276
FXUS02 KWBC 150657
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
257 AM EDT SAT AUG 15 2026
VALID 12Z TUE AUG 18 2026 - 12Z SAT AUG 22 2026
..LONG-DURATION HEAT WAVE CONTINUES ACROSS THE SOUTH/SOUTHEAST
..MIDWEST HEAVY RAIN/FLOODING THREAT RENEWS TUESDAY
..OVERVIEW
SOUTHERN TIER UPPER RIDGING WILL BE WELL ESTABLISHED AS THE MEDIUM
RANGE PERIOD BEGINS TUESDAY AND CONTINUE TO PRODUCE HAZARDOUS HEAT.
TO THE NORTH OF THE RIDGE, RELATIVELY ACTIVE FLOW WILL PRODUCE
ROUNDS OF RAIN AND THUNDERSTORMS ACROSS THE NORTHERN TIER. RAINFALL
AMOUNTS ARE FORECAST TO PEAK IN THE MIDWEST TUESDAY WITH A FLASH
FLOODING THREAT. SHORTWAVES SHOULD GRADUALLY DEEPEN TROUGHING
ACROSS THE EASTERN U.S. INTO LATE WEEK AND SLOWLY EASE THE
MAGNITUDE AND SCOPE OF THE HEAT IN THE SOUTHEAST, WHILE MAINTAINING
ROUNDS OF RAIN/THUNDERSTORMS THERE. HOWEVER, THE STRONG UPPER
RIDGE/HIGH WILL KEEP A HOLD ON THE FOUR CORNERS STATES AND HIGH
PLAINS, GROWING NORTH ACROSS THE INTERMOUNTAIN WEST. SOME MONSOONAL
MOISTURE SHOULD STREAM NORTH UNDER THE RIDGE FOR SCATTERED CONVECTION.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT MODEL GUIDANCE IS GENERALLY AGREEABLE WITH THE PATTERN
DESCRIBED ABOVE. THE MOST QUESTIONABLE ASPECT OF THE PATTERN WILL
BE WITH SHORTWAVES MOVING THROUGH THE NORTHERN TIER, PARTICULARLY
AS THEY SERVE TO DEEPEN THE TROUGH OVER THE EASTERN HALF OF THE
U.S., AND HOW THEY AFFECT FRONTAL POSITIONS AND RAINFALL POTENTIAL.
AFTER THE 18Z GFS SEEMED TO BE AN OUTLIER WITH THE SLOW TIMING AND
STRENGTH OF A SHORTWAVE MOVING THROUGH THE CENTRAL PLAINS AND
VICINITY BY THURSDAY, THE 00Z GFS HAS TRENDED MORE LIKE OTHER
GUIDANCE, I.E. SHALLOW AND MORE PROGRESSIVE. SO THE 00Z MODEL CYCLE
SHOWS A BETTER CONSENSUS, AND A MULTI-MODEL BLEND OF THE DYNAMICAL
AND AI GUIDANCE WORKED WELL FOR THE PATTERN.
MODELS ARE ALSO AGREEABLE WITH THE UPPER RIDGE GROWING NORTHWARD
ACROSS THE WEST INTO LATE WEEK. BUT THIS WILL BE SOMEWHAT DEPENDENT
ON A LESS CERTAIN EASTERN PACIFIC TROUGH APPROACHING THE WEST
COAST. 00Z MODEL GUIDANCE HAS SLOWED A BIT WITH THE TROUGH JUST
REACHING THE WEST BY EARLY SATURDAY. THE 00Z GFS TROUGH IS
CONSIDERABLY WEAKER THAN OTHER GUIDANCE WITH THAT TROUGH, AS IT
FAVORS A STRONGER UPPER LOW WELL NORTHWEST IN THE NORTHEAST PACIFIC
INSTEAD, UNLIKE THE NON-NCEP GUIDANCE. MEANWHILE THE 00Z AIGFS
SHOWS THE TROUGH MOVING INTO THE WEST FASTER THAN CONSENSUS. SO AS
THE MEDIUM RANGE PERIOD PROGRESSED, FAVORED INCREASING PROPORTIONS
OF THE ENSEMBLE MEANS AND LESS GFS-BASED GUIDANCE.
