094
FXUS02 KWBC 131850
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
250 PM EDT TUE AUG 13 2024
VALID 12Z FRI AUG 16 2024 - 12Z TUE AUG 20 2024
...HAZARDOUS HEAT LIKELY ACROSS THE SOUTHERN PLAINS AND CENTRAL
GULF COAST STATES THROUGH THIS WEEKEND AND EARLY NEXT WEEK...
..OVERVIEW
AN UPPER HIGH/RIDGE WILL REMAIN NEARLY STATIONARY OVER THE
SOUTHERN HIGH PLAINS LATE THIS WEEK INTO EARLY NEXT WEEK AND WILL
MAINTAIN HIGH, POTENTIALLY HAZARDOUS, HEAT INDICES ACROSS THE
SOUTHERN PLAINS TO THE GULF COAST/LOWER MISSISSIPPI VALLEY AND 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. MEANWHILE, AN UPPER TROUGH PUSHING
ACROSS THE MIDWEST FRIDAY WILL SLOW AND DEEPEN OVER THE GREAT
LAKES THIS WEEKEND. THIS FEATURE AND A SURFACE LOW PRESSURE/FRONTAL
SYSTEM WILL PROMOTE SCATTERED TO WIDESPREAD THUNDERSTORMS WITH THE
POTENTIAL FOR HEAVY RAIN ACROSS THE EAST-CENTRAL TO EASTERN U.S.
THROUGH THE PERIOD. TROPICAL CYCLONE ERNESTO IS FORECAST TO TRACK
NORTH OVER THE WESTERN ATLANTIC AND REMAIN OFFSHORE OF THE EAST
COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE SCALE
PATTERN THROUGH THE MEDIUM RANGE PERIOD WITH AN EXPECTED AMOUNT OF
MODEL SPREAD/UNCERTAINTY. THE 06Z GFS CONTINUES TO BE A LITTLE
FASTER THAN OTHER GUIDANCE WITH THE PROGRESSION OF UPPER LEVEL
LOWS, WHILE THE 00Z ECMWF FALLS MORE IN THE MIDDLE OF THE GUIDANCE
SPREAD. THE 00Z CMC IS IN GOOD AGREEMENT WITH THE GFS AND ECMWF
THROUGH THE FIRST HALF OF THE PERIOD, BUT UPSTREAM DIFFERENCES IN
THE GULF OF ALASKA RESULTED IN IT FALLING AWAY FROM THE GENERAL
MODEL CONSENSUS IN THE SECOND HALF OF THE PERIOD. FOR THESE
REASONS, MORE WEIGHT WAS PUT ON THE ECMWF AND LESS WAS PUT ON THE
CMC IN THE WPC FORECAST BLEND.
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 06Z GFS AND 00Z ECMWF EARLY IN THE PERIOD. THE PROPORTION OF
DETERMINISTIC RUNS WAS GRADUALLY REDUCED IN FAVOR OF THE GEFS AND
EC ENSEMBLE MEANS BY DAYS 6 AND 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 INTO THE EAST THIS WEEKEND AND EARLY NEXT WEEK. ABOVE
NORMAL MOISTURE AND INSTABILITY WILL BE IN PLACE AND COULD LEAD TO
HEAVY RAIN WITH POTENTIAL FLOODING CONCERNS. A MARGINAL RISK OF
EXCESSIVE RAINFALL IS IN PLACE ON 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
WET ANTECEDENT CONDITIONS IN PORTIONS OF THE EAST COULD CONTRIBUTE
TO A HIGHER FLOOD RISK. RAIN LOOKS TO PERSIST ACROSS THE EASTERN
SEABOARD INTO EARLY NEXT WEEK.
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.
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. EXPECT
CHANGES TO THE RAINFALL FORECAST IN FUTURE CYCLES, AND THERE IS
CERTAINLY NOT ENOUGH AGREEMENT FOR ANY EXCESSIVE RAINFALL RISK.
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 LOCALLY HEAVY RAIN,
ESPECIALLY FRIDAY. THE GREATEST CONCERN FOR POSSIBLE FLOODING
ISSUES WOULD BE IF HEAVY RAIN RATES AFFECT THE MIAMI/VICINITY URBAN
AREA.
UPPER RIDGING OVER THE SOUTH-CENTRAL U.S. WILL MAINTAIN
TEMPERATURES AROUND 10 DEGREES 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
110 DEGREES 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, MOST AREAS SHOULD SEE NEAR TO
SLIGHTLY BELOW NORMAL HIGHS, WITH THE MOST NOTABLE BELOW AVERAGE
TEMPERATURES IN THE NORTHWEST LATE WEEK INTO THE WEEKEND IN
RESPONSE TO THE LOW ALOFT. LOW TEMPERATURES SHOULD BE NEAR TO A BIT
ABOVE AVERAGE IN MOST AREAS.
DOLAN/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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