169
FXUS21 KWNC 151834
PMDTHR
US HAZARDS OUTLOOK
NWS CLIMATE PREDICTION CENTER COLLEGE PARK MD
300 PM EDT JULY 15 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, AND NORTHERN HIGH PLAINS. MID-LEVEL LOW PRESSURE FOCUSED OVER
THE LOWER PLAINS AND MISSISSIPPI VALLEY IS LIKELY TO PROMOTE COOLER AND WETTER
CONDITIONS, WITH INCREASED CHANCES FOR HEAVY PRECIPITATION FROM THE SOUTHERN
PLAINS TO THE EASTERN SEABOARD EARLY IN WEEK-2. TIED TO AN ACTIVE NORTH AMERICA
MONSOON CIRCULATION EARLY IN THE PERIOD, THERE IS ALSO AN INCREASED RISK OF
LOCALLY HEAVY PRECIPITATION THROUGHOUT THE DESERT SOUTHWEST.
HAZARDS
MODERATE RISK OF EXCESSIVE HEAT ACROSS PORTIONS OF CALIFORNIA GREAT BASIN,
ROCKIES, AND NORTHERN HIGH PLAINS, TUE-WED, JUL 23-24.
SLIGHT RISK OF EXCESSIVE HEAT FOR MUCH OF THE WESTERN CONUS AND NORTHERN HIGH
PLAINS, TUE-FRI, JUL 23-26.
SLIGHT RISK OF EPISODIC HIGH WINDS FOR MUCH OF THE WESTERN CONUS, TUE-SAT, JUL
23-27.
SLIGHT RISK OF HEAVY PRECIPITATION FOR PARTS OF THE SOUTHERN PLAINS, LOWER
MISSISSIPPI, TENNESSEE AND OHIO VALLEYS, APPALACHIANS, AND THE SOUTHEAST,
TUE-THU, JUL 23-25.
SLIGHT RISK OF PERIODS OF HEAVY PRECIPITATION FOR PORTIONS OF THE DESERT
SOUTHWEST, TUE-THU, JUL 23-25.
RAPID ONSET DROUGHT RISK FOR PORTIONS OF THE NORTHERN PLAINS.
DETAILED SUMMARY
FOR THURSDAY JULY 18 - MONDAY JULY 22:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
FOR TUESDAY JULY 23 - MONDAY JULY 29: SINCE LATE LAST WEEK, THERE HAS BEEN GOOD
CONSISTENCY IN THE MEAN WEEK-2 500-HPA HEIGHT FORECASTS FROM THE GEFS AND ECMWF
ENSEMBLES FEATURING AN AMPLIFIED RIDGE CENTER OVER SOUTHERN CANADA, WITH
ANOMALOUS 500-HPA TROUGHING FAVORED OVER THE SOUTHERN TIER OF THE U.S. MAINLY
EAST OF THE ROCKIES. THE FORMER MID-LEVEL FEATURE LOOKS TO BE THE MAIN HAZARDS
DRIVER FOR THE CONTINUATION OF THE EXCESSIVE HEAT RISK FROM WEEK-1 INTO WEEK-2
FOR MUCH OF THE INTERIOR WEST, HOWEVER THIS ELEVATED HEAT POTENTIAL MAY BE
SUBDUED BY THE DEVELOPMENT OF ANOMALOUS MID-LEVEL TROUGHING FAVORED OVER
WESTERN CANADA TOWARDS THE MIDDLE OF THE PERIOD. WITH LOWERED MEAN 500-HPA
HEIGHTS FAVORED DOWNSTREAM IN THE LOWER PLAINS AND MISSISSIPPI VALLEY,
RESULTANT NORTHWESTERLY FLOW IS LIKELY TO KEEP TEMPERATURES ON THE COOL SIDE,
BUT ALSO PROMOTE THE RISK OF HEAVY PRECIPITATION GIVEN CONTINUED FORCING FROM A
STATIONARY FRONT FAVORED ACROSS THE SOUTHEASTERN CONUS AND ENHANCED MOISTURE
ADVECTION TIED TO GULF OF MEXICO RETURN FLOW.
LATE IN WEEK-1, 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. BY THE
OUTSET OF WEEK-2 (JUL 23), THESE ANOMALIES EASE SOMEWHAT, BUT RAW TOOLS
CONTINUE TO MAINTAIN DOUBLE-DIGIT POSITIVE MAXIMUM TEMPERATURE DEPARTURES
ACROSS CALIFORNIA AND OVER PARTS OF THE NORTHERN INTERMOUNTAIN AND NORTHERN
PLAINS DURING THE FIRST FEW DAYS OF THE PERIOD. THIS EVOLUTION IS WELL
REFLECTED IN THE PROBABILISTIC EXTREMES TOOLS (PETS), WHERE BOTH THE GEFS AND
ECMWF DEPICT ELEVATED CHANCES (30-50%) FOR DAYTIME TEMPERATURES EXCEEDING THE
85TH CLIMATOLOGICAL PERCENTILE EARLY IN THE PERIOD. BASED ON GOOD AGREEMENT
BETWEEN THE GEFS AND ECMWF FAVORING THE DEAMPLIFYING RIDGE ALOFT WITH
DIMINISHING HEAT SIGNALS IN THE PETS BEYOND DAY 10, THE CORRESPONDING HIGH RISK
IS TIMED OFF FOR WEEK-2, HOWEVER A MODERATE RISK FOR EXCESSIVE HEAT IS
CONTINUED FOR PARTS OF CALIFORNIA, GREAT BASIN, INTERMOUNTAIN WEST AND HIGH
PLAINS FOR JUL 23-24. A BROADER SLIGHT RISK AREA FOR EXCESSIVE HEAT REMAINS
ISSUED AND INCLUDES PARTS OF THE PACIFIC NORTHWEST, DESERT SOUTHWEST AND
CENTRAL PLAINS FOR JUL 23-26. ALTHOUGH WEAKER, MUCH OF THE ANOMALOUS 500-HPA
RIDGING OVER CANADA IS FAVORED TO SLIDE EASTWARD TO THE SOUTH OF THE HUDSON BAY
WHERE THE EXCESSIVE HEAT RISK COULD EMERGE OVER THE PLAINS AND UPPER MIDWEST
TOWARDS THE MIDDLE OF THE PERIOD. HOWEVER, NO CORRESPONDING HAZARDS ARE ISSUED
GIVEN UNCERTAINTIES ASSOCIATED WITH THE AFOREMENTIONED TROUGHING UPSTREAM OVER
WESTERN CANADA AND GROWING DISPARITIES IN THE PETS AT THIS LEAD.
