND
SOUTHERN GREAT PLAINS AND MISSISSIPPI VALLEY, SAT-MON, SEP 9-11.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHWEST, CENTRAL AND
SOUTHERN GREAT PLAINS, MISSISSIPPI, AND TENNESSEE VALLEYS, AND THE SOUTHEAST,
SAT-WED, SEP 9-13.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE MID-ATLANTIC AND THE
NORTHEAST, SAT-MON, SEP 9-11.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF THE WESTERN CONUS, SAT-FRI, SEP 9-15.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF SOUTHEASTERN MAINLAND AND
SOUTHEAST ALASKA, SAT-TUE, SEP 9-12.
SLIGHT RISK OF HIGH WINDS FOR PORTIONS OF SOUTHERN ALASKA, SAT-TUE, SEP 9-12.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR MONDAY SEPTEMBER 04 - FRIDAY SEPTEMBER 08:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY SEPTEMBER 09 - FRIDAY SEPTEMBER 15: AT THE START OF WEEK-2, THERE
IS GOOD AGREEMENT BETWEEN THE GEFS AND ECMWF 500-HPA HEIGHT FORECASTS FEATURING
AMPLIFIED RIDGING THROUGHOUT MUCH OF WESTERN NORTH AMERICA, WITH THE STRONGEST
POSITIVE MID-LEVEL HEIGHT DEPARTURES ESTABLISHED ALONG THE ROCKY MOUNTAINS. ON
THE OPPOSING SIDES OF THIS RIDGE AXIS, WEAK 500-HPA TROUGHING LOOKS TO DEVELOP
ACROSS WESTERN CANADA AND THE PACIFIC NORTHWEST, AS WELL AS MORE TROUGHING
FAVORED IN THE ENSEMBLES OVER THE EASTERN CONUS AND WESTERN ATLANTIC. THIS
LATTER DEVELOPMENT IS NOTEWORTHY, AS ANY CYCLONIC FLOW AIDED BY ANOMALOUS
EASTERLIES UNDERNEATH THE RIDGE FAVORED OVER THE NORTHEAST WOULD INCREASE
CHANCES FOR DISTURBED WEATHER OVER PARTS OF THE EASTERN SEABOARD.
FOR MANY PARTS OF THE SOUTHWESTERN CONUS AND THE GREAT PLAINS, THE FAVORED
ABOVE NORMAL MID-LEVEL HEIGHTS ARE EXPECTED TO BRING A RENEWED RISK OF
EXCESSIVE HEAT NEXT WEEKEND FOR MANY AREAS THAT HAVE BEEN PERPETUALLY AFFECTED
BY STIFLING TEMPERATURES THIS SUMMER. THIS IS REFLECTED IN THE PROBABILISTIC
EXTREMES TOOLS (PETS), WHICH CONTINUE TO DEPICT A BROAD AREA WITH AT LEAST A
20% CHANCE OF MAXIMUM TEMPERATURES EXCEEDING THE 90TH PERCENTILE THROUGH MUCH
OF THE PERIOD TO SUPPORT A SLIGHT RISK OF EXCESSIVE HEAT FOR SEP 9-13.
CONSISTENT WITH THE PREVIOUS THINKING, CAUTION IS TAKEN IN REGARDS TO THE
HIGHER PROBABILITIES DEPICTED IN THE GEFS PET, AS THIS TOOL HAS CONSISTENTLY
EXHIBITED A WARM BIAS THROUGHOUT MUCH OF THE MIDWEST, AND THE OUTLOOK RELIES
MORE ON THE ECMWF PET FOR ELEVATED EXCESSIVE HEAT POTENTIAL FOR AREAS MAINLY
EAST OF THE ROCKIES. BASED ON THIS TOOL, UNCALIBRATED TEMPERATURE TOOLS, AND
THE PREVAILING MID-LEVEL PATTERN, A MODERATE RISK OF EXCESSIVE HEAT REMAINS
ISSUED ACROSS CENTRAL AND SOUTHERN PLAINS, LOWER MISSISSIPPI VALLEY AND
PORTIONS OF THE DESERT SOUTHWEST THROUGH DAY 10 (SEP 11). A HIGH RISK OF
EXCESSIVE HEAT WAS CONSIDERED FOR PARTS OF THE SOUTHERN HIGH PLAINS DUE TO
HIGHER PROBABILITIES (50%) FOR TEMPERATURES EXCEEDING THE 90TH PERCENTILE IN
THE ECMWF PET. HOWEVER, THE GEFS PET IS LESS SUPPORTIVE OF THIS POTENTIAL, AND
THERE IS MORE UNCERTAINTY IN REGARDS TO WHETHER ACTUAL TEMPERATURES WILL REACH
HAZARD THRESHOLDS LATER IN SEPTEMBER. WITHIN THE MODERATE RISK AREA, THE
NATIONAL BLEND OF MODELS (NBM) ANALYSIS CONTINUES TO INDICATE AN INCREASED
NUMBER OF STATIONS APPROACHING NEAR RECORD DAYTIME TEMPERATURES THROUGH NEXT
WEEKEND OVER THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY. RELATIVE TO
YESTERDAY, THE PET TOOLS ARE INDICATING LOWER CHANCES OF EXCESSIVE HEAT IN THE
NORTHERN PLAINS AND MIDWEST, THEREFORE, THE SLIGHT AND MODERATE RISK SHAPES DO
NOT EXTEND AS FAR NORTH IN THESE REGIONS.
FOR PRECIPITATION, THE AFOREMENTIONED TROUGHING FAVORED ACROSS THE EASTERN
CONUS AND INCREASED SIGNS OF A STATIONARY BOUNDARY ALONG FROM THE MID-ATLANTIC
TO THE NORTHEAST BRING INCREASED CHANCES FOR HEAVY PRECIPITATION TO THESE
REGIONS. THERE REMAIN SOME ENSEMBLE MEMBERS FROM THE GEFS AND ECMWF THAT BRING
A REMNANT LOW FROM TROPICAL STORM IDALIA INTO THE NORTHEAST EARLY IN WEEK-2,
ALTHOUGH THIS SEEMS LESS LIKELY THAN YESTERDAY. REGARDLESS OF POTENTIAL
TROPICAL IMPACTS, PETS FROM THE ECMWF AND GEFS ARE IN GOOD AGREEMENT SHOWING
INCREASED CHANCES FOR 3-DAY PRECIPITATION AMOUNTS EXCEEDING THE 85TH
PERCENTILE. THEREFORE, A SLIGHT RISK OF HEAVY PRECIPITATION IS ISSUED FROM THE
DELMARVA TO MAINE FOR SEP 9-11.
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. BASED ON DECLINING PERCENT OF NORMALS, THE ROD AREA
WAS EXPANDED YESTERDAY TO INCLUDE ADDITIONAL PORTIONS OF NEW MEXICO, OKLAHOMA
AND SOUTHWESTERN KANSAS.
IN ALASKA, AN AREA OR MULTIPLE AREAS OF SURFACE LOW PRESSURE LOOK 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 9-12. FURTHER, THE ECMWF PET AND TO SOME EXTENT THE GEFS HAVE ELEVATED
CHANCES FOR HIGH WINDS INTO MUCH OF THE GULF OF ALASKA. WHILE THESE WINDS ARE
LIKELY TO BE MOSTLY A MARITIME HAZARD, THERE IS A CHANCE FOR INCREASED HIGH
WINDS EXTENDING INTO PORTIONS OF SOUTH-CENTRAL ALASKA FOR SEP 9-12.
FORECASTER: RYAN BOLT
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