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FXUS21 KWNC 161823
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 16 2024
SYNOPSIS: MID-LEVEL HIGH PRESSURE PERSISTING FROM WEEK-1 INTO WEEK-2 OVER THE
WESTERN CONTIGUOUS U.S. (CONUS) IS EXPECTED TO SUSTAIN AN ELEVATED RISK FOR
EXCESSIVE HEAT OVER MANY PARTS OF CALIFORNIA, GREAT BASIN, NORTHERN
INTERMOUNTAIN WEST, AND NORTHERN HIGH PLAINS. MID-LEVEL LOW PRESSURE FOCUSED
OVER THE LOWER PLAINS AND MISSISSIPPI VALLEY IS LIKELY TO PROMOTE COOLER AND
WETTER CONDITIONS EARLY IN WEEK-2, WITH INCREASED CHANCES FOR HEAVY
PRECIPITATION FROM THE SOUTHERN PLAINS TO THE EASTERN SEABOARD. AN ACTIVE NORTH
AMERICA MONSOON CIRCULATION SHOULD LEAD TO AN INCREASED RISK OF LOCALLY HEAVY
PRECIPITATION THROUGHOUT THE DESERT SOUTHWEST EARLY IN WEEK-2.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT ACROSS PORTIONS OF CALIFORNIA, THE GREAT BASIN,
THE ROCKIES, AND THE NORTHERN HIGH PLAINS, WED, JUL 24.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE WESTERN CONUS AND NORTHERN HIGH
PLAINS, WED-FRI, JUL 24-26.
SLIGHT RISK OF EPISODIC HIGH WINDS FOR MUCH OF THE WESTERN CONUS, WED-SAT, JUL
24-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, THE LOWER
MISSISSIPPI, TENNESSEE AND OHIO VALLEYS, THE APPALACHIANS, SOUTHEAST,
MID-ATLANTIC, AND NORTHEAST, WED-THU, JUL 24-25.
SLIGHT RISK OF PERIODS OF HEAVY PRECIPITATION FOR PORTIONS OF THE DESERT
SOUTHWEST, WED-FRI, JUL 24-26.
RAPID ONSET DROUGHT RISK FOR PORTIONS OF THE NORTHERN PLAINS.
FLOODING POSSIBLE OVER THE EASTERN CAROLINAS AND SOUTHEASTERN VIRGINIA.
DETAILED SUMMARY
FOR FRIDAY JULY 19 - TUESDAY JULY 23:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR WEDNESDAY JULY 24 - TUESDAY JULY 30: BY THE MIDDLE OF NEXT WEEK, THE GEFS
AND ECMWF ENSEMBLE MEAN 500-HPA HEIGHT ANOMALY FORECASTS ARE IN FAIR AGREEMENT
FEATURING POSITIVE HEIGHT DEPARTURES OVERSPREADING CENTRAL CANADA TO THE
WESTERN CONUS. THIS MID-LEVEL FEATURE LOOKS TO REMAIN THE MAIN HAZARDS DRIVER
FOR THE CONTINUATION OF THE EXCESSIVE HEAT RISK FROM WEEK-1 INTO EARLY WEEK-2
OVER MUCH OF THE INTERIOR WEST, THOUGH THIS RISK IS EXPECTED TO BE QUELLED BY
THE DEVELOPMENT OF TROUGHING SHIFTING EASTWARD FROM THE NORTHEASTERN PACIFIC
INTO WESTERN CANADA, LEADING TO TO A DECREASE IN HEIGHTS THROUGHOUT THE WESTERN
CONUS TOWARDS THE END OF NEXT WEEK. EAST OF THE ROCKIES, THE GEFS AND ECMWF
CONTINUE TO FEATURE DOWNSTREAM TROUGHING CENTERED OVER THE GREAT PLAINS AND
MISSISSIPPI VALLEY. WHILE BOTH ENSEMBLES HAVE TRENDED IN DEEPENING THIS FEATURE
IN RECENT MODEL RUNS TO BRING UNSEASONABLY COLDER THAN NORMAL SUMMERTIME
TEMPERATURES TO THE CENTRAL CONUS EARLY IN WEEK-2, THERE ARE SIGNIFICANT
DISPARITIES AMONG THE MEAN SOLUTIONS WITH THE EVOLUTION OF THIS MID-LEVEL
FEATURE, WHICH HAS IMPLICATIONS ON THE POTENTIAL EMERGENCE OF HEAT CONDITIONS
OVER PARTS OF THE NORTHEASTERN CONUS LATER IN JULY.
