709
FXUS21 KWNC 171814
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 17 2024
SYNOPSIS: AN EASTWARD SHIFTING AREA OF MID-LEVEL HIGH PRESSURE DURING WEEK-2 IS
EXPECTED TO SUSTAIN AN INCREASED RISK FOR EXCESSIVE HEAT OVER MANY PARTS OF THE
WESTERN CONTIGUOUS U.S. (CONUS) EARLY IN THE PERIOD, WITH INCREASING CHANCES
FOR HEAT ACROSS THE NORTHERN GREAT PLAINS AND THE UPPER MIDWEST LATER IN JULY.
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. CONTINUED ABOVE-NORMAL AND POTENTIALLY HEAVY PRECIPITATION FAVORED
EAST OF THE APPALACHIANS DURING THE NEXT TWO WEEKS MAY LEAD TO FLOODING OVER
PARTS OF THE EASTERN CAROLINAS AND SOUTHEASTERN VIRGINIA.
HAZARDS
SLIGHT RISK OF EXCESSIVE HEAT FOR PARTS OF CALIFORNIA, DESERT SOUTHWEST, GREAT
BASIN, ROCKIES AND NORTHERN INTERMOUNTAIN, THU-FRI, JUL 25-26.
SLIGHT RISK OF EXCESSIVE HEAT FOR PARTS OF THE NORTHERN AND CENTRAL PLAINS, AND
UPPER MISSISSIPPI VALLEY, THU-WED, JUL 25-31.
SLIGHT RISK OF EPISODIC HIGH WINDS FOR MUCH OF THE WESTERN CONUS, THU-SAT, JUL
25-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, THE LOWER
MISSISSIPPI, TENNESSEE AND OHIO VALLEYS, THE APPALACHIANS, SOUTHEAST, AND
MID-ATLANTIC, THU-SAT, JUL 25-27.
RAPID ONSET DROUGHT RISK FOR PORTIONS OF THE NORTHERN PLAINS.
FLOODING POSSIBLE OVER THE EASTERN CAROLINAS AND SOUTHEASTERN VIRGINIA.
DETAILED SUMMARY
FOR SATURDAY JULY 20 - WEDNESDAY JULY 24:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR THURSDAY JULY 25 - WEDNESDAY JULY 31: EARLY IN WEEK-2, DYNAMICAL MODEL
HEIGHT FORECASTS CONTINUE TO FEATURE ANOMALOUS 500-HPA RIDGING EXTENDING FROM
CENTRAL CANADA TO THE WESTERN CONUS, WITH A FAIRLY PERSISTENT, POSITIVELY
TILTED TROUGH CENTERED OVER THE PLAINS AND MISSISSIPPI VALLEY. THE FORMER
MID-LEVEL FEATURE IS EXPECTED TO BE THE DRIVER FOR A CONTINUED, BUT DECLINING
EXCESSIVE HEAT RISK FROM WEEK-1 INTO EARLY WEEK-2 FOR PARTS OF CALIFORNIA AND
THE INTERIOR WEST, AS MID-LEVEL TROUGHING IS FAVORED TO DEVELOP OVER WESTERN
CANADA AND THE WEST COAST OF THE CONUS TO MODERATE TEMPERATURES. DOWNSTREAM,
MODELS HAVE COME INTO BETTER AGREEMENT WITH A WEAKENING TROUGH OVER THE
MIDWEST, HOWEVER DISPARITIES STILL EXIST WITH THE EVOLUTION OF THE ANOMALOUS
RIDGE CENTER TO THE NORTH IN CANADA. THE ECMWF FAVORS A MORE WESTERLY AND
SLUGGISH SOLUTION OF THE MID-LEVEL FEATURE, WHEREAS BOTH THE GEFS AND CANADIAN
ARE MORE PROGRESSIVE, PLACING THE RIDGE CENTER TO THE SOUTH OF HUDSON BAY AFTER
THE FIRST FEW DAYS OF THE PERIOD, AS A RESULT, THERE IS INCREASED UNCERTAINTY
IN THE TIMING AND STRENGTH OF WARMING TEMPERATURES AND THE POTENTIAL EMERGENCE
OF EXCESSIVE HEAT OVER PORTIONS OF THE NORTHERN CONUS.
