804
FXUS02 KWBC 120656
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM 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/SOUTHEAST THROUGH THE WEEK INTO NEXT WEEKEND...
..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 INTO THE SOUTHEAST.
AN UPPER LOW IS FORECAST TO DIG INTO THE EASTERN PACIFIC AND WEST
COAST LATE WEEK INTO THE WEEKEND, LEADING TO A COOLER AND WETTER
PATTERN FOR THE NORTHWEST WHILE THE SOUTHERN STREAM RIDGE BUILDS
AND RETROGRADES A BIT IN RESPONSE. A SHORTWAVE TROUGH AND A SURFACE
LOW PRESSURE SYSTEM ARE FORECAST TO TRACK ACROSS THE NORTH-CENTRAL
TO EASTERN U.S. AND PROMOTE SCATTERED TO WIDESPREAD THUNDERSTORMS
WITH POTENTIAL FOR HEAVY RAIN AND FLOODING, GRADUALLY SHIFTING EAST
WITH TIME. MEANWHILE, THE TROPICAL LOW PRESSURE SYSTEM THAT NHC
HAS NOW DESIGNATED POTENTIAL TROPICAL CYCLONE FIVE WILL CONTINUE TO
BE MONITORED, BUT FORTUNATELY MOST MODELS AND LATEST NHC GUIDANCE
CONTINUE TO TAKE THE LOW SYSTEM WELL EAST OF THE LOWER 48 INTO THE
WESTERN ATLANTIC.
..GUIDANCE/PREDICTABILITY ASSESSMENT
GUIDANCE REMAINS GENERALLY AGREEABLE REGARDING THE LARGE SCALE
PATTERN, INCLUDING TROUGHING DIVING SOUTH IN THE EASTERN PACIFIC
LATE WEEK AND FORMING A CLOSED LOW THAT MEANDERS JUST OFFSHORE OF
THE PACIFIC NORTHWEST THROUGH THE WEEKEND/EARLY NEXT WEEK. THERE IS
GOOD AGREEMENT FOR AN UPPER HIGH TO BECOME CENTERED OVER THE
SOUTHERN HIGH PLAINS WITH RIDGING BUILDING NORTH, THOUGH THE
EASTERN EXTENT WILL DEPEND ON THE RELATIVELY MORE UNCERTAIN
TROUGHING TO ITS EAST. THIS (INITIALLY SHORTWAVE) TROUGH SUPPORTING
A SURFACE LOW WILL TRACK THROUGH THE NORTH-CENTRAL U.S. THURSDAY-
FRIDAY AND IS LIKELY TO SLOW ATOP THE GREAT LAKES/EAST-CENTRAL U.S.
THIS WEEKEND INTO EARLY NEXT WEEK AS THE PATTERN AMPLIFIES
OVERALL. THE DEPTH OF THE TROUGH IS IN QUESTION EVEN INTO LATE
WEEK, WHICH IMPACTS THE SURFACE LOW PLACEMENT AND QPF PLACEMENT.
THE 12Z ECMWF WAS ON THE SHALLOWER SIDE WITH THE TROUGH, RESULTING
IN FASTER MOVEMENT OF THE QPF EAST THAN OTHER GUIDANCE, LIMITING
QPF AND ERO CONFIDENCE. THE NEWER 00Z ECMWF IS A BIT SLOWER/FARTHER
WEST THAN ITS PREVIOUS RUN, BUT THERE IS STILL SOME SPREAD IN
RAINFALL PLACEMENT. MODEL GUIDANCE GENERALLY SHOWS THE TROUGH
DEEPENING SOMEWHAT AND SLOWING, MAINTAINING AN AXIS OVER THE EAST-
CENTRAL U.S. INTO EARLY NEXT WEEK. THIS IS BEHIND AN INITIAL TROUGH
OVER THE EAST COAST/WESTERN ATLANTIC LATE THIS WEEK, WHICH LOOKS
TO STEER POTENTIAL T.C. FIVE EAST OF THE LOWER 48.
THE WPC FORECAST USED A BLEND OF MAINLY DETERMINISTIC MODELS EARLY
IN THE PERIOD, USING THE 12Z ECMWF AND CMC AND 18Z GFS ALONG WITH
A BIT OF THE EC ENSEMBLE MEAN. AS THE PERIOD PROGRESSED, LESSENED
THE PROPORTION OF DETERMINISTIC GUIDANCE IN FAVOR OF THE ENSEMBLE
MEANS TO REDUCE THE INDIVIDUAL MODEL DIFFERENCES.
..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. MODEL DIFFERENCES IN PLACEMENT OF HEAVY
RAIN PRECLUDED ANY UPGRADE IN THE ERO, BUT GIVEN THE POTENTIAL FOR
HIGH RAIN RATES IN AN UNSTABLE ENVIRONMENT, A SLIGHT RISK IN FUTURE
CYCLES IS CERTAINLY A POSSIBILITY. 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.
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page