466
FXUS21 KWNC 291832
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 29 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 MAINLY ACROSS
CALIFORNIA AND THE DESERT SOUTHWEST. 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,
FRI-SUN, SEP 6-8.
SLIGHT RISK OF EXCESSIVE HEAT FOR THE PARTS OF THE DESERT SOUTHWEST,
CALIFORNIA, PACIFIC NORTHWEST, INTERIOR WEST, AND NORTHERN HIGH PLAINS,
FRI-TUE, SEP 6-10.
SLIGHT RISK OF HIGH WINDS FOR PARTS OF CALIFORNIA, GREAT BASIN AND DESERT
SOUTHWEST, FRI-SUN, SEP 6-8
SLIGHT RISK OF HEAVY PRECIPITATION FOR PORTIONS OF THE GULF COAST, SOUTHEAST,
AND MID-ATLANTIC, FRI-TUE, SEP 6-10.
SLIGHT RISK OF HEAVY PRECIPITATION FOR THE ALASKA PANHANDLE, FRI-SUN, SEP 6.
RISK FOR RAPID ONSET DROUGHT FOR PORTIONS OF THE OHIO AND MIDDLE MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR SUNDAY SEPTEMBER 01 - THURSDAY SEPTEMBER 05:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR FRIDAY SEPTEMBER 06 - THURSDAY SEPTEMBER 12: BY LATE NEXT WEEK, DYNAMICAL
MODELS CONTINUE TO ADVERTISE AN AMPLIFYING MID-LEVEL RIDGE CENTERED OVER
SOUTHWESTERN CANADA, FLANKED BY A PAIR OF TROUGHS OVER THE GULF OF ALASKA AND
THE EASTERN U.S. MODEL SOLUTIONS EARLIER THIS WEEK HAD BEEN FAIRLY PROGRESSIVE
WITH THE MID-LEVEL HEIGHT ANOMALIES, HOWEVER THE CURRENT PRESENTATION IN THE
ENSEMBLES IS SUGGESTIVE OF A MORE STABLE PATTERN TAKING SHAPE, WHERE SUCH
ANOMALIES COULD BE MORE PERSISTENT AND BRING A LINGERING THREAT OF LATE SUMMER
EXCESSIVE HEAT CONDITIONS FOR MANY PARTS OF THE WEST DURING THE WEEK-2 PERIOD.
RAW TEMPERATURE TOOLS SHOW THE GREATEST ANOMALIES ALONG THE MID-LEVEL RIDGE
AXIS EXTENDING FROM THE DESERT SOUTHWEST INTO NORTHERN HIGH PLAINS. THIS IS
ALSO REFLECTED IN BOTH THE GEFS AND ECMWF PROBABILISTIC EXTREMES TOOLS (PETS)
WHICH DEPICT ELEVATED CHANCES FOR MAXIMUM DAYTIME TEMPERATURES EXCEEDING THE
85TH CLIMATOLOGICAL PERCENTILE. WHILE THE HIGHEST SIGNALS (>50% CHANCES) IN
PERCENTILE SPACE ARE IN PROXIMITY TO THE ANOMALOUS RIDGE CENTER OVER
SOUTHWESTERN CANADA, CHANCES FOR ACTUAL TEMPERATURES EXCEEDING HAZARD CRITERIA
ARE LOWER, MOST LIKELY DUE TO DECREASING SUN ANGLES AT THE HIGHER LATITUDES OF
THE NORTHWESTERN CONUS DURING SEPTEMBER. HOWEVER, THIS IS LESS THE CASE FURTHER
SOUTH, WHERE BOTH RAW AND CALIBRATED TOOLS FAVOR DAYTIME TEMPERATURES WELL INTO
THE TRIPLE DIGITS ACROSS THE CALIFORNIA VALLEY AND THE DESERT SOUTHWEST, WITH
INCREASED SUPPORT IN THE NATIONAL BLEND OF MODELS (NBM) FOR A FEW LOCATIONS IN
THE SOUTHWEST WITH NEAR RECORD BREAKING TEMPERATURES. WITH 30-40% CHANCES FOR
TEMPERATURES EXCEEDING THE 85TH PERCENTILE IN THE PETS, THIS SUPPORTS THE
ADDITION OF A MODERATE RISK OF EXCESSIVE HEAT BEING ISSUED FOR THESE REGIONS,
AND IS VALID THROUGH DAY 10 (SEP 8) BEFORE MUCH OF THE RIDING ALOFT BEGINS TO