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 NORTHWESTERN CONUS.
THEREFORE, A SLIGHT RISK OF HIGH WINDS IS ISSUED FOR JUL 23-27 WHERE PETS ALSO
SLOW INCREASED CHANCES FOR WIND SPEEDS EXCEEDING THE 85TH PERCENTILE OVER A
BROADER REGION IN THE WESTERN CONUS COMPARED TO THE PREVIOUS HAZARDS OUTLOOK.
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.
OVER THE LOWER PLAINS AND MISSISSIPPI VALLEY, ANOMALOUS TROUGHING FLANKED BY A
PAIR OF MID-LEVEL RIDGE CENTERS OVER THE WESTERN CONUS AND WESTERN ATLANTIC IS
ANTICIPATED TO PROLONG THE CHANCES OF DISTURBED WEATHER LATE IN WEEK-1 AND THE
RISK FOR HEAVY PRECIPITATION EARLY IN THE PERIOD. 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. WHILE THE GEFS PET
IS SPOTTY IN REGARDS TO ITS CHANCES FOR 3-DAY AMOUNTS EXCEEDING AN INCH
COMPARED TO THE ECMWF PET, BOTH TOOLS FEATURE AN EXPANDED AREA WITH AT LEAST
20% CHANCES FOR AMOUNTS EXCEEDING THE 85TH PERCENTILE, ALIGNING WELL WITH THE
PREDICTED SYNOPTIC FEATURE. NOTABLY, THE UNCALIBRATED ECMWF DEPICTS WIDESPREAD
PROBABILITIES OF AT LEAST 40% FOR 3-DAY AMOUNTS EXCEEDING AN INCH (WITH 60-70%
CHANCES FOCUSED OVER THE TENNESSEE VALLEY) SUPPORTIVE OF A MODERATE RISK
DESIGNATION, HOWEVER A BROAD SLIGHT RISK AREA IS POSTED (JUL 23-25) IN THE
UPDATED OUTLOOK IN DEFERENCE TO THE DRIER GEFS AND CANADIAN GUIDANCE.
WITH ABOVE-NORMAL AND POTENTIALLY HEAVY PRECIPITATION FAVORED OVER PARTS OF THE
OHIO AND TENNESSEE VALLEYS DURING THE NEXT TWO WEEKS, THE RISK FOR RAPID ONSET
DROUGHT (ROD) HAS BEEN REMOVED OVER THESE AREAS IN THE UPDATED OUTLOOK.
HOWEVER, THE ONGOING RISK FOR EXCESSIVE HEAT AS WELL AS BELOW-NORMAL
PRECIPITATION FORECAST UPSTREAM OF THE HIGH PLAINS SUPPORTS THE CONTINUATION OF
A 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.
THERE CONTINUES TO BE GOOD MODEL SUPPORT FOR A HEALTHY NORTH AMERICA MONSOON
CIRCULATION WITH ABOVE-NORMAL PRECIPITATION ANTICIPATED OVER THE DESERT
SOUTHWEST HEADING INTO WEEK-2. THE GEFS AND ECMWF ENSEMBLE MEAN RIDGE CENTER
(594DM) IS INITIALLY FAVORED OVER THE WESTERN FOUR CORNERS, THOUGH GUIDANCE
SHOWS MORE OF A SUPPRESSED AND SOUTHWARD DISPLACED RIDGE AXIS LATER IN THE
PERIOD. AS A RESULT, SIGNALS FOR HEAVY PRECIPITATION ARE REDUCED BY THE MIDDLE
OF WEEK-2 IN THE PETS, BUT THESE TOOLS MAINTAIN AT LEAST A 20% CHANCE FOR
AMOUNTS EXCEEDING THE 85TH PERCENTILE THROUGH DAY 10 (JUL 25) AND A SLIGHT RISK
OF HEAVY PRECIPITATION REMAINS POSTED WITH ITS COVERAGE NOW FOCUSED MORE TO THE
WEST OVER THE SONORAN DESERT. WHILE THE EASTERN PACIFIC LOOKS TO REMAIN
INACTIVE IN TERMS OF ANY ASSOCIATED TROPICAL CYCLONE TRACKS NEAR THE GULF OF
CALIFORNIA, DETERMINISTIC SOLUTIONS SHOW INCREASES IN DEWPOINT TEMPERATURES
OVER SOUTHERN ARIZONA CONSISTENT WITH GULF SURGE ACTIVITY, WHICH MAY POSSIBLY
TRIGGER LOCALLY HEAVY PRECIPITATION. THE UNCALIBRATED GEFS AND ECENS GUIDANCE
INDICATE AREAS OF SPORADIC CONVECTION THROUGHOUT THE PERIOD 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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