804
FXUS21 KWNC 151910
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 15 2023
SYNOPSIS: MID-LEVEL HIGH PRESSURE IS PREDICTED FOR THE CENTRAL AND EASTERN
CONTIGUOUS U.S. (CONUS) EXCLUDING THE NORTHEAST DURING WEEK-2, BRINGING AN
INCREASED RISK OF EXCESSIVE HEAT TO MUCH OF THIS AREA. HOWEVER, THIS HIGH
PRESSURE IS ANTICIPATED TO WEAKEN THROUGHOUT WEEK-2 AS MID-LEVEL LOW PRESSURE
EXPANDS ACROSS THE EASTERN CONUS. THIS PATTERN SUPPORTS INCREASED POTENTIAL FOR
EXCESSIVE HEAT INITIALLY ACROSS A BROAD AREA EAST OF THE ROCKIES (EXCLUDING THE
NORTHEASTERN CONUS) GRADUALLY BEING LIMITED TO THE SOUTH-CENTRAL AND
SOUTHEASTERN U.S. BY THE MIDDLE OF THE PERIOD. A POTENTIAL STORM TRACKING FROM
EASTERN CANADA TO THE NORTHEAST MAY BRING HEAVY RAINFALL TO THE NORTHEAST AND
HIGH WINDS TO COASTAL PORTIONS OF THE MID-ATLANTIC AND NORTHEAST AT THE
BEGINNING OF THE PERIOD.
HAZARDS
HIGH RISK OF EXCESSIVE HEAT FOR PORTIONS OF EASTERN TEXAS, THE LOWER
MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, SOUTHERN APPALACHIANS, AND
SOUTHEAST, WED, AUG 23.
MODERATE RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE SOUTHERN PLAINS, LOWER AND
MIDDLE MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, SOUTHERN APPALACHIANS,
SOUTHERN MID-ATLANTIC, AND SOUTHEAST, WED-FRI, AUG 23-25.
SLIGHT RISK OF EXCESSIVE HEAT FOR PARTS OF THE CENTRAL AND NORTHERN PLAINS,
UPPER AND MIDDLE MISSISSIPPI, AND OHIO VALLEYS, AND APPALACHIANS, WED-FRI, AUG
23-25.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE SOUTH-CENTRAL AND SOUTHEASTERN
CONUS, WED-TUE, AUG 23-29.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE NORTHEAST, THU-FRI, AUG
24-25.
SLIGHT RISK OF HIGH WINDS FOR COASTAL PORTIONS OF THE NORTHEAST AND
MID-ATLANTIC, THU-FRI, AUG 24-25.
FLOODING POSSIBLE FOR PARTS OF THE WESTERN FOUR CORNERS, NEVADA, SOUTHERN
CALIFORNIA, AND SOUTHERN IDAHO.
RAPID ONSET DROUGHT RISK FOR PARTS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI
VALLEY.
DETAILED SUMMARY
FOR FRIDAY AUGUST 18 - TUESDAY AUGUST 22:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY AUGUST 23 - TUESDAY AUGUST 29: THERE CONTINUES TO BE GOOD MODEL
AGREEMENT INDICATING MID-LEVEL RIDGING CENTERED OVER THE CENTRAL CONUS
WEAKENING THROUGHOUT WEEK-2 TRANSLATING TO DECREASING COVERAGE AND
PROBABILITIES OF EXCESSIVE HEAT ACROSS THE EASTERN TWO-THIRDS OF THE CONUS. A
HIGH RISK OF EXCESSIVE HEAT IS POSTED FOR PORTIONS OF EASTERN TEXAS, THE LOWER
MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, SOUTHERN APPALACHIANS, AND SOUTHEAST
FOR DAY 8 ONLY (AUG 23) PRIMARILY BASED ON WHERE THE ECMWF PROBABILISTIC
EXTREMES TOOL (PET) INDICATES AT LEAST A 60% CHANCE OF EXCEEDING THE 85TH
PERCENTILE CLIMATOLOGICALLY, FURTHER SUPPORTED BY THE CALIBRATED HEAT INDEX
TOOL. THE NATIONAL BLEND OF MODELS (NBM) INDICATES NEAR RECORD BREAKING MAXIMUM
(TRIPLE DIGITS DEG F) AND MINIMUM TEMPERATURES (UPPER 70S/LOW 80S) PRIMARILY
ACROSS THE LOWER MISSISSIPPI VALLEY, ALBEIT WITH LESS COVERAGE COMPARED TO
YESTERDAY. A MODERATE RISK (>40% CHANCE) OF EXCESSIVE HEAT IS POSTED FOR
PORTIONS OF THE SOUTHERN PLAINS, LOWER AND MIDDLE MISSISSIPPI, OHIO, AND
TENNESSEE VALLEYS, SOUTHERN APPALACHIANS, SOUTHERN MID-ATLANTIC, AND SOUTHEAST,
AUG 23-25. TWO SLIGHT RISK OF EXCESSIVE HEAT AREAS ARE DESIGNATED, WITH THE
FIRST RISK AREA HIGHLIGHTED FOR PARTS OF THE CENTRAL AND NORTHERN PLAINS, UPPER
AND MIDDLE MISSISSIPPI, AND OHIO VALLEYS, AND APPALACHIANS, AUG 23-25, AND AN
ADJACENT ONE SOUTH OF IT FOR MUCH OF THE SOUTH-CENTRAL AND SOUTHEASTERN CONUS,
THROUGHOUT WEEK-2 WHERE EXCESSIVE HEAT MAY LINGER FOR A LONGER DURATION.
