946
FXUS21 KWNC 301831
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 30 2024
SYNOPSIS: STRENGTHENING MID-LEVEL HIGH PRESSURE OVER THE WESTERN CONTIGUOUS
U.S. (CONUS) IS FAVORED TO PROMOTE WIDESPREAD ABOVE-NORMAL TEMPERATURES, WITH
AN ELEVATED RISK OF LATE SUMMER EXCESSIVE HEAT CONDITIONS ACROSS CALIFORNIA AND
THE DESERT SOUTHWEST, WITH THIS THREAT EXPANDING INTO THE NORTHERN HIGH PLAINS
LATER IN THE PERIOD. DOWNSTREAM, STRONG MID-LEVEL LOW PRESSURE IS EXPECTED TO
BRING UNSEASONABLY COOLER TEMPERATURES FOR MUCH OF THE EAST-CENTRAL CONUS. A
STALLED FRONTAL BOUNDARY DRAPED ACROSS THE SOUTHERN TIER IS EXPECTED TO BRING
INCREASED CHANCES FOR HEAVY PRECIPITATION FOR MUCH OF THE SOUTHEASTERN U.S.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT FOR THE CALIFORNIA VALLEY AND DESERT SOUTHWEST,
SAT-SUN, SEP 7-8.
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE NORTHERN HIGH PLAINS,
SUN-TUE, SEP 8-10.
SLIGHT RISK OF EXCESSIVE HEAT FOR THE DESERT SOUTHWEST, CALIFORNIA, PACIFIC
NORTHWEST, INTERIOR WEST, AND NORTHERN HIGH PLAINS, SAT-WED, SEP 7-11.
SLIGHT RISK OF HIGH WINDS FOR PARTS OF CALIFORNIA, GREAT BASIN AND DESERT
SOUTHWEST, SAT-MON, SEP 7-9.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE GULF COAST, SOUTHEAST,
AND MID-ATLANTIC, SAT-TUE, SEP 7-10.
RISK FOR RAPID ONSET DROUGHT FOR PORTIONS OF THE OHIO AND MIDDLE MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR MONDAY SEPTEMBER 02 - FRIDAY SEPTEMBER 06:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR SATURDAY SEPTEMBER 07 - FRIDAY SEPTEMBER 13: HEADING INTO NEXT WEEKEND,
500-HPA HEIGHT FIELDS FROM THE DYNAMICAL MODELS APPEAR TO BE ON TRACK FAVORING
AN AMPLIFIED MID-LEVEL PATTERN OVER NORTH AMERICA, CONSISTING OF A POTENT RIDGE
AXIS FROM THE DESERT SOUTHWEST TO SOUTH-CENTRAL CANADA, FLANKED BY A PAIR OF
TROUGHS OVER THE NORTHEASTERN PACIFIC AND THE EASTERN CONUS. SINCE YESTERDAY,
MODEL GUIDANCE HAS BECOME SLIGHTLY MORE PROGRESSIVE WITH THESE MID-LEVEL HEIGHT
FEATURES, WITH THE ECMWF BEING COMPARABLY STRONGER THAN THE GEFS IN THE
PATTERN, PARTICULARLY WITH THE TROUGHING FEATURES EARLY IN THE PERIOD. WHILE
THIS TRANSLATES INTO SOME ADDED UNCERTAINTY IN REGARDS TO INCREASED COLD AIR
(PRECIPITATION) POTENTIAL IN THE EAST (PACIFIC NORTHWEST), THE MAIN HAZARDS
FOCUS CONTINUES TO CENTER ON THE AMPLIFIED RIDGING WHICH IS LIKELY TO PROMOTE
UNSEASONABLY HOT, LATE SUMMER TEMPERATURES, WITH THE EXCESSIVE HEAT CONDITIONS
POSSIBLY UNFOLDING ACROSS THE WESTERN AND NORTH-CENTRAL CONUS.