EASE. RELATIVE TO YESTERDAY, THE SLIGHT RISK THAT WAS POSTED FOR THE CALIFORNIA
VALLEY IS EXPANDED TO INCLUDE MUCH OF THE WEST AND INTO THE NORTHERN PLAINS
BASED ON THE EVOLVING PET SIGNAL WITH THE RIDGING POTENTIALLY SHIFTING
EASTWARD. DUE TO INCREASED SIGNS FOR THERMAL TROUGHING IN THE CENTRAL
CALIFORNIA VALLEY AND SONORAN DESERT, WITH SUPPORT FROM THE ECMWF PET, A SLIGHT
RISK OF HIGH WINDS IS ALSO POSTED FOR SEP 6-8. 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 THE
EAST-CENTRAL CONUS. THE ASSOCIATED NORTHWESTERLY FLOW TRANSLATES INTO HIGHER
SIGNALS IN THE PETS FOR MINIMUM TEMPERATURES FALLING BELOW THE 15TH PERCENTILE,
WITH SOME LOCATIONS IN THE GREAT LAKES HAVING THE POTENTIAL FOR SEEING
NIGHTTIME MINIMUMS DROPPING INTO THE UPPER 30S. WHILE ANOMALOUS, THE COLDER AIR
IS NOT EXPECTED TO BE IMPACTFUL AT THIS TIME, AS SOME LOCATIONS IN THE NORTHERN
GREAT LAKES TYPICALLY EXPERIENCE NEAR FREEZING TEMPERATURES BY MID-SEPTEMBER.
NO CORRESPONDING TEMPERATURE HAZARDS ARE POSTED, BUT THIS AREA WILL BE
MONITORED IN SUBSEQUENT OUTLOOKS.
ACROSS THE SOUTHERN TIER, 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. GIVEN CONTINUED SUPPORT IN THE PETS FOR 3-DAY
AMOUNTS EXCEEDING THE 85TH PERCENTILE AND 1 INCH, THE SLIGHT RISK OF HEAVY
PRECIPITATION REMAINS ISSUED, BUT IS EXTENDED THROUGH DAY 12 (SEP 10) BASED ON
UNCALIBRATED ECMWF GUIDANCE. OF NOTE, SOME ENSEMBLE MEMBERS FROM THE GEFS AND
ECMWF SUGGEST SOME OF THE HEAVY PRECIPITATION SIGNALS STEM FROM A TROPICAL
DISTURBANCE IN THE MAIN DEVELOPMENT REGION OF THE ATLANTIC. WHILE THE NATIONAL
HURRICANE CENTER (NHC) HAS INCREASED THEIR CHANCES FOR DEVELOPMENT ASSOCIATED
WITH THIS DISTURBANCE (CURRENTLY 40% FOR THE NEXT SEVEN DAYS), IT IS MUCH TOO
EARLY TO ATTRIBUTE ANY OF THE HIGHLIGHTED HEAVY PRECIPITATION RISK TO THIS
POTENTIAL SYSTEM. HOWEVER, THE TROPICAL SITUATION DOES BEAR CLOSE MONITORING IN
THE COMING DAYS.
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. RELATIVE TO YESTERDAY, MUCH OF THE DESIGNATED ROD AREA HAS BEEN REMOVED
OVER THE SOUTHERN PLAINS, MISSISSIPPI AND TENNESSEE VALLEYS DUE TO
ABOVE-AVERAGE RAINFALL FAVORED DURING WEEK-1, WHICH IS EXPECTED TO HELP
ALLEVIATE ONGOING MOISTURE DEFICITS.
TIED TO STRONG TROUGHING OVER THE GULF OF ALASKA, PETS CONTINUE TO INDICATE
INCREASED CHANCES FOR HEAVY PRECIPITATION FOR SOUTHEASTERN ALASKA EARLY IN
WEEK-2, SUPPORTING A SLIGHT RISK FOR HEAVY PRECIPITATION FOR THE ALASKA
PANHANDLE FOR SEP 6. THE SOIL IN THIS REGION IS ALREADY SATURATED DUE TO HEAVY
ANTECEDENT RAINFALL AND LANDSLIDES HAVE BEEN AN ONGOING CONCERN, A PROBLEM
WHICH MAY BE EXACERBATED BY THIS POTENTIALLY HAZARDOUS PRECIPITATION.
FORECASTER: NICK NOVELLA
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