THERE IS GOOD MODEL AGREEMENT AND MODEL RUN CONSISTENCY INDICATING MID-LEVEL
TROUGHING DEVELOPING OVER THE NORTHEASTERN CONUS, SUPPORTING A SURFACE LOW
TRACKING FROM EASTERN CANADA TO THE NORTHEAST AT THE BEGINNING OF THE PERIOD.
THIS LOW AND ASSOCIATED FRONTAL BOUNDARY MAY BRING UNSETTLED WEATHER AND
INCREASED CHANCES FOR HEAVY RAIN AND HIGH WINDS TO PORTIONS OF THE EAST, MOST
LIKELY DAYS 9 TO 10 (AUG 24-25). A SLIGHT RISK OF HEAVY PRECIPITATION IS
DESIGNATED FOR THE NORTHEAST, WHILE A SLIGHT RISK OF HIGH WINDS IS POSTED FOR
COASTAL PORTIONS OF THE NORTHEAST AND MID-ATLANTIC FOR THESE DAYS, PRIMARILY
BASED ON THE ECMWF PET. THE ECMWF PET SHOWS AT LEAST A 20% CHANCE OF 3-DAY
RAINFALL TOTALS (DURING DAYS 8-10) EXCEEDING THE 85TH PERCENTILE WITH THE GEFS
INDICATING THIS SIGNAL REMAINING OFFSHORE. HOWEVER, RECENT DETERMINISTIC GFS
AND CANADIAN MODEL SOLUTIONS SHOW THIS HEAVY PRECIPITATION POTENTIAL ASSOCIATED
WITH THE ANTICIPATED STORM. THE ECMWF AND CANADIAN PETS INDICATE AT LEAST A 20%
CHANCE OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND 25 MPH WHERE THE SLIGHT
RISK OF HIGH WINDS IS DESIGNATED.
SEVERAL MODEL SOLUTIONS CONTINUE TO SHOW THE POSSIBILITY OF A TROPICAL CYCLONE
TRACKING NORTHWESTWARD NEAR BAJA CALIFORNIA AT THE END OF WEEK-1, BEFORE MOVING
WESTWARD BY THE ONSET OF WEEK-2. THIS STORM IS LIKELY TO TRIGGER A GULF OF
CALIFORNIA MOISTURE SURGE, BRINGING ENHANCED LOWER-LEVEL MOISTURE AND
POTENTIALLY HEAVY RAINFALL TO PORTIONS OF THE WESTERN CONUS. THE ECMWF AND
CANADIAN PETS SHOW AT LEAST A 20% CHANCE OF 3-DAY RAINFALL TOTALS EXCEEDING THE
85TH PERCENTILE ACROSS PARTS OF THE FOUR CORNERS, SOUTHERN CALIFORNIA, NEVADA,
AND IDAHO AT THE BEGINNING OF WEEK-2. DUE TO THE HIGH UNCERTAINTY OF WHERE THE
HEAVIEST RAINFALL WILL OCCUR, A HEAVY PRECIPITATION HAZARD IS NOT SPECIFIED AT
THIS TIME. HOWEVER, LOCALIZED HEAVY RAINFALL COULD TRIGGER FLOODING IN SOME
AREAS. THE POSSIBLE FLOOD RISK HAS BEEN EXTENDED FURTHER NORTH TO COVER MORE
AREAS OF IDAHO, WHERE THERE IS A CONCERN FOR WET THUNDERSTORMS. IT IS WORTH
NOTING THAT FLASH FLOODING CAN OCCUR EVEN IN AREAS THAT DO NOT DIRECTLY RECEIVE
RAINFALL DUE TO RUNOFF FROM PRECIPITATION UPSTREAM.
FORECASTER: MELISSA OU
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