034
FXUS02 KWBC 232028
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
427 PM EDT WED AUG 23 2023
VALID 12Z SAT AUG 26 2023 - 12Z WED AUG 30 2023
...RELIEF FROM THE EXTREME HEAT EXPECTED TO SPREAD FARTHER SOUTH
INTO THE GULF COAST STATES BEGINNING THIS WEEKEND AS HEAVY RAIN
THREAT EMERGES OVER THE CENTRAL HIGH PLAINS AND THE CAROLINAS...
..SYNOPTIC OVERVIEW
WELCOMED RELIEF FROM THE EXTREME HEAT IS IN STORE FOR THE
NORTH-CENTRAL U.S. BY THE WEEKEND AS THE ONGOING MAJOR LATE SUMMER
HEATWAVE ABATES AND THE UPPER RIDGE IS SUPPRESSED TO THE SOUTH.
THE AREAL COVERAGE OF EXCESSIVE AND RECORD-BREAKING HEAT WILL
SHRINK WITH IT, BECOMING MORE LIMITED TO THE GULF COAST STATES BY
THE WEEKEND WHERE ADDITIONAL DAILY RECORD HIGHS ARE LIKELY. THE
UPPER TROUGH THAT IS FORECAST TO AMPLIFY OVER THE NORTHEASTERN
QUADRANT OF THE COUNTRY NEXT WEEK NOT ONLY WILL BE RESPONSIBLE FOR
BRINGING THE COOLER TEMPERATURES BUT ALSO A THREAT OF HEAVY RAIN
FOR THE CENTRAL HIGH PLAINS THIS WEEKEND, FOLLOWED BY THE
CAROLINAS AND POSSIBLY FARTHER UP THE EAST COAST DURING THE FIRST
HALF OF NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
OVERALL, THE 06Z AND 12Z MODEL GUIDANCE SUITE IS IN RELATIVELY
GOOD AGREEMENT ACROSS THE MAINLAND U.S. THROUGH MUCH OF THE
MEDIUM-RANGE PERIOD. THE MOST NOTICEABLE DIFFERENCES ARE FOUND
ACROSS THE GREAT LAKES WHERE THERE APPEARS TO BE A TENDENCY FOR
THE MODELS TO AMPLIFY A SHORTWAVE DIGGING INTO THE LONGWAVE TROUGH
DOWNSTREAM. THE MORE AMPLIFIED TROUGH APPEARS TO SLOW DOWN THE
EASTWARD AND SOUTHWARD PROGRESSION OF THE FRONT ACROSS THE EASTERN
U.S. EARLY NEXT WEEK, LEADING TO A BETTER CHANCE FOR THE
ASSOCIATED SHOWERS AND THUNDERSTORMS TO LINGER OVER THE EAST COAST
ESPECIALLY OVER THE CAROLINAS. THE BETTER-DEFINED TROUGHING ALSO
HELPS TO BRING A BETTER CHANCE OF PRECIPITATION INTO THE NORTHEAST
BY MIDWEEK. OVER THE CENTRAL PLAINS, THE ECMWF AND CMC FAVOR MORE
PRECIPITATION AHEAD OF A SHORTWAVE TROUGH COMPARED WITH THE GFS.
ON THE OTHER HAND, THE GFS FAVORS MORE PRECIPITATION OVER THE
CAROLINAS EARLY NEXT WEEK RELATIVE TO THE DRIER SOLUTION GIVEN BY
THE ECMWF.
THE WPC MEDIUM-RANGE FORECAST PACKAGE WAS BASED ON A COMPOSITE
BLEND OF 40% FROM THE 06Z GFS/ GEFS, 40% FROM THE 00Z ECMWF/EC
MEAN, AND 20% FROM THE CMC/CMC MEAN, WITH A HIGHER PERCENTAGE FROM
THE ENSEMBLE MEANS TOWARD THE END OF THE FORECAST PERIOD. THIS
SOLUTION MAINTAINS GOOD CONTINUITY WITH THE PREVIOUS WPC FORECASTS
BUT WITH A SLOWER PROGRESSION OF THE FRONT ACROSS THE EASTERN U.S.
NEXT WEEK.
..WEATHER/HAZARDS HIGHLIGHTS
SHOWERS AND THUNDERSTORMS ARE EXPECTED TO BE PREVALENT ACROSS MUCH
OF COLORADO AND EXTENDING INTO PORTIONS OF THE CENTRAL PLAINS THIS
WEEKEND AS A COUPLE OF CONVECTIVE COMPLEXES ARE LIKELY TO DEVELOP,
ALTHOUGH THE HEAVIEST RAINFALL WILL LIKELY HAPPEN BEFORE SATURDAY.
A MARGINAL RISK AREA IS MAINTAINED FOR BOTH WEEKEND DAYS FROM
CENTRAL COLORADO TO ADJACENT PORTIONS OF NEBRASKA AND KANSAS.
HOWEVER, THE 12Z ECMWF NOW INDICATES A FASTER EASTWARD EJECTION OF
THE SHORTWAVE, WHICH LOWERS THE HEAVY RAIN THREAT OVER THE CENTRAL
HIGH PLAINS BUT RAISES THE THREAT OVER THE CENTRAL PLAINS.
