470
FXUS21 KWNC 311801
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 31 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 CHANCES FOR EPISODES OF
HIGH WINDS, AND COULD WORSEN ONGOING WILDFIRES THROUGHOUT THE FOUR CORNERS.
ENHANCED ONSHORE FLOW IS EXPECTED TO BRING ABOVE-NORMAL PRECIPITATION ACROSS
MANY PARTS OF THE ALASKA MAINLAND.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHWEST, CENTRAL AND
SOUTHERN GREAT PLAINS AND MISSISSIPPI VALLEY, FRI-TUE, SEP 8-12.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHWEST, GREAT PLAINS,
MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS, AND THE SOUTHEAST, FRI-THU, SEP 8-14.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE MID-ATLANTIC AND THE
NORTHEAST, FRI-SUN, SEP 8-10.
SLIGHT RISK OF HIGH WINDS FOR MANY PORTIONS OF THE WESTERN CONUS, FRI-THU, SEP
8-14.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR SUNDAY SEPTEMBER 03 - THURSDAY SEPTEMBER 07:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY SEPTEMBER 08 - THURSDAY SEPTEMBER 14: 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 NORTH AMERICA, WITH THE STRONGEST POSITIVE
MID-LEVEL HEIGHT DEPARTURES ESTABLISHED ALONG AN AXIS FROM THE SOUTHWESTERN
CONUS TO THE CANADIAN MARITIMES. ON THE OPPOSING SIDES OF THIS RIDGE AXIS, WEAK
500-HPA TROUGHING LOOKS TO PERSIST ACROSS WESTERN CANADA AND THE PACIFIC
NORTHWEST, AS WELL AS MORE TROUGHING FAVORED IN THE ENSEMBLES OVER THE
SOUTHEASTERN 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
RETURN OF BUILDING MID-LEVEL HEIGHTS IS EXPECTED TO BRING A RENEWED RISK OF
EXCESSIVE HEAT NEXT WEEKEND FOR MANY AREAS THAT HAVE BEEN CONTINUALLY 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 THROUGHOUT THE
PERIOD TO SUPPORT A SLIGHT RISK OF EXCESSIVE HEAT FOR WEEK-2. 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 THROUGH DAY 12 (SEP 12)
AND NOW COVERS PORTIONS OF THE DESERT SOUTHWEST GIVEN INCREASED HEAT SIGNALS IN
THE GUIDANCE OVER PARTS OF SOUTHERN ARIZONA. 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.
FOR PRECIPITATION, THE AFOREMENTIONED TROUGHING FAVORED AND INCREASED SIGNS OF
DEEPENING MEAN SURFACE PRESSURE OVER THE MID-ATLANTIC AND WESTERN ATLANTIC
SUPPORTS AN INCREASING RISK OF HEAVY PRECIPITATION. SINCE YESTERDAY, SEVERAL
DETERMINISTIC SOLUTIONS FROM THE GFS AND CANADIAN SHOW TROPICAL STORM IDALIA'S
LOW CENTER BEGINNING TO TRACK BACK TO THE WEST AND APPROACH THE EASTERN
SEABOARD. NOTABLY, THE LATEST 0Z ECMWF DETERMINISTIC AND ENSEMBLE SHOW BETTER
SUPPORT FOR THIS REALIZATION, HOWEVER, THE TIMING, POSITION, AND EVENTUAL TRACK
OF THE LOW REMAINS QUITE VARIED AMONG THESE SOLUTIONS LEADING TO HIGH
UNCERTAINTY. REGARDLESS OF POTENTIAL TROPICAL IMPACTS, THERE IS GOOD AGREEMENT
IN THE PETS SHOWING INCREASED CHANCES FOR 3-DAY PRECIPITATION AMOUNTS EXCEEDING
THE 85TH PERCENTILE FROM THE MID-ATLANTIC TO THE NORTHEAST, WITH INCREASED
CHANCES FOR AMOUNTS EXCEEDING AN INCH OVER PORTIONS OF NEW ENGLAND. THEREFORE,
A SLIGHT RISK OF HEAVY PRECIPITATION IS ISSUED FROM THE DELMARVA TO MAINE FOR
SEP 8-10. FOR ANY HAZARDOUS WEATHER CONCERNS AND UPDATES RELATED TO TROPICAL
STORM IDALIA, PLEASE REFER TO THE NATIONAL HURRICANE CENTER.
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 SUPPORTS 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 IS EXPANDED TO INCLUDE MORE
OF THE WESTERN 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
HAS BEEN EXPANDED TO INCLUDE ADDITIONAL PORTIONS OF NEW MEXICO, OKLAHOMA AND
SOUTHWESTERN KANSAS IN THE UPDATED OUTLOOK.
OVER THE HIGHER LATITUDES OF NORTH AMERICA, ENHANCED ONSHORE FLOW IS FAVORED TO
BRING ABOVE-NORMAL PRECIPITATION OVER THE ALASKA MAINLAND TIED TO ANOMALOUS
RIDGING OVER THE ALEUTIANS AND TROUGHING TO THE NORTH OVER THE BERING SEA.
THERE IS FAIR AGREEMENT BETWEEN THE GEFS AND ECMWF PETS INDICATING 3-DAY
AMOUNTS EXCEEDING THE 85TH PERCENTILE, HOWEVER TOTALS ARE EXPECTED TO REMAIN
BELOW HAZARD THRESHOLDS. ELEVATED WIND SPEEDS ARE ALSO POSSIBLE, THOUGH THE
HIGHEST WINDS ARE EXPECTED TO REMAIN OFFSHORE.
FORECASTER: NICK NOVELLA
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page