066
FXUS21 KWNC 141908
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT AUGUST 14 2023
SYNOPSIS: MID-LEVEL HIGH PRESSURE IS PREDICTED FOR THE CENTRAL AND EASTERN
CONTIGUOUS U.S. 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 LEADING TO DECREASING CHANCES FOR
EXCESSIVE HEAT CONDITIONS BY THE END OF WEEK-2. THE EXCESSIVE HEAT POTENTIAL
COUPLED WITH ABNORMALLY DRY CONDITIONS ACROSS THE SOUTH-CENTRAL CONUS SUPPORTS
THE ELEVATED POTENTIAL FOR RAPID ONSET DROUGHT ACROSS PARTS OF TEXAS AND THE
LOWER MISSISSIPPI VALLEY.
HAZARDS
HIGH RISK OF EXCESSIVE HEAT FOR PARTS OF THE LOWER MISSISSIPPI AND TENNESSEE
VALLEYS, SOUTHERN APPALACHIANS, AND SOUTHEAST, TUE-WED, AUG 22-23.
MODERATE RISK OF EXCESSIVE HEAT FOR PARTS OF THE SOUTHEASTERN U.S., TUE-THU,
AUG 22-24.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE CENTRAL AND NORTHERN PLAINS,
UPPER AND MIDDLE MISSISSIPPI AND OHIO VALLEYS, GREAT LAKES REGION, CENTRAL
APPALACHIANS, AND MID-ATLANTIC, TUE-MON, AUG 22-23.
SLIGHT RISK OF EXCESSIVE HEAT FOR PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS,
MIDDLE AND LOWER MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, APPALACHIANS,
SOUTHEAST, AND MID-ATLANTIC, TUE-MON, AUG 22-28.
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 THURSDAY AUGUST 17 - MONDAY AUGUST 21:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY AUGUST 22 - MONDAY AUGUST 28: THERE IS MULTI-MODEL DEPICTION OF
POSITIVE 500-HPA HEIGHTS ACROSS THE CENTRAL AND EASTERN CONUS EXCLUDING NEW
ENGLAND THROUGHOUT WEEK-2. HOWEVER, THESE POSITIVE HEIGHT ANOMALIES ARE
ANTICIPATED TO BE WEAKER COMPARED TO LATE WEEK-1 AND WEAKEN NOTICEABLY
ESPECIALLY BY DAY 10 (AUG 24). THIS TRANSLATES TO THE HEAT TOOLS INDICATING THE
GREATEST CHANCES FOR HEAT AT THE BEGINNING OF WEEK-2, MODERATING BY THE MIDDLE
OF THE PERIOD.
A HIGH RISK FOR EXCESSIVE HEAT IS DESIGNATED FOR PARTS OF THE LOWER MISSISSIPPI
AND TENNESSEE VALLEYS, SOUTHERN APPALACHIANS, AND SOUTHEAST, AUG 22-23,
PRIMARILY BASED ON WHERE THE ECMWF PROBABILISTIC EXTREMES TOOL (PET) INDICATES
AT LEAST A 60% CHANCE OF EXCEEDING THE 85TH PERCENTILE CLIMATOLOGICALLY. THIS
IS FURTHER SUPPORTED BY THE NATIONAL BLEND OF MODELS (NBM) INDICATING NEAR
RECORD BREAKING HIGH TEMPERATURES EXCEEDING TRIPLE DIGITS (DEG F), AS WELL AS
THE CALIBRATED ECMWF HEAT TOOL. THE NBM ALSO SHOWS WIDESPREAD COVERAGE WHERE
THE HIGH RISK IS POSTED OF NIGHTTIME TEMPERATURES NEARING RECORDS, IN THE UPPER
70S TO LOWER 80S EVEN UNTIL AUG 25. A MODERATE RISK (>40% CHANCE) OF EXCESSIVE
HEAT IS POSTED FOR PARTS OF THE SOUTHEASTERN U.S. BASED ON SIGNALS FROM THE