THEREFORE, THE MARGINAL RISK ON DAY 5 FOR THIS REGION HAS BEEN
SPLIT INTO TWO GIVEN INCREASING MODEL INDICATIONS OF A SEPARATE
HIGH QPF OVER THE CENTRAL PLAINS. ACROSS THE EASTERN U.S., AS A
COLD FRONT INTERCEPTS A MOIST AND UNSTABLE AIRMASS ACROSS THE
CAROLINAS, SCATTERED TO NUMEROUS SHOWERS AND STORMS ARE EXPECTED
TO DEVELOP. GIVEN A SLOWER EASTWARD PROGRESSION OF THE FRONT IN
THIS FORECAST CYCLE, A MARGINAL RISK AREA IS IN EFFECT FOR THIS
REGION FOR THIS WEEKEND. THERE IS GROWING MODEL SIGNALS TO
SUSTAIN AN AREA OF MODERATE TO HEAVY RAIN ACROSS THE CAROLINAS
INTO EARLY NEXT WEEK, POSSIBLY SPREADING FARTHER UP THE EAST COAST
AS AN UPPER TROUGH APPROACHES. IN ADDITION, TROPICAL CONVECTION
APPEARS TO STREAM TOWARD FLORIDA FROM THE SOUTHWEST AS THE
NATIONAL HURRICANE CENTER IS MONITORING THE POTENTIAL FOR A
TROPICAL SYSTEM TO DEVELOP OVER THE EASTERN GULF OF MEXICO NEXT
WEEK. IN THE MEAN TIME, MAINLY COOL AND DRY, ALMOST FALL-LIKE,
CONDITIONS CAN BE EXPECTED ACROSS MOST OF THE MIDWEST AND THE
TENNESSEE RIVER VALLEY.
THERE IS SOME GOOD NEWS TO REPORT ABOUT THE CENTRAL U.S. HEATWAVE
GOING INTO THE WEEKEND. A PATTERN CHANGE WILL LEAD TO THE BIG
UPPER RIDGE DECAYING SOME AS THE NORTHERN STREAM FLOW ACROSS THE
GREAT LAKES AND NEW ENGLAND WEAKENS IT. THIS WILL RESULT IN A
COLD FRONT SLOWLY SETTLING SOUTHWARD ACROSS THE DEEP SOUTH AND
THEN THE GULF COAST REGION BY EARLY NEXT WEEK. ALTHOUGH THE
HEATWAVE WILL HAVE CONCLUDED ACROSS THE MIDWEST, IT WILL STILL BE
QUITE HOT AND UNCOMFORTABLE FROM TEXAS TO SOUTH CAROLINA THIS
WEEKEND, WITH WIDESPREAD 100-105 DEGREE HIGHS EXPECTED ON SATURDAY
SOUTH OF THE INTERSTATE 40 CORRIDOR, AND THEN THE TRIPLE DIGITS
BECOME MORE CONFINED TO THE INTERSTATE 20 CORRIDOR AND POINTS
SOUTH ON SUNDAY. THIS WILL LIKELY ESTABLISH ADDITIONAL DAILY
RECORD HIGHS AND PROBABLY SECURE THE HOTTEST AUGUST ON RECORD FOR
MANY GULF COAST AND DEEP SOUTH CITIES. THE HEAT ABATES ACROSS THE
GULF COAST GOING INTO TUESDAY AND WEDNESDAY WITH MAINLY 90S FOR
HIGHS EXPECTED FOR THESE SAME AREAS. UNFORTUNATELY HEAT BUILDS
AGAIN ACROSS THE LOWER ELEVATIONS OF THE DESERT SOUTHWEST WITH
SOME 110S LIKELY BY THE WEEKEND. ELSEWHERE, TEMPERATURES WILL BE
PLEASANT ACROSS MUCH OF THE NORTHEASTERN QUADRANT OF THE NATION
WITH HIGHS MAINLY IN THE 70S AND 80S AND COMFORTABLE HUMIDITY
LEVELS.
KONG/HAMRICK
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
HAZARDS:
- HEAVY RAIN ACROSS PORTIONS OF THE CENTRAL ROCKIES AND THE
CENTRAL PLAINS, SAT, AUG 26.
- HEAVY RAIN ACROSS PORTIONS OF THE SOUTHEAST AND THE
MID-ATLANTIC, SUN-TUE, AUG 27-AUG 29.
- HAZARDOUS HEAT ACROSS PORTIONS OF CALIFORNIA AND THE SOUTHWEST,
SUN-MON, AUG 27-AUG 28.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE SOUTHEAST, THE LOWER
MISSISSIPPI VALLEY, AND THE SOUTHERN
PLAINS, SAT-TUE, AUG 26-AUG 29.
- HAZARDOUS HEAT ACROSS PORTIONS OF THE LOWER MISSISSIPPI VALLEY,
THE TENNESSEE VALLEY, THE MIDDLE
MISSISSIPPI VALLEY, THE MID-ATLANTIC, THE SOUTHERN APPALACHIANS,
THE SOUTHEAST, THE SOUTHERN
PLAINS, AND THE OHIO VALLEY, SAT-SUN, AUG 26-AUG 27.
- HEAVY RAIN ACROSS PORTIONS OF THE ALASKA PANHANDLE AND MAINLAND
ALASKA, SAT-SUN, AUG 26-AUG 27.
- HEAVY RAIN ACROSS PORTIONS OF MAINLAND ALASKA, TUE-WED, AUG
29-AUG 30.
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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