919
FXUS02 KWBC 130658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 AM EDT TUE AUG 13 2024
VALID 12Z FRI AUG 16 2024 - 12Z TUE AUG 20 2024
...EXCESSIVE HEAT LIKELY ACROSS THE SOUTHERN PLAINS AND CENTRAL
GULF COAST STATES THROUGH THE WEEKEND INTO EARLY NEXT WEEK...
..OVERVIEW
AN UPPER HIGH/RIDGE MEANDERING NEAR THE SOUTHERN HIGH PLAINS LATE
THIS WEEK INTO EARLY NEXT WEEK WILL MAINTAIN THE POTENTIAL FOR HIGH
HEAT INDICES THAT COULD BE HAZARDOUS ACROSS THE SOUTHERN PLAINS TO
GULF COAST/LOWER MISSISSIPPI VALLEY AND TOWARD THE SOUTHEAST. AN
UPPER LOW IS FORECAST TO DIG INTO THE EASTERN PACIFIC OFF THE WEST
COAST THIS WEEKEND, LEADING TO A COOLER AND WETTER PATTERN FOR THE
NORTHWEST WHILE THE RIDGE BUILDS AND RETROGRADES A BIT IN RESPONSE.
MEANWHILE, AN UPPER TROUGH PUSHING ACROSS THE MIDWEST FRIDAY WILL
SLOW AND DEEPEN OVER THE WEEKEND. THIS FEATURE AND A SURFACE LOW
PRESSURE/FRONTAL SYSTEM WILL PROMOTE SCATTERED TO WIDESPREAD
THUNDERSTORMS WITH POTENTIAL FOR HEAVY RAIN ACROSS THE EAST-CENTRAL
TO EASTERN U.S. THROUGH THE PERIOD. MEANWHILE, TROPICAL STORM
ERNESTO IS CURRENTLY FORECAST TO INTENSIFY AND TRACK OFFSHORE OF
THE U.S. EAST COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE IS RATHER AGREEABLE ON THE LARGE SCALE THROUGHOUT
THE MEDIUM RANGE PERIOD, AS THE EASTERN PACIFIC TO WEST COAST
TROUGH, WEST-CENTRAL TO CENTRAL U.S. RIDGE, AND EAST-CENTRAL TO
EASTERN U.S. TROUGH PATTERN SETS UP AND AMPLIFIES. EXACT UPPER
LOW/TROUGH AND SURFACE LOW POSITIONS VARY SOMEWHAT, AS IS TYPICAL.
THE GREAT LAKES SURFACE LOW SHOWS SOME SPREAD IN POSITION BY EARLY
SUNDAY, WITH THE 12Z CMC TAKING IT QUICKLY NORTH AND NOW THE 00Z
GFS ACTING SIMILARLY, DEPENDENT ON SHORTWAVE AND TROUGH STRUCTURE
DETAILS. WITH THE EASTERN PACIFIC UPPER LOW, THE 12Z GFS WAS
FARTHER WEST THAN OTHER GUIDANCE, BUT THE 18Z AND 00Z GFS RUNS SHOW
BETTER ALIGNMENT. THE 12Z ECMWF WAS ON THE FASTER SIDE BY LATE IN
THE PERIOD IN BRINGING THE LOW/TROUGH EAST, BUT EC-BASED
AI/MACHINE LEARNING MODELS WERE EVEN FASTER, SO THE OPERATIONAL EC
MAY BE A GOOD MIDDLE GROUND. ADDITIONALLY, SMALLER DETAILS LIKE
SHORTWAVE ENERGIES ROUNDING THE MEAN FLOW VARY AS WELL, AND THESE
DIFFERENCES AFFECT QPF PLACEMENT. SO WHILE CONFIDENCE IN THE
OVERALL PATTERN IS PRETTY HIGH, SENSIBLE WEATHER DETAILS LIKE QPF
HAVE LOWER CONFIDENCE. UKMET RUNS CONTINUE TO BE OUTLIERS IN
PRODUCING VERY HEAVY QPF ACROSS THE CENTRAL U.S. INTO FRIDAY, WHILE
ECMWF RUNS HAVE BEEN CONSISTENTLY FASTER WITH EASTERN U.S. QPF
COMPARED TO OTHER GUIDANCE, BUT FORTUNATELY THE 00Z ECMWF IS
FAVORABLY SLOWER.
THE TREND OVER THE PAST COUPLE OF DAYS FOR THE EASTERN TROUGH TO
AMPLIFY AND STALL DOES ALLOW FOR WHAT IS CURRENTLY TROPICAL STORM
ERNESTO TO EDGE A BIT CLOSER TO THE U.S. EAST COAST, BUT ON THE
WHOLE MODELS STILL KEEP IT EAST OF THE U.S. OVER THE WESTERN
ATLANTIC. HOWEVER, THE CANADIAN MARITIMES SHOULD KEEP A CLOSE EYE
ON THE FORECAST AS GUIDANCE DOES HAVE ERNESTO PASSING OVER OR
NEARBY THOSE AREAS AS A NOTABLE LOW (WHETHER IT IS TROPICAL OR NOT
BY THEN).
THE WPC FORECAST USED A MULTI-MODEL DETERMINISTIC BLEND FAVORING
THE 18Z GFS AND 12Z ECMWF EARLY IN THE PERIOD. GRADUALLY REDUCED
THE PROPORTION OF DETERMINISTIC RUNS IN FAVOR OF THE GEFS AND EC
ENSEMBLE MEANS AS THE PERIOD PROGRESSED, REACHING HALF MODELS/HALF
MEANS BY DAYS 6-7 TO REDUCE THE INDIVIDUAL MODEL DIFFERENCES IN THE
OVERALL AGREEABLE PATTERN.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER TROUGH IN THE GREAT LAKES BUILDING SOUTH AND EAST WITH
TIME, ALONG WITH A SURFACE LOW PRESSURE SYSTEM, WILL PROVIDE
FORCING FOR RAIN AND THUNDERSTORMS ACROSS THE EAST-CENTRAL U.S.
