019
FXUS02 KWBC 140659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM 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 STRETCH NORTHWARD FROM AN UPPER HIGH
IN THE SOUTHERN HIGH PLAINS AND PROMOTE HAZARDOUS HEAT FROM THE
SOUTHERN PLAINS TO THE CENTRAL GULF COAST. AN UPPER LOW IS FORECAST
TO DIG INTO THE EASTERN PACIFIC 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 EASTERN 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 FIRST PART OF THE MEDIUM RANGE PERIOD WITH AN
EXPECTED AMOUNT OF MODEL SPREAD/UNCERTAINTY THAT INCREASES OVER
TIME. BY SUNDAY, THE 12Z CMC AND 18Z GFS JUMP FARTHER NORTH INTO
CANADA WITH THE GREAT LAKES SURFACE LOW COMPARED TO THE BROADER
AGREEMENT OF OTHER MODELS AND ENSEMBLE MEANS KEEPING IT FARTHER
SOUTH. THE NEWER 00Z GFS LOOKS MORE FAVORABLE, WHILE THE 00Z CMC IS
STILL ON THE NORTHERN SIDE BUT NOT AS FAR OFF AS THE 12Z RUN. A
LARGER SCALE DIFFERENCE THAT EMERGES INTO EARLY NEXT WEEK IS THE
POTENTIAL FOR A CENTRAL CANADA UPPER LOW TO THE NORTH OR NORTHEAST
OF THE RIDGE AXIS. THE 12Z ECMWF HAS THIS FEATURE EAST OF OTHER
GUIDANCE, SERVING TO DEEPEN THE EASTERN TROUGH FOR LONGER THAN
OTHER GUIDANCE. OTHER MODELS LIKE THE GFS RUNS AND THE 12Z CMC SHOW
THIS UPPER LOW TO THE WEST WHERE IT INSTEAD SUPPRESSES THE
NORTHERN SIDE OF THE RIDGE. THE NEW 00Z GFS MAY TAKE THIS
SUPPRESSION OF THE RIDGE A BIT TOO FAR AS IT IS QUITE STRONG WITH
THE LOW. MEANWHILE FARTHER WEST, MODELS SHOW REASONABLY GOOD
AGREEMENT WITH THE EASTERN PACIFIC UPPER LOW THROUGH ABOUT MONDAY,
WITH MORE SPREAD AFTER THAT WITH IF/WHEN THE TROUGHING COMES INLAND
OR CONTINUES TO HOVER OFFSHORE. THIS DEPENDS ON LOW CONFIDENCE
UPSTREAM ENERGY COMING FROM ALASKA, SO A MIDDLE GROUND
MODEL/ENSEMBLE BLEND SEEMS BEST THERE.
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, BERMUDA AND THE CANADIAN MARITIMES SHOULD KEEP A
CLOSE EYE ON THE FORECAST AS SOME 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 12Z ECMWF AND 12/18Z GFS EARLY IN THE PERIOD. SINCE EACH
DETERMINISTIC RUN HAD SOME AFOREMENTIONED ISSUES, THE PROPORTION OF
DETERMINISTIC RUNS WAS GRADUALLY REDUCED IN FAVOR OF THE GEFS AND
EC ENSEMBLE MEANS TO OVER HALF 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/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 FROM THE
CENTRAL/NORTHERN APPALACHIANS AND LOWER GREAT LAKES INTO THE
NORTHERN MID-ATLANTIC AND PORTIONS OF THE NORTHEAST, AND DAY
5/SUNDAY A BIT EAST. 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. RAIN MAY PERSIST ACROSS THE EASTERN SEABOARD INTO
EARLY NEXT WEEK.
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.
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.
The Nexlab HPC Page Main Text Page