GEFS AND ECMWF PETS AND CALIBRATED HEAT INDEX FORECASTS, AUG 22-24. THE ECMWF
PET DOES INDICATE AT LEAST 40% CHANCES FOR MAXIMUM TEMPERATURES EXCEEDING THE
85TH PERCENTILE LINGERING ACROSS PORTIONS OF GULF COAST STATES FOR ANOTHER DAY,
ALBEIT LESS WIDESPREAD THAN THE IDENTIFIED RISK DAYS. A BROADER SLIGHT RISK
AREA REMAINS HIGHLIGHTED ACROSS PORTIONS OF THE CENTRAL AND NORTHERN PLAINS,
UPPER AND MIDDLE MISSISSIPPI AND OHIO VALLEYS, GREAT LAKES REGION, CENTRAL
APPALACHIANS, AND MID-ATLANTIC THROUGHOUT WEEK-2. AN AREA OF SLIGHT RISK IS
ALSO DESIGNATED FOR PARTS OF THE CENTRAL AND SOUTHERN PLAINS, MIDDLE AND LOWER
MISSISSIPPI, OHIO, AND TENNESSEE VALLEYS, APPALACHIANS, SOUTHEAST, AND
MID-ATLANTIC FOR THE BEGINNING OF WEEK-2 (AUG 22-23). THERE IS SIGNIFICANT
UNCERTAINTY REGARDING HOW FAR NORTH EXCESSIVE HEAT WOULD OCCUR AT THE BEGINNING
OF THE PERIOD. THE NBM SHOWS A STATION POTENTIALLY NEARING RECORD HIGH
TEMPERATURES FOR NORTHWESTERN WISCONSIN. DUE TO THIS BEING A RELATIVELY
LOCALIZED SIGNAL AND CALIBRATED TOOLS NOT BEING AS FAVORABLE, THE NORTHERN
EXTENT IS NOT EXPANDED TO THIS AREA.
IN THE TROPICS, THE MADDEN-JULIAN OSCILLATION (MJO) IS FAVORED TO REMAIN
DISORGANIZED THROUGH LATE AUGUST. HOWEVER, UPPER-LEVEL VELOCITY POTENTIAL
ANOMALY FORECASTS SHOW AN ENVELOPE OF ENHANCED DIVERGENCE ALOFT CENTERED OVER
THE WESTERN HEMISPHERE, WITH EQUATORIAL KELVIN AND ROSSBY WAVE ACTIVITY
SUPPORTIVE OF TROPICAL CYCLONE DEVELOPMENT IN THE EASTERN PACIFIC. THIS IS
REFLECTED IN THE ENSEMBLE AND DETERMINISTIC GUIDANCE WHICH REMAIN BULLISH ON AN
AREA OF DEEPENING LOW PRESSURE TO THE WEST OF BAJA CALIFORNIA LATE IN WEEK-1.
SEVERAL MODEL SOLUTIONS TRACK THE LOW NORTHWESTWARD WHICH IS LIKELY TO TRIGGER
A GULF OF CALIFORNIA MOISTURE SURGE, BRINGING ENHANCED LOWER-LEVEL MOISTURE AND
POTENTIALLY HEAVY RAINFALL TO PORTIONS OF THE SOUTHWESTERN CONUS. MODEL
SOLUTIONS FAVOR THE HEAVIEST PRECIPITATION FOR CALIFORNIA AND SOUTHERN NEVADA,
WHICH IS FURTHER WEST THAN PREVIOUS RUNS (OVER THE DESERT SOUTHWEST). THERE IS
STILL RELATIVELY HIGH UNCERTAINTY ABOUT WHETHER THE GREATEST LIKELIHOOD FOR
THIS FEATURE WOULD OCCUR BEFORE THE ONSET OF WEEK-2. THE ECMWF AND CANADIAN
PETS SHOW AT LEAST A 20% CHANCE OF 3-DAY RAINFALL TOTALS EXCEEDING THE 85TH
PERCENTILE, PRIMARILY ACROSS ARIZONA, WITH THE ECMWF SHOWING THIS SIGNAL ALSO
ACROSS SOUTHERN IDAHO. DUE TO THE GEFS' LACK OF AGREEMENT WITH THESE TOOLS AND
HIGH UNCERTAINTY OF WHERE THE HEAVIEST RAINFALL WILL OCCUR, A HEAVY
PRECIPITATION HAZARD IS NOT SPECIFIED AT THIS TIME.