A MODERATE RISK OF EXCESSIVE HEAT REMAINS ISSUED FOR THE CENTRAL CALIFORNIA
VALLEY AND DESERT SOUTHWEST FOR SEP 7-8 WHERE GOOD AGREEMENT EXISTS IN THE
UNCALIBRATED TEMPERATURE GUIDANCE, AND THE PROBABILISTIC EXTREMES TOOLS (PETS)
WHICH CONTINUE TO SHOW 30-40% CHANCES OF MAXIMUM TEMPERATURES EXCEEDING THE
85TH PERCENTILE. IN THE HIGHLIGHTED AREA, THERE ARE ELEVATED CHANCES FOR
DAYTIME TEMPERATURES TO EXCEED 100 (105) DEGREES F IN THE CALIFORNIA VALLEY
(SONORAN DESERT). PETS CONTINUE TO SHOW THE GREATEST SIGNALS IN PERCENTILE
SPACE FURTHER NORTH ACROSS THE INTERIOR WEST AND INTO THE NORTHERN HIGH PLAINS
IN PROXIMITY TO AN ANOMALOUS RIDGE CENTER EARLY IN WEEK-2. COMPARED TO PREVIOUS
ENSEMBLE RUNS, THOUGH, THE STRONGER MEAN TROUGHING FAVORED UPSTREAM IN THE GEFS
AND ECMWF OVER THE NORTHEASTERN PACIFIC LEADS TO DEEPER SOUTHWESTERLY FLOW OVER
THE NORTHWESTERN CONUS WHERE RAW TEMPERATURE TOOLS DEPICT STRONGER TEMPERATURE
ANOMALIES THROUGH THE MIDDLE OF WEEK-2. THIS TREND IS REFLECTED IN THE PETS,
WHERE THE ECMWF IN PARTICULAR, NOW FAVORS 50-60% (20-30%) CHANCES FOR
TEMPERATURES EXCEEDING THE 85TH (95TH) PERCENTILE, WITH ELEVATED CHANCES FOR
THESE DAYTIME HIGHS EXCEEDING 95 DEGREES F OVER PORTIONS OF MONTANA AND THE
WESTERN DAKOTAS. DESPITE SHORTER DAYLIGHT HOURS AT THIS LATITUDE LATER IN
SEPTEMBER, THIS SUPPORTS THE ADDITION OF A MODERATE RISK OF EXCESSIVE HEAT FOR
SEP 8-10. ENCOMPASSING THE MODERATE RISK AREAS, A BROAD SLIGHT RISK AREA OF
EXCESSIVE HEAT REMAINS POSTED FOR MUCH OF THE WEST AND NORTH-CENTRAL CONUS,
WITH ITS COVERAGE EXPANDED TO INCLUDE MORE OF THE NORTHERN AND CENTRAL PLAINS
TO ACCOUNT FOR THE EVOLUTION OF THE HEAT POTENTIAL. BASED ON CONTINUED SIGNS IN
THE GUIDANCE FOR THERMAL TROUGHING TO INDUCE STRENGTHENING PRESSURE GRADIENTS
IN THE CENTRAL CALIFORNIA VALLEY AND SONORAN DESERT, A SLIGHT RISK OF HIGH
WINDS IS ALSO POSTED FOR SEP 7-9, WHICH IS ALSO SUPPORTED IN THE PETS. THE
COMBINATION OF ABOVE-NORMAL TEMPERATURES AND EPISODES OF HIGH WINDS ALSO
ELEVATES THE RISK FOR WILDFIRE ACTIVITY, AND POTENTIALLY WORSEN ANY ACTIVE
INCIDENTS IN THE WEST.
DOWNSTREAM OF THE RIDGE AXIS IN THE WEST, A DIGGING TROUGH FAVORED EARLY IN
WEEK-2 IS EXPECTED TO BRING UNSEASONABLY COOL TEMPERATURES THROUGHOUT MUCH OF
THE EAST-CENTRAL CONUS. WITH THE ECMWF BEING MORE PRONOUNCED WITH THIS
MID-LEVEL FEATURE, THE TEMPERATURE RESPONSE HAS LIKEWISE BECOME STRONGER IN
CALIBRATED GUIDANCE WHERE THERE ARE HIGHER CHANCES (30-50%) FOR MINIMUM
TEMPERATURES FALLING BELOW THE 15TH PERCENTILE THROUGH MUCH OF THE MIDWEST
EARLY IN WEEK-2. THESE PET SIGNALS CORRESPOND TO HIGHER CHANCES FOR NIGHT TIME
TEMPERATURES DIPPING INTO THE UPPER 30S (F) FOR MANY PARTS OF THE GREAT LAKES
SUGGESTIVE OF LATE SUMMER FROST. HOWEVER, DUE TO THE LACK OF AGREEMENT WITH THE
GEFS PET, AND LITTLE TO NO SUPPORT FOR THIS POTENTIAL IN ANY OF THE RAW TOOLS,
NO CORRESPONDING HAZARDS ARE ISSUED.