..WEATHER/HAZARDS HIGHLIGHTS
A STRONG UPPER RIDGE REMAINING POSITIONED OVER THE SOUTHERN U.S.
WILL MAINTAIN HOT CONDITIONS ACROSS THE SOUTHERN PLAINS THROUGH THE
LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, AND SOUTHEAST.
AFTERNOON TEMPERATURES WILL COMMONLY REACH THE UPPER 90S TO LOWER
100S ACROSS THE INTERIOR SOUTH, WHILE HIGH HUMIDITY AND PERSISTENT
ELEVATED DEWPOINTS WILL PRODUCE OPPRESSIVE HEAT INDICES OF 105-115
DEGREES. RECORD-BREAKING WARM OVERNIGHT TEMPERATURES WILL FURTHER
LIMIT RECOVERY, PARTICULARLY WHERE SEVERAL CONSECUTIVE DAYS OF HEAT
OCCUR. WIDESPREAD MAJOR TO EXTREME HEATRISK CONDITIONS SHOULD
PERSIST INTO MUCH OF NEXT WEEK, AND SEVERAL CONSECUTIVE DAYS OF
HIGH HEAT STRESS CAN LEAD TO HEAT- RELATED ILLNESSES. THERE IS SOME
GRADUAL MINOR RELIEF IN SIGHT FOR EASTERN AREAS LATE NEXT
WEEK/WEEKEND DUE TO THE TROUGH ALOFT AND WEAK COLD FRONTS, BUT THE
SOUTH-CENTRAL U.S. AND BACK TOWARD THE SOUTHWEST WILL LIKELY
CONTINUE TO SEE DANGEROUS HEAT. SEE WPC'S KEY MESSAGES FOR MORE
INFORMATION.
SHORTWAVE ENERGY MOVING THROUGH THE NORTHERN TIER WILL CARRY
FRONTAL SYSTEMS THAT COULD FOCUS MOISTURE FOR ROUNDS OF SHOWERS AND
THUNDERSTORMS NEXT WEEK. ONE STRONG SHORTWAVE AND COLD FRONT ARE
FORECAST TO BRING INCREASING RAIN AMOUNTS TO THE UPPER MIDWEST IN
PARTICULAR BY TUESDAY. A SLIGHT RISK REMAINS IN PLACE IN THE DAY
4/TUESDAY EXCESSIVE RAINFALL OUTLOOK FOR STRONG THUNDERSTORMS/MCS
ACTIVITY THAT COULD CAUSE FLASH FLOODING. RAIN AND THUNDERSTORM
CHANCES WILL PUSH ACROSS THE GREAT LAKES TO NORTHEAST BACK ACROSS
THE OHIO VALLEY AND CENTRAL PLAINS BY WEDNESDAY WITH AN ISOLATED
FLASH FLOOD THREAT. FARTHER SOUTH, A WEAK COLD FRONT SHOULD PROVIDE
A FOCUS FOR SCATTERED THUNDERSTORMS ACROSS THE SOUTHEAST TUESDAY-
WEDNESDAY IN THE HOT AND UNSTABLE AIRMASS. BY LATE WEEK, THIS
ACTIVITY MAY ALL COMBINE INTO CHANCES FOR SCATTERED SHOWERS/STORMS
ACROSS THE EASTERN SEABOARD, AND WEST ACROSS THE MID-SOUTH AND
MISSISSIPPI VALLEY WITH THE TAIL END OF THE FRONT. MEANWHILE, SOME
MONSOONAL MOISTURE SHOULD BRING SCATTERED, DIURNALLY DRIVEN
CONVECTION TO THE SOUTHWEST ESPECIALLY BY WEDNESDAY AND BEYOND.
MOISTURE MAY CONTINUE SPREADING NORTH ACROSS THE INTERMOUNTAIN WEST
BY FRIDAY, ALLOWING FOR RAIN AND THUNDERSTORM CHANCES THERE.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY MESSAGES
ARE AT:
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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