041
FXUS02 KWBC 141838
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
238 PM EDT WED AUG 14 2024
VALID 12Z SAT AUG 17 2024 - 12Z WED AUG 21 2024
...HAZARDOUS HEAT LIKELY ACROSS THE SOUTHERN PLAINS AND CENTRAL
GULF COAST STATES THROUGH THIS WEEKEND AND EARLY NEXT WEEK...
..OVERVIEW
A SLOW-MOVING UPPER PATTERN WILL BE IN PLACE THIS WEEKEND INTO
EARLY NEXT WEEK. RIDGING WILL GENERALLY HOLD OVER THE CENTRAL U.S.,
ANCHORED BY AN UPPER HIGH CENTERED OVER THE SOUTHERN HIGH PLAINS
TO PROMOTE HAZARDOUS HEAT FROM THE SOUTHERN PLAINS TO THE CENTRAL
GULF COAST. AN UPPER LOW IS FORECAST TO MEANDER OFF THE WEST COAST,
LEADING TO A COOLER AND WETTER PATTERN FOR THE NORTHWEST.
MEANWHILE, AN UPPER TROUGH 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 NORTHEAST U.S. THROUGH MUCH OF
THE PERIOD. TROPICAL CYCLONE ERNESTO IS FORECAST TO TRACK NORTH
OVER THE WESTERN ATLANTIC AND REMAIN OFFSHORE OF THE U.S. EAST
COAST.
..GUIDANCE/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE CONTINUES TO SHOW GOOD AGREEMENT ON THE LARGE SCALE
PATTERN THROUGH THE PERIOD WITH SOME FAIRLY TYPICAL MODEL
SPREAD/UNCERTAINTY IN THE DETAILS. MOST OF THE PATTERN UNCERTAINTY
SURROUNDS AN UPPER SHORTWAVE (OR LOW) INTO CENTRAL/SOUTHEAST CANADA
THAT MAY ERODE THE TOP PORTION OF THE CENTRAL U.S. RIDGE. THE GFS
IS THE MOST BULLISH ON THIS, SUGGESTING A DEEP CLOSED LOW OVER
CENTRAL CANADA NEXT WEEK AND SHORTWAVE TROUGHING THROUGH THE
NORTHERN PLAINS/UPPER MIDWEST. THE ECMWF AND CMC (WITH SUPPORT FROM
THE MEANS) HOLD ONTO MORE RIDGING ACROSS THAT AREA. ACROSS THE
EAST, MOST OF THE GUIDANCE HAS TRENDED TOWARDS MORE AMPLIFIED
TROUGHING OVER THE EAST, BUT A LOT OF UNCERTAINTY ABOUT IF/HOW
ERNESTO MAY GET INTERTWINED AS IT MOVES NORTHWARD THROUGH THE
WESTERN ATLANTIC. CONSENSUS IS FOR ERNESTO TO STAY WELL ENOUGH
EAST, BUT THERE IS SOME GUIDANCE WHICH BRINGS THE SYSTEM CLOSE TO
OR THROUGH THE CANADIAN MARITIMES.
WPC PROGS USED A GENERAL BLEND OF THE OPERATIONAL GUIDANCE FOR THE
FIRST HALF OF THE PERIOD. TRENDED TOWARDS JUST OVER HALF OF THE
ENSEMBLE MEANS BY THE END OF THE PERIOD TO ACCOUNT FOR INCREASING
SPREAD IN THE GUIDANCE. THE CMC WAS NOT INCLUDED LATE PERIOD DUE TO
IT BEING SLIGHTLY FASTER/EAST WITH THE EASTERN U.S. TROUGH.
..WEATHER/HAZARDS HIGHLIGHTS
THE UPPER TROUGH IN THE GREAT LAKES BUILDING SOUTH AND EAST WITH
TIME, ALONG WITH A SURFACE LOW PRESSURE/FRONTAL SYSTEM, WILL
PROVIDE FORCING FOR RAIN AND THUNDERSTORMS ACROSS THE EASTERN U.S.