ACROSS THE GULF STATES, THE WEATHER PREDICTION CENTER (WPC) SURFACE PROGNOSTIC
MAPS CONTINUE TO DEPICT A STALLED FRONTAL BOUNDARY ESTABLISHED FROM THE
SOUTHERN PLAINS TO THE SOUTHEAST LATE IN WEEK-1. THIS FEATURE LOOKS TO PERSIST
INTO WEEK-2, WHERE ENSEMBLES FAVOR A BROAD ZONE OF ENHANCED PRECIPITATION FROM
THE GULF OF MEXICO INTO THE SOUTHEAST AND MID-ATLANTIC THROUGH THE MIDDLE OF
THE PERIOD. ALTHOUGH THE GEFS PET IS MORE MODEST WITH THE HEAVY PRECIPITATION
RISK, THE ECMWF PET AND ITS UNCALIBRATED COUNTERPART CONTINUES TO FAVOR
INCREASED CHANCES FOR 3-DAY AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1 INCH TO
SUPPORT THE CONTINUATION OF A SLIGHT RISK OF HEAVY PRECIPITATION (SEP 7-10) FOR
MUCH OF THE SOUTHEAST. BASED ON THE LATEST TROPICAL WEATHER OUTLOOK (TWO) FROM
THE NATIONAL HURRICANE CENTER (NHC), THERE IS AN AREA JUST OFFSHORE IN
SOUTHEASTERN TEXAS WITH 20% CHANCES OF TROPICAL CYCLONE (TC) FORMATION DURING
THE NEXT WEEK. AND GIVEN THE INCREASE IN PRECIPITATION AMOUNTS OFFSHORE IN THE
LATEST GUIDANCE, THE SLIGHT RISK AREA IS EXPANDED TO INCLUDE THE WESTERN GULF
COAST IN THE UPDATED OUTLOOK. THE WPC SHOWS ENHANCED PRECIPITATION FORECAST
ALONG PARTS OF THE WESTERN AND CENTRAL GULF COAST DURING WEEK-1, AND ANY
LOCALLY HEAVY RAINFALL REALIZED DURING WEEK-2 MAY LEAD TO URBAN FLASH FLOODING
DURING WEEK-2. IT ALSO BEARS MENTIONING THAT SOME PORTION OF THE HEAVY
PRECIPITATION SIGNALS IN THE GULF OF MEXICO ARE ASSOCIATED WITH A FEW ENSEMBLE
MEMBERS LATCHING ONTO A TROPICAL DISTURBANCE THAT IS CURRENTLY LOCATED IN THE
MAIN DEVELOPMENT REGION OF THE ATLANTIC. THE NHC GIVES THIS DISTURBANCE 40%
CHANCES OF FORMATION FROM THE SOUTHERN CARIBBEAN TO JUST EAST OF THE LESSER
ANTILLES DURING THE NEXT SEVEN DAYS, AND WHILE THESE ODDS HAVE INCREASED FOR
DEVELOPMENT SINCE YESTERDAY, THERE IS TOO MUCH UNCERTAINTY AT THIS LEAD AS TO
WHERE AND WHEN THIS POTENTIAL SYSTEM WOULD TRACK SHOULD IT FORM.
ALTHOUGH TEMPERATURES ARE ANTICIPATED TO BE COOLER THAN NORMAL IN THE EASTERN
U.S. TO REDUCE EVAPOTRANSPIRATION RATES IN WEEK-2, NEAR TO BELOW NORMAL
PRECIPITATION FORECAST FOR THE NEXT TWO WEEKS OVER PARTS OF THE OHIO AND
TENNESSEE VALLEYS SUPPORTS THE CONTINUATION OF RAPID ONSET DROUGHT (ROD) IN THE
REGION. THE HIGHLIGHTED AREA CONTINUES TO REGISTER VERY POOR SOIL MOISTURE
PERCENTILES WITH MANY AREAS HAVING OBSERVED LESS THAN HALF OF THEIR NORMAL
RAINFALL ACCUMULATION SINCE THE BEGINNING OF AUGUST.
NO HAZARDS ARE ISSUED OVER ALASKA IN THE UPDATED OUTLOOK. WHILE ABOVE-NORMAL
PRECIPITATION REMAINS FAVORED FOR MUCH OF THE SOUTHEAST, THE AXIS OF THE
ANOMALOUS TROUGHING FAVORED IN NORTHEASTERN PACIFIC IS FAVORED TO SHIFT
EASTWARD WHICH SHOULD ALLOW THE HEAVY PRECIPITATION RISK EASE AND TIME OFF
DURING WEEK-2.
FORECASTER: NICK NOVELLA
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