THERE IS GOOD CONTINUITY IN THE PROBABILISTIC EXTREMES TOOLS (PETS) DEPICTING
GRADUALLY WANING HEAT SIGNALS BY LATE NEXT WEEK OVER PARTS OF CALIFORNIA,
INTERIOR WEST, AND NORTHERN PLAINS. WHILE THESE TOOLS SHOW SOME AREAS WITH
ELEVATED PROBABILITIES (30-50%) FOR MAXIMUM TEMPERATURES EXCEEDING THE 85TH
PERCENTILE, THE GEFS AND ECMWF LARGELY DISAGREE IN REGARDS TO THEIR LOCATION.
GIVEN THIS, AND GOOD AGREEMENT IN THE HEIGHT FORECASTS FEATURING LOWERED
MID-LEVEL HEIGHTS OVER THE WESTERN CONUS, THE MODERATE RISK FOR EXCESSIVE HEAT
IS DISCONTINUED, BUT THE CORRESPONDING SLIGHT RISK AREA REMAINS POSTED FOR JUL
25-26. BEYOND DAY 9 (JUL 26), PETS GENERALLY SHOW AN EXPANSION OF THE HEAT
SIGNALS ALONG THE NORTHERN TIER OF THE CONUS, WITH AT LEAST 20% CHANCES OF
MAXIMUM TEMPERATURES EXCEEDING THE 85TH PERCENTILE. HOWEVER, THESE TOOLS
CONTINUE TO DISAGREE IN REGARDS TO THE EASTERN EXTENT OF THESE HEAT SIGNALS
TIED TO THE VARIED POSITION AND STRENGTH OF THE ANOMALOUS 500-HPA RIDGING IN
THE ENSEMBLES OVER CANADA. TO ADDRESS THESE DIFFERENCES, A SLIGHT RISK FOR
EXCESSIVE HEAT IS ADDED FOR THE NORTHERN AND CENTRAL PLAINS, AND INTO THE UPPER
MISSISSIPPI VALLEY MAINLY WEST OF THE RIDGE AXIS, WHERE THE GREATEST DEGREE OF
AGREEMENT EXISTS BETWEEN THE RAW AND CALIBRATED GUIDANCE. BASED ON THE
PREVAILING HEIGHT PATTERN TOWARDS THE END OF JULY, THE SLIGHT RISK IS VALID FOR
THE ENTIRETY OF WEEK-2.
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 OVER THE NORTHERN PLAINS AND ENSEMBLE MEAN SURFACE
ANALYSES CONTINUE TO SHOW PERIODS OF STRENGTHENING PRESSURE GRADIENTS IN THE
REGION, AS WELL AS FURTHER SOUTH IN THE DESERT SOUTHWEST. BASED ON CONTINUED
SUPPORT FROM THE ECMWF PET FAVORING INCREASED CHANCES FOR WIND SPEEDS EXCEEDING
THE 85TH PERCENTILE, A SLIGHT RISK OF HIGH WINDS REMAINS ISSUED FOR JUL 25-27
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.
WITH TROUGHING ESTABLISHED OVER THE LOWER PLAINS AND MISSISSIPPI VALLEY, 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 AN INTO WEEK-2. RELATIVE TO YESTERDAY'S GUIDANCE, THESE
ENHANCED TOTALS PERSIST SLIGHTLY LONGER THAN INITIALLY FAVORED, WHERE ENSEMBLES
MAINTAIN GOOD RETURN FLOW FROM THE GULF OF MEXICO TO ADVECT AMPLE MOISTURE INTO
THE SOUTHEASTERN CONUS THROUGH DAY 10 (JUL 27). 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 AGREE IN DEPICTING 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
REMAINS AND IS EXTENDED THROUGH JUL 27 IN THE UPDATED OUTLOOK.
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 MID-ATLANTIC THROUGH THE
ENTIRETY OF WEEK-2 TO SUPPORT THE FLOODING POSSIBLE HAZARD. 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.
AN ACTIVE NORTH AMERICAN MONSOON CIRCULATION IS FAVORED EARLY IN WEEK-2 BEFORE
A MORE SUPPRESSED AND SOUTHWARD DISPLACED RIDGE AXIS DEVELOPS LATER IN THE
PERIOD. WHILE PRECIPITATION TOOLS FAVOR ABOVE-NORMAL PRECIPITATION THROUGHOUT
THE LOWER FOUR CORNERS THROUGH THE MIDDLE OF WEEK-2, THE GEFS AND ECWMF PETS NO
LONGER SUPPORT THE POTENTIAL FOR HEAVY PRECIPITATION, AND THE CORRESPONDING
SLIGHT RISK HAZARD IS REMOVED IN THE OUTLOOK.
FORECASTER: NICK NOVELLA
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