THIS WEEKEND. INSTABILITY AND ABOVE NORMAL MOISTURE WILL BE IN
PLACE AND COULD LEAD TO LOCALLY HEAVY RAIN. MARGINAL RISKS OF
EXCESSIVE RAINFALL ARE IN PLACE DAY 4/SATURDAY AND DAY 5/SUNDAY
FROM THE CENTRAL/NORTHERN APPALACHIANS AND LOWER GREAT LAKES INTO
THE NORTHERN MID-ATLANTIC AND PORTIONS OF THE NORTHEAST. WHILE
WIDESPREAD HEAVY RAIN IS NOT PARTICULARLY LIKELY, EMBEDDED HEAVY
TOTALS COULD CAUSE FLOODING CONCERNS ESPECIALLY CONSIDERING WET
ANTECEDENT CONDITIONS IN MUCH OF THE REGION.
MONSOONAL CONVECTION WILL RETURN THIS WEEKEND WITH MOIST SOUTHERLY
FLOW WEST OF THE UPPER RIDGE AXIS. A MARGINAL RISK IS IN PLACE FOR
DAY 4/SATURDAY FOR MUCH OF ARIZONA AND UTAH INTO SURROUNDING
AREAS. MOISTURE AND INSTABILITY LOOKS TO MAKE IT EVEN FARTHER NORTH
BY SUNDAY, REACHING THE NORTHERN ROCKIES, SO THE MARGINAL RISK FOR
SUNDAY STRETCHES FARTHER NORTH AND EAST. CONVECTION MAY CONTINUE
ACROSS THESE AREAS INTO EARLY NEXT WEEK BUT GRADUALLY REDUCE IN
SCOPE.
FARTHER NORTH, THE UPPER LOW SETTING UP IN THE EASTERN PACIFIC
WILL PROMOTE ROUNDS OF MODEST RAIN ACROSS THE NORTHWEST. 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 ACROSS THE PLAINS, MISSISSIPPI VALLEY, AND
TENNESSEE VALLEY TO SOUTHEAST. EXPECT CHANGES TO THE RAINFALL
FORECAST IN FUTURE CYCLES, AND THERE IS CERTAINLY NOT ENOUGH
AGREEMENT FOR ANY EXCESSIVE RAINFALL RISK AT THIS TIME. MEANWHILE,
A FRONT SLOWLY MOVING THROUGH FLORIDA AND ABOVE NORMAL PRECIPITABLE
WATER VALUES COULD LEAD TO LOCALLY HEAVY RAIN, ESPECIALLY INTO
SATURDAY. 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 ABOVE
NORMAL TEMPERATURES BY 5-15 DEGREES FROM THE CENTRAL AND SOUTHERN
HIGH PLAINS EAST INTO THE LOWER MISSISSIPPI VALLEY/CENTRAL GULF
COAST. HIGHS WELL INTO THE 100S IN MUCH OF TEXAS COULD SET DAILY
RECORDS. FROM EASTERN TEXAS INTO THE LOWER MISSISSIPPI VALLEY, HIGH
DEWPOINTS WILL COMBINE WITH THE HOT TEMPERATURES TO PRODUCE HEAT
INDICES LIKELY ABOVE 110 DEGREES. LOW TEMPERATURES IN THE UPPER 70S
TO LOW 80S WILL EXACERBATE HEAT STRESS AND COULD SET RECORDS FOR
WARM MINIMUM TEMPERATURES. EXPERIMENTAL HEATRISK CONTINUES TO SHOW
MAJOR TO EXTREME IMPACTS FROM THIS LENGTHY HEAT WAVE. NORTH OF THIS
REGION, MOST AREAS SHOULD SEE NEAR NORMAL HIGHS AND NEAR TO
SLIGHTLY ABOVE AVERAGE LOWS. THE PACIFIC NORTHWEST INTO NORTHERN
CALIFORNIA CAN EXPECT TEMPERATURES A BIT BELOW NORMAL IN RESPONSE
TO THE LOW ALOFT.
SANTORELLI/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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