FRIDAY AND GRADUALLY MOVING EAST INTO THE WEEKEND AND EARLY NEXT
WEEK. ABOVE NORMAL MOISTURE AND INSTABILITY IN PLACE COULD LEAD TO
HEAVY RAIN WITH POTENTIAL FLOODING CONCERNS. A MARGINAL RISK IS IN
PLACE DAY 4/FRIDAY FROM THE GREAT LAKES REGION SOUTH INTO THE OHIO
AND TENNESSEE VALLEYS, AND ACROSS THE NORTHERN MID-ATLANTIC TO
PORTIONS OF THE NORTHEAST FOR DAY 5/SATURDAY. THERE IS GENERALLY
LESS SIGNAL FOR HEAVY RAIN CAUSING WIDESPREAD FLOODING COMPARED TO
THE SHORT RANGE PERIOD, BUT ESPECIALLY AS RAIN GETS INTO THE EAST
WHERE THERE ARE WET ANTECEDENT CONDITIONS, ERO UPGRADES IN FUTURE
CYCLES ARE NOT OUT OF THE QUESTION. RAIN LOOKS TO PERSIST ACROSS
THE EASTERN SEABOARD INTO EARLY NEXT WEEK.
ROUNDS OF STORMS ARE ALSO POSSIBLE ACROSS THE CENTRAL U.S. AS
IMPULSES "RIDE THE RIDGE" IN NORTHWEST FLOW. THERE IS PARTICULARLY
LOW CONFIDENCE IN WHERE THESE STORMS FORM ON ANY GIVEN DAY/NIGHT AS
MODEL GUIDANCE RANGES FROM THE CENTRAL PLAINS, MIDDLE OR LOWER
MISSISSIPPI VALLEY, TENNESSEE VALLEY, AND/OR SOUTHEAST. SO EXPECT
CHANGES TO THE RAINFALL FORECAST IN FUTURE CYCLES, AND THERE IS
CERTAINLY NOT ENOUGH AGREEMENT ON ANY EXCESSIVE RAINFALL RISK.
MONSOONAL CONVECTION COULD CREEP BACK INTO ARIZONA FRIDAY BUT BEGIN
IN EARNEST BY SATURDAY PARTICULARLY ACROSS ARIZONA AND UTAH,
FOCUSING WESTWARD A BIT COMPARED TO EARLIER IN THE WEEK AS THE
UPPER RIDGE RETROGRADES. A MARGINAL RISK HAS BEEN DELINEATED IN THE
ERO FOR DAY 5/SATURDAY FOR LOCALIZED FLOODING POTENTIAL. SOME
STORMS MAY MAKE IT NORTH AND EAST BY SUNDAY-MONDAY AS MOISTURE
ADVECTS AROUND THE RIDGE. FARTHER NORTH, THE UPPER LOW SETTING UP
IN THE EASTERN PACIFIC WILL PROMOTE ROUNDS OF MODEST RAIN ACROSS
THE NORTHWEST. ADDITIONALLY, A FRONT SLOWLY MOVING THROUGH FLORIDA
AND ABOVE NORMAL PRECIPITABLE WATER VALUES COULD LEAD TO HEAVY RAIN
ESPECIALLY FRIDAY. THE GREATEST CONCERN FOR POSSIBLE FLOODING
ISSUES WOULD BE IF HEAVY RAIN RATES AFFECT THE MIAMI/VICINITY URBAN
AREA, BUT WILL HOLD OFF ON ANY MARGINAL RISK AREA SINCE THERE IS
NOT AGREEMENT FOR EXCEPTIONALLY HEAVY RAIN.
UPPER RIDGING OVER THE SOUTH-CENTRAL U.S. WILL MAINTAIN
TEMPERATURES AROUND 10F ABOVE (ALREADY HOT) NORMAL VALUES ACROSS
THE SOUTHERN PLAINS INTO THE LOWER MISSISSIPPI VALLEY. COMBINED
WITH THE HIGH DEWPOINTS IN MOST AREAS, HEAT INDICES ABOVE 110F ARE
EXPECTED, AND EXPERIMENTAL HEATRISK SHOWS MAJOR TO EXTREME IMPACTS.
LOW TEMPERATURES IN THE UPPER 70S TO LOW 80S WILL EXACERBATE HEAT
STRESS AND COULD SET RECORDS FOR WARM MINIMUM TEMPERATURES. NORTH
OF THIS REGION THOUGH, MOST AREAS SHOULD SEE NEAR TO SLIGHTLY BELOW
NORMAL HIGHS, WITH THE MOST NOTABLE BELOW AVERAGE TEMPERATURES IN
THE NORTHWEST LATE WEEK-WEEKEND IN RESPONSE TO THE LOW ALOFT. LOW
TEMPERATURES SHOULD BE NEAR TO A BIT ABOVE AVERAGE IN MOST AREAS.
TATE
ADDITIONAL 3-7 DAY HAZARD INFORMATION CAN BE FOUND ON THE WPC MEDIUM
RANGE HAZARDS OUTLOOK CHART AT:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,
QUANTITATIVE PRECIPITATION FORECAST (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=ERO
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML
HTTPS://WWW.WPC.NCEP.NOAA.GOV/#PAGE=OVW
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