ACROSS THE WEST, RAW TEMPERATURE TOOLS SHOW THE GREATEST MAXIMUM DAYTIME
DEPARTURES FROM NORMAL COINCIDING WITH THE MID-LEVEL RIDGE AXIS EXTENDING FROM
THE CALIFORNIA VALLEY NORTHEASTWARD INTO THE NORTHERN HIGH PLAINS LATE IN
WEEK-1. THESE TOOLS SHOW THE POSITIVE ANOMALIES WEAKENING HEADING INTO WEEK-2
TO SIGNAL THE PEAK OF THE HEAT EVENT TIMING OFF, THOUGH PROBABILISTIC EXTREMES
TOOLS (PETS) MAINTAIN ELEVATED (30-50%) CHANCES FOR MAXIMUM TEMPERATURES
EXCEEDING THE 85TH PERCENTILE AT THE OUTSET OF WEEK-2. BY DAY 9 (JUL 25), BOTH
PETS CONTINUE TO SHOW A MARKED REDUCTION IN THE HEAT SIGNALS THROUGHOUT THE
WEST TIED TO MORE ZONAL FLOW INDUCED BY THE AFOREMENTIONED TROUGHING SHIFTING
INTO WESTERN CANADA. THEREFORE, A MODERATE RISK OF EXCESSIVE HEAT IS CONTINUED
FROM THE CENTRAL CALIFORNIA VALLEY TO THE NORTHERN HIGH PLAINS FOR DAY 8, AND A
BROADER SLIGHT RISK AREA FOR EXCESSIVE HEAT REMAINS POSTED WHERE PETS RETAIN
SOME HEAT RISK (20% CHANCES FOR EXCEEDING THE 85TH PERCENTILE) THROUGH DAY 10
(JUL 26).
FROM DAY 10 AND BEYOND, THE PICTURE FOR HAZARDOUS HEAT BECOMES MUCH LESS CLEAR.
THE GEFS FAVORS MUCH OF THE ANOMALOUS RIDGING TO SHIFT EASTWARD AND REMAIN
FAIRLY AMPLIFIED TO THE SOUTH OF THE HUDSON BAY, WITH A NARROW AND RAPIDLY
WEAKENING TROUGH FARTHER SOUTH. BY COMPARISON, THE ECMWF IS LESS PROGRESSIVE
WITH THE RIDGING, AND FEATURES A ZONALLY BROADER TROUGH FOCUSED OVER THE
MIDWEST TO ALLOW MORE NORTHEASTERLY FLOW OVER THE GREAT LAKES, KEEPING
ANOMALOUSLY WARM TEMPERATURES AT BAY. THESE DISCREPANCIES ARE REFLECTED IN THE
PETS WHERE THE GEFS FEATURES INCREASING CHANCES FOR TEMPERATURES EXCEEDING THE
85TH PERCENTILE OVERSPREADING THE GREAT LAKES AND THE MISSISSIPPI, OHIO AND
TENNESSEE VALLEYS LATER IN WEEK-2, WHEREAS THE ECMWF DEPICTS VIRTUALLY NONE OF
THESE SIGNALS. GIVEN THESE UNCERTAINTIES INTRODUCED BY DIFFERENCES IN THE
GUIDANCE, NO ADDITIONAL TEMPERATURE HAZARDS ARE POSTED, BUT THE EVOLVING
FORECAST PATTERN WILL CONTINUE TO BE MONITORED IN UPCOMING OUTLOOKS.
THE COMBINATION OF CONTINUED ABOVE-NORMAL TEMPERATURES AND BELOW-NORMAL
PRECIPITATION SUPPORTS THE CONTINUATION OF A RAPID ONSET DROUGHT (ROD) RISK
OVER PARTS OF EASTERN MONTANA, WYOMING AND THE DAKOTAS. DRY SOILS, STRESSED
VEGETATION, AND REDUCED WATER AVAILABILITY DUE TO LOW PONDS AND STREAMS MAY
LEAD TO INCREASED IMPACTS TO AGRICULTURE AND LIVESTOCK, WITH SEVERAL LOCATIONS
ALREADY EXPERIENCING IMPACTS.
AT THE BASE OF THE TROUGHING ALOFT OVER SOUTHWESTERN CANADA, SURFACE LOW
FORMATION REMAINS FAVORED AND ENSEMBLE MEAN SURFACE ANALYSES CONTINUE TO SHOW
PERIODS OF STRENGTHENING PRESSURE GRADIENTS OVER THE WESTERN CONUS. BASED ON
CONTINUED SUPPORT FROM THE PETS FAVORING INCREASED CHANCES FOR WIND SPEEDS
EXCEEDING THE 85TH PERCENTILE, A SLIGHT RISK OF HIGH WINDS REMAINS ISSUED FOR
JUL 24-27 BEFORE THE PATTERN DEAMPLIFIES OVER THE WESTERN CONUS. HOT, DRY AND
WINDY CONDITIONS ARE LIKELY TO LEAD TO ENHANCED RISK OF WILDFIRES IN THE
HIGHLIGHTED REGION, AND INHIBIT EFFORTS TO CONTAIN ANY ACTIVE WILDFIRES.