DESPITE THE LACK OF RAINFALL HAZARDS POSTED TODAY, THE ANTICIPATED MOISTURE
SURGE AND THE POTENTIAL FOR LOCALIZED HEAVY RAINFALL, THE POSSIBLE FLOOD AREA
HAS BEEN EXPANDED TO INCLUDE THE WESTERN FOUR CORNERS, NEVADA, SOUTHERN
CALIFORNIA, AND SOUTHERN IDAHO. MULTIPLE ENSEMBLE MEANS INDICATE AMPLIFIED
MID-LEVEL TROUGHING AT THE END OF WEEK-1 WITH WEAKENING BY WEEK-2. THERE
REMAINS THE POSSIBILITY OF THIS AMPLIFIED TROUGHING TO LINGER INTO WEEK-2 WHICH
WOULD ALSO SUPPORT THE INCREASED LIKELIHOOD OF POSSIBLE HEAVY PRECIPITATION AND
FLOODING EXTENDING INTO THE NORTHERN GREAT BASIN. IT IS WORTH NOTING THAT FLASH
FLOODING CAN OCCUR EVEN IN AREAS THAT DO NOT DIRECTLY RECEIVE RAINFALL DUE TO
RUNOFF FROM PRECIPITATION UPSTREAM.
SOME MODELS INDICATE THE POSSIBILITY FOR SURFACE LOW FORMATION OVER THE
NORTHERN PLAINS AHEAD OF THE MID-LEVEL TROUGH OVER THE NORTHWEST EARLY IN
WEEK-2. THIS PATTERN COULD BRING HEAVY PRECIPITATION TO SOME OF THIS REGION.
THE ECMWF PET IS PRIMARILY SUPPORTIVE OF THIS ENHANCED PRECIPITATION SIGNAL,
SHOWING AT LEAST A 50% CHANCE OF 3-DAY RAINFALL EXCEEDING THE 85TH PERCENTILE
AND HALF AN INCH ACROSS EASTERN MONTANA AND THE DAKOTAS AT THE BEGINNING OF THE
PERIOD. DESPITE TOOL SIGNALS NOT REACHING HAZARDS CRITERIA AT THIS TIME, THERE
EXISTS THE POSSIBILITY OF LOCALIZED FLOODING ACROSS THE REGION.
PREDICTED MID-LEVEL TROUGHING ACROSS THE NORTHEAST MAY LEAD TO SURFACE LOW
FORMATION AND FRONTAL ACTIVITY ACROSS THE REGION, WHICH COULD SUPPORT INCREASED
CHANCES FOR HIGH WINDS AT THE BEGINNING OF WEEK-2. THE ECMWF AND CANADIAN PETS
SHOW AT LEAST A 20% CHANCE OF WIND SPEEDS EXCEEDING THE 85TH PERCENTILE AND 20
MPH ACROSS SOME COASTAL AREAS OF THE MID-ATLANTIC. DUE TO HIGH UNCERTAINTY
REGARDING THE DETAILS OF THIS FEATURE, A RELATED HAZARD IS NOT DESIGNATED AT
THIS TIME.
A RAPID ONSET DROUGHT (ROD) RISK REMAINS POSTED ACROSS PARTS OF THE LOWER
MISSISSIPPI VALLEY AND TEXAS DUE TO THE COMBINATION OF INCREASED 30-DAY
PRECIPITATION DEFICITS, LITTLE TO NO RAINFALL FAVORED IN THE NEXT TWO WEEKS,
AND HIGH EVAPOTRANSPIRATION RATES ASSOCIATED WITH THE ABOVE-NORMAL TEMPERATURES.
OVER ALASKA, NO HAZARDS ARE POSTED. ANOMALOUS TROUGHING FAVORED OVER THE BERING
SEA IS EXPECTED TO BRING ABOVE-NORMAL PRECIPITATION FOR PORTIONS OF THE WESTERN
AND NORTHERN MAINLAND, ALTHOUGH TOTALS ARE NOT EXPECTED TO REACH HAZARD
THRESHOLDS.
FORECASTER: MELISSA OU
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