528
FXUS02 KWBC 211923
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
323 PM EDT MON AUG 21 2023
VALID 12Z THU AUG 24 2023 - 12Z MON AUG 28 2023
...MAJOR HEAT WAVE CONTINUES THURSDAY IN THE MIDWEST WITH NUMEROUS
RECORD HIGHS LIKELY, EASING THERE LATE WEEK BUT PERSISTING ACROSS
THE GULF COAST STATES...
..OVERVIEW
THE MONSTER UPPER RIDGE THAT WILL BE ANCHORED IN PLACE ACROSS THE
MIDWEST STATES THROUGH MIDWEEK WILL START DECAYING SOME GOING INTO
THE END OF THE WORKWEEK, AND THEN GETS DISPLACED TOWARDS THE SOUTH
AND THEN SOUTHWEST AS A STRONG TROUGH AMPLIFIES ACROSS THE GREAT
LAKES AND THEN THE NORTHEAST. THE AREAL COVERAGE OF EXCESSIVE AND
RECORD-BREAKING HEAT WILL SHRINK WITH IT, BECOMING MORE LIMITED TO
THE GULF COAST STATES BY THE WEEKEND AFTER A HOT DAY IN THE
MIDWEST THURSDAY AND AS FAR NORTH AS THE OHIO VALLEY FRIDAY. THE
DEEP TROUGH SHOULD ALLOW FOR THE GREAT LAKES REGION TO THE
MID-ATLANTIC/NORTHEAST TO BE OVERALL COOLER THAN AVERAGE AND WILL
ALSO PUSH A FRONT OR TWO ACROSS THE EASTERN HALF OF THE U.S. WITH
SHOWERS AND STORMS AHEAD OF IT. COOLER TEMPERATURES AND WIDESPREAD
RAIN AND STORMS WITH POTENTIAL FOR FLASH FLOODING ARE LIKELY
ACROSS THE INTERMOUNTAIN WEST AND ROCKIES WITH MONSOONAL MOISTURE
IN PLACE LATE THIS WEEK INTO THE WEEKEND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE REMAINS IN GOOD AGREEMENT THAT THE ANOMALOUSLY
STRONG UPPER HIGH WILL HOLD ON THROUGH THURSDAY, BUT THEN A
SHORTWAVE TRACKING NEAR THE CANADA/U.S. BORDER EAST-SOUTHEASTWARD
SHOULD HELP SUPPRESS IT ON THE NORTHEAST SIDE WHILE DEVELOPING
AMPLIFIED TROUGHING ATOP THE NORTHEAST BY THE WEEKEND. WITH
REASONABLY HIGH CONFIDENCE IN THIS PATTERN, A MULTI-MODEL BLEND OF
THE DETERMINISTIC GUIDANCE FROM THE 06Z GFS AND 00Z
ECMWF/CMC/UKMET WAS USED FOR THE FORECAST EARLY IN THE PERIOD
(EXCEPT FOR TEMPERATURES AND DEWPOINTS, AS THE GFS CONTINUES TO
SHOW WAY TOO MUCH BOUNDARY LAYER MIXING, PRODUCING TOO HIGH
TEMPERATURES AND TOO LOW DEWPOINTS, WHICH IS A KNOWN ISSUE). THEN
MODELS HAVE BEEN WAFFLING SOME WITH THE RIDGE-TROUGH INTERFACE
AROUND THE MIDWEST BY EARLY NEXT WEEK. SOME GUIDANCE INCLUDING THE
00Z GFS MAINTAINED MORE RIDGING, BUT THE LARGER CLUSTER OF
GUIDANCE INCLUDING THE 00Z ECMWF AND CMC AND THE 06Z GFS BROUGHT
IN ADDITIONAL REINFORCING SHORTWAVE ENERGY, SO THE LATTER WAS
FAVORED. THE 00Z GFS ALSO SEEMED DISPLACED WITH AN EASTERN PACIFIC
UPPER LOW THAT WAS BECOMING MORE AGREEABLE WITH THE OTHER
GUIDANCE, INCLUDING THE INCOMING 12Z GUIDANCE. BUT OTHER THAN THAT
RUN, THE MULTI-MODEL BLEND WAS ABLE TO BE MAINTAINED, WITH MINOR
INCORPORATION OF THE EC AND GEFS MEANS WITH TIME TO SMOOTH OUT
GENERALLY SMALL DIFFERENCES.
..WEATHER/HAZARDS HIGHLIGHTS
THE REGION WITH THE GREATEST COVERAGE OF SHOWERS AND THUNDERSTORMS
IS EXPECTED TO BE ACROSS THE FOUR CORNERS STATES IN RESPONSE TO
MONSOONAL MOISTURE IN PLACE ON THE BACKSIDE OF THE UPPER RIDGE.