TIED TO THE TROUGHING FAVORED OVER THE LOWER PLAINS AND MISSISSIPPI VALLEY
EARLY IN WEEK-2, THE GEFS, ECMWF AND CANADIAN ENSEMBLES CONTINUE TO FEATURE THE
PERSISTENCE OF A STALLED FRONTAL BOUNDARY DRAPED FROM THE SOUTHERN PLAINS TO
THE EASTERN SEABOARD, WITH ENHANCED PRECIPITATION ALONG ITS PERIPHERY IN THE
RAW DAILY TOTALS LATE IN WEEK-1 BEFORE BECOMING DRIER BY DAY 10 (JUL 26). WHILE
THE GEFS PET REMAINS SPOTTY IN REGARDS TO ITS CHANCES FOR 3-DAY AMOUNTS
EXCEEDING AN INCH COMPARED TO THE ECMWF PET, BOTH TOOLS MAINTAIN A WIDESPREAD
AREA WITH AT LEAST 20% CHANCES FOR AMOUNTS EXCEEDING THE 85TH PERCENTILE,
ALIGNING WELL WITH THE PREDICTED SYNOPTIC FEATURE. THEREFORE, A SLIGHT RISK OF
HEAVY PRECIPITATION IS ISSUED FOR JUL 24-25 AND IS EXPANDED INTO PARTS OF THE
NORTHEAST WHERE THERE IS INCREASED SUPPORT IN THE ECMWF PET AND ITS
UNCALIBRATED COUNTERPART FOR 3-DAY AMOUNTS EXCEEDING AN INCH.
DUE TO HEAVY PRECIPITATION FAVORED OVER EASTERN CAROLINAS AND SOUTHEASTERN
VIRGINIA, AND POSSIBLE FLOODING (ACCORDING TO OWPS EXPERIMENTAL FLOOD HAZARD
OUTLOOK) DURING WEEK-1, ANY ADDITIONAL PRECIPITATION DURING WEEK-2 IS LIKELY TO
WORSEN SATURATED GROUND CONDITIONS AND POSSIBLY TRIGGER ADDITIONAL FLOODING IN
THE REGION. ABOVE-NORMAL PRECIPITATION REMAINS FAVORED OVER THE MID-ATLANTIC
AND THE SOUTHEAST, WHERE THE RAW ECMWF IN PARTICULAR MAINTAINS ELEVATED
PRECIPITATION SIGNALS ALONG AND JUST OFFSHORE OF THE CAROLINAS THROUGH THE
ENTIRETY OF WEEK-2 TO SUPPORT THE ADDITION OF A FLOODING POSSIBLE HAZARD IN THE
UPDATED OUTLOOK. REGARDLESS OF FLOODING, THE FAVORED CONTINUATION OF
ABOVE-NORMAL PRECIPITATION IS EXPECTED TO MITIGATE ANOMALOUS DRYNESS, STRONG
SOIL MOISTURE DEFICITS, AND POSSIBLY LEAD TO DROUGHT RECOVERY FOR PARTS OF THE
COASTAL CAROLINAS THAT ARE CURRENTLY REGISTERING SEVERE (D2) TO EXTREME (D3)
DROUGHT CONDITIONS.
THERE CONTINUES TO BE GOOD MODEL SUPPORT FOR A HEALTHY NORTH AMERICA MONSOON
CIRCULATION, AT LEAST AT THE OUTSET OF WEEK-2, BEFORE A MORE SUPPRESSED AND
SOUTHWARD-DISPLACED RIDGE AXIS IS FAVORED LATER IN THE PERIOD. A SLIGHT RISK
OF HEAVY PRECIPITATION REMAINS ISSUED OVER THE SONORAN DESERT FOR JUL 24-26
BASED ON GENERAL AGREEMENT IN THE PETS DEPICTING 20% CHANCES FOR 3-DAY AMOUNTS
EXCEEDING THE 85TH PERCENTILE. DETERMINISTIC SOLUTIONS SHOW INCREASES IN
DEWPOINT TEMPERATURES OVER SOUTHERN ARIZONA CHARACTERISTIC WITH GULF SURGE
ACTIVITY, WHICH MAY TRIGGER LOCALLY HEAVY PRECIPITATION. THE UNCALIBRATED GEFS
AND ECENS GUIDANCE INDICATE AREAS OF SPORADIC CONVECTION WHICH COULD RESULT IN
LOCALIZED AREAS OF FLOODING, ALTHOUGH THERE IS TOO MUCH UNCERTAINTY REGARDING
THE EXACT PLACEMENT TO DESIGNATE A FLOOD RISK.
FORECASTER: NICK NOVELLA
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