553
FXUS02 KWBC 121900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT MON AUG 12 2024
VALID 12Z THU AUG 15 2024 - 12Z MON AUG 19 2024
...EXCESSIVE HEAT LIKELY ACROSS THE SOUTHERN PLAINS AND CENTRAL
GULF COAST STATES THROUGH THE WEEKEND INTO EARLY NEXT WEEK...
...HEAVY RAIN POTENTIAL ACROSS THE MIDWEST TO PORTIONS OF THE OHIO
VALLEY AND APPALACHIANS THURSDAY TO FRIDAY...
..OVERVIEW
SUMMERTIME UPPER RIDGING ACROSS THE SOUTHERN TIER WILL LEAD TO
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 LATE IN THE PERIOD, LEADING TO A COOLER AND
WETTER PATTERN FOR THE NORTHWEST WHILE THE SOUTHERN STREAM RIDGE
BUILDS AND RETROGRADES A BIT IN RESPONSE. A MORE AMPLIFIED TROUGH
AND ASSOCIATED LOW PRESSURE SYSTEM WILL PROMOTE SCATTERED TO
WIDESPREAD THUNDERSTORMS WITH POTENTIAL FOR HEAVY RAIN AND
FLOODING ACROSS THE MIDWEST TO PORTIONS OF THE OHIO VALLEY AND
APPALACHIANS THURSDAY TO FRIDAY. MEANWHILE, POTENTIAL TROPICAL
CYCLONE FIVE IS CURRENTLY FORECAST TO INTENSIFY AND TRACK WELL OFF
THE EAST COAST OF THE U.S.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GUIDANCE REMAINS GENERALLY AGREEABLE WITH ONE ANOTHER REGARDING
THE LARGE SCALE PATTERN EVOLUTION ACROSS THE MAINLAND U.S. THROUGH
EARLY NEXT WEEK. THE SHORTWAVE TROUGH THAT IS FORECAST TO MOVE
ACROSS THE NORTHERN AND CENTRAL PLAINS LATER THIS WEEK HAS BEEN A
TRICKY FEATURE TO HANDLE. IN THE PAST COUPLE OF DAYS, MODELS HAVE
GENERALLY TRENDED TOWARD A MORE AMPLIFIED SOLUTION, LEADING TO A
SHARPER UPPER TROUGH AND A STRONGER SURFACE LOW PRESSURE SYSTEM
TO MOVE ACROSS THE GREAT LAKES BY AROUND FRIDAY. THE IMPACT OF THE
MORE AMPLIFIED PATTERN IS A BETTER-DEFINED FORCING TO SUPPORT HEAVY
RAIN POTENTIAL OVER THE MIDWEST ON THURSDAY AHEAD OF THE SYSTEM.
HOWEVER, THE ECMWF QPFS REMAIN NOTICEABLY FARTHER NORTH THAN THE
GFS/GEFS QPFS. THIS DISCREPANCY CONTINUES DOWNSTREAM INTO THE
EASTERN U.S. BY LATE IN THE FORECAST PERIOD. MEANWHILE, THE
CORRESPONDING UPPER RIDGE OVER THE SOUTHERN ROCKIES TO THE SOUTHERN
PLAINS SHOWS A TENDENCY TO BE STRONGER AS WELL. THE AMPLIFIED
PATTERN SLOWS THE EASTWARD PROGRESSION OF THE TROUGH OVER THE
EASTERN U.S. BY EARLY NEXT WEEK. THE SLOW-MOVING AND MORE AMPLIFIED
TROUGH DID HELP TO STEER POTENTIAL T.C. FIVE CLOSER TO THE EAST
COAST WITH THE 12Z GFS RUN BUT NOT WITH THE 12Z ECMWF. THERE IS
ALSO A TENDENCY FOR THE CLOSED LOW OFF THE PACIFIC NORTHWEST TO BE
BROADER/MORE AMPLIFIED AND A BIT FURTHER OFF THE COAST.
THE WPC FORECAST WAS DERIVED FROM A BLEND OF THE DETERMINISTIC
AND ENSEMBLE MEANS FROM THE 00Z ECMWF, 06Z GFS, AND 00Z CMC,
TRANSITIONING TOWARD A LARGER PROPORTION FROM THE ENSEMBLE MEANS
BY DAY 7.
..WEATHER/HAZARDS HIGHLIGHTS
UPPER TROUGHING AND SURFACE CYCLOGENESIS WILL PROVIDE FORCING FOR
RAIN AND THUNDERSTORMS ACROSS THE MIDWEST THURSDAY IN AN UNSTABLE
AND MOIST ENVIRONMENT. THERE MAY BE MULTIPLE MESOSCALE CONVECTIVE
SYSTEMS PRODUCING HEAVY RAIN RATES ACROSS THE REGION POSSIBLY
CAUSING FLOODING CONCERNS. A MARGINAL RISK REMAINS IN PLACE FOR THE
DAY 4 ERO FOR PORTIONS OF THE UPPER/MIDDLE MISSISSIPPI VALLEY AND
THE WESTERN OHIO VALLEY. A SLIGHT RISK HAS BEEN INTRODUCED INSIDE
THE MARGINAL GIVEN BETTER-DEFINED FORCING WITH A MORE AMPLIFIED
PATTERN. THUNDERSTORM CHANCES MOVE EAST ON FRIDAY, REACHING THE
UPPER GREAT LAKES REGION TO OHIO AND TENNESSEE VALLEYS AND INTO THE
CENTRAL/SOUTHERN APPALACHIANS. A BROAD MARGINAL RISK IS IN PLACE
FOR THOSE AREAS FOR THE DAY 5 ERO TO COVER THE FLOODING THREAT WITH
EMBEDDED HEAVY RAINS. CONVECTION SHOULD PUSH SLOWLY EAST TOWARD
THE EASTERN SEABOARD OVER THE WEEKEND.
THE MONSOON MAY TAKE A RELATIVE BREAK INTO THURSDAY, BUT EXPECT
MONSOONAL CONVECTION TO GET RENEWED FRIDAY AND ESPECIALLY INTO THE
WEEKEND PARTICULARLY ACROSS ARIZONA AND UTAH, FOCUSING WESTWARD A
BIT COMPARED TO EARLIER IN THE WEEK AS THE UPPER RIDGE RETROGRADES.
SOME STORMS MAY MAKE IT NORTH AND EAST BY SUNDAY 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, ABOVE NORMAL MOISTURE POOLING NEAR A
FRONT MAY ENHANCE STORM CHANCES OVER FLORIDA LATER THIS WEEK, BUT
THERE IS LIKELY NOT ENOUGH FLOODING RISK TO DELINEATE ANY AREA IN
THE ERO, SINCE FLORIDA IS NOT SENSITIVE TO FLOODING UNLESS THERE
ARE EXCEPTIONALLY HEAVY RAIN RATES/TOTALS. LIGHT TO MODEST SHOWERS
ARE FORECAST IN THE NORTHEAST INTO THURSDAY AHEAD OF MORE
WIDESPREAD RAIN APPROACHING INTO THE WEEKEND.
UPPER RIDGING OVER THE SOUTH-CENTRAL U.S. WILL MAINTAIN
TEMPERATURES UP TO 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 FROM THE WESTERN TO NORTH-CENTRAL TO EASTERN U.S.,
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.
KONG/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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