SOUTHERLY LOW-MID LEVEL FLOW WILL ENSURE A CONTINUED TRANSPORT OF
MOISTURE FROM WESTERN MEXICO THROUGH THE END OF THE WEEK AND
CONTINUING INTO THE WEEKEND, WITH PRECIPITABLE WATER ANOMALIES
WELL ABOVE THE 90TH PERCENTILE. MOISTURE LEVELS OF THIS MAGNITUDE
AND SCATTERED TO OCCASIONALLY WIDESPREAD COVERAGE OF CONVECTION
WARRANTS LARGE MARGINAL RISKS IN THE EROS FOR BOTH THURSDAY AND
FRIDAY, WITH AN EMBEDDED SLIGHT RISK IN UTAH THURSDAY WHERE
ENHANCED QPF FALLS OVER SENSITIVE AREAS LIKE SLOT CANYONS, AND A
SLIGHT RISK FRIDAY INTO PORTIONS OF COLORADO AND WYOMING WHERE
THERE IS MCS POTENTIAL. ACROSS THE EASTERN U.S., A COUPLE OF
CONVECTIVE COMPLEXES EMBEDDED WITHIN NORTHWEST FLOW ALOFT AROUND
THE RIDGE MAY AFFECT PORTIONS OF THE GREAT LAKES AND EXTENDING
ACROSS NORTHERN PORTIONS OF THE MID-ATLANTIC FOR THURSDAY. A
MARGINAL RISK IS IN PLACE FOR THIS POTENTIAL, AND EMBEDDED SLIGHT
RISKS CANNOT BE RULED OUT IN FUTURE CYCLES IF MODELS AGREE BETTER
ON PLACEMENT OF THE QPF AND IF IT IS IN SENSITIVE AREAS. BY FRIDAY
THE FRONT AND RAIN SHOULD PUSH INTO NEW ENGLAND. THE GRAY AND
CARIBOU MAINE WEATHER FORECAST OFFICES PREFERRED NOT TO HAVE ANY
MARGINAL RISK AT THIS POINT IN THE FORECAST, BUT THIS WILL
CONTINUE TO BE MONITORED AS THERE IS GOOD DYNAMICS FOR ASCENT IN
PLACE, BUT INSTABILITY COULD BE A LIMITING FACTOR. SCATTERED
CONVECTION IS POSSIBLE FARTHER SOUTH ALONG AND AHEAD OF THE COLD
FRONT, THROUGH THE CENTRAL APPALACHIANS TO MID-ATLANTIC FRIDAY AND
THE CAROLINAS AND SOUTHEAST OVER THE WEEKEND.
A MAJOR LATE SUMMER HEAT WAVE WILL CONTINUE TO MAKE ALL KINDS OF
WEATHER HEADLINES ACROSS THE MIDWEST STATES THROUGH THURSDAY WITH
THE MASSIVE UPPER RIDGE IN PLACE AND SUPPRESSING CLOUD
DEVELOPMENT, FOLLOWED BY MUCH WELCOMED RELIEF AS A STRONG COLD
FRONT HERALDS THE ARRIVAL OF MORE SEASONAL TEMPERATURES.
WIDESPREAD 100+ DEGREE HIGH TEMPERATURES ARE EXPECTED FROM
NEBRASKA AND IOWA AND POINTS SOUTHWARD, WITH OPPRESSIVE DEWPOINTS
ABOVE 75 DEGREES IN MANY CASES. THESE READINGS ACROSS THE MIDWEST
EQUATE TO 15-20 DEGREES ABOVE AVERAGE, WITH THE CORE OF THE
ANOMALIES FROM IOWA TO NORTHERN INDIANA ON THURSDAY. MEANWHILE,
THE INTENSE HEAT IS EXPECTED TO PERSIST UNABATED ACROSS TEXAS AND
THE CENTRAL GULF COAST REGION THROUGH ABOUT SATURDAY BEFORE A
GRADUAL MODERATING TREND COMMENCES. HIGHS ARE EXPECTED TO BE 10-15
DEGREES ABOVE AVERAGE, WITH MANY LOCATIONS ACROSS THE DEEP SOUTH
LIKELY HAVING THEIR WARMEST AUGUST ON RECORD. NUMEROUS DAILY
RECORDS AND PERHAPS SOME ALL-TIME RECORD HIGHS ARE EXPECTED,
PARTICULARLY ACROSS PORTIONS OF MISSISSIPPI AND LOUISIANA.
THE OPPOSITE HOLDS TRUE ACROSS MUCH OF THE INTERIOR WESTERN U.S.
WITH TEMPERATURES RUNNING 5 TO 15 DEGREES BELOW AVERAGE OWING TO
GREATER CLOUD COVER AND SHOWERS THROUGH THE END OF THE WEEK.
TEMPERATURES SLOWLY MODERATE GOING INTO THE WEEKEND ACROSS THAT
REGION AND RETURN CLOSER TO LATE AUGUST AVERAGES. MUCH OF THE
NORTHEAST U.S. SHOULD ALSO AVOID THE BRUTAL HEAT WITH MULTIPLE
SHORTWAVE PASSAGES REINFORCING A COMFORTABLE AIRMASS ACROSS THE
REGION.
TATE/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
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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