300
FXUS02 KWBC 290701
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT TUE AUG 29 2023
VALID 12Z FRI SEP 01 2023 - 12Z TUE SEP 05 2023
...HEAT LOOKS TO INCREASE ACROSS THE PLAINS TO MIDWEST LATE THIS
WEEK INTO NEXT WEEK...
..OVERVIEW
AS IDALIA SHOULD BE TRACKING AWAY FROM THE U.S. AS THE MEDIUM
RANGE PERIOD BEGINS FRIDAY, FOCUS WILL SHIFT TO AN AMPLIFYING
UPPER-LEVEL PATTERN COMPOSED MAINLY OF AN UPPER TROUGH (WITH AN
EMBEDDED CLOSED LOW LATE WEEK) OVER THE WEST COAST, WHILE UPPER
RIDGING DOMINATES EAST OF IT ACROSS THE CENTRAL CONUS. THESE
FEATURES SHOULD SHIFT GRADUALLY EASTWARD INTO EARLY NEXT WEEK.
THIS PATTERN WILL ALLOW FOR WELL ABOVE AVERAGE TEMPERATURES TO
BUILD ACROSS THE PLAINS AND UPPER MIDWEST. MEANWHILE RAIN CHANCES
ARE POSSIBLE ACROSS MUCH OF THE WEST UNDERNEATH AND EAST OF THE
UPPER TROUGH WITHIN MOIST MEAN SOUTHERLY FLOW. ADDITIONALLY,
SCATTERED STORMS COULD PERSIST LATE WEEK INTO EARLY NEXT WEEK
ALONG THE GULF COAST AND FLORIDA AS A FRONTAL BOUNDARY STALLS
ACROSS THE REGION.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE SHOWS REASONABLY GOOD AGREEMENT WITH THE PATTERN
DESCRIBED ABOVE, ESPECIALLY EARLY IN THE PERIOD/LATE THIS WEEK.
SOME SPREAD IN THE TROUGH AXIS/LOW ALONG THE WEST COAST IS MAINLY
WITH THE 12Z CMC ON THE WESTERN SIDE OF THE ENVELOPE. THE 00Z
MODEL SUITE SHOWS EVEN BETTER AGREEMENT IN ITS POSITION. BY EARLY
NEXT WEEK THE TROUGH SHOULD BE SHIFTING EASTWARD BUT THE DETAILS
BECOME MORE UNCERTAIN WITHIN THE TROUGH, ESPECIALLY AS SECONDARY
ROUNDS OF ENERGY COME INTO IT FROM UPSTREAM. FOR NOW THE WPC
FORECAST FAVORED A DETERMINISTIC BLEND EARLY WITH MORE WEIGHTING
ON THE ECMWF AND GFS, WITH GRADUALLY INCREASING PROPORTIONS OF THE
GEFS AND EC ENSEMBLE MEANS WITH TIME TO SMOOTH OUT DIFFERENCES OF
SHORTWAVES WITHIN THE FLOW, WHICH CAN HOPEFULLY BE BETTER RESOLVED
WITH TIME.
SOMETHING WE ARE MONITORING, THOUGH CERTAINLY NOT BITING ON AT
THIS POINT, IS THE POTENTIAL FOR AT LEAST A PORTION OF IDALIA'S
ENERGY TO DRIFT SOUTH IN THE WESTERN ATLANTIC TO THE EAST OF
FLORIDA AND THEN ACTUALLY TRACK BACK WESTWARD INTO THE ATLANTIC
COAST OF FLORIDA. THE 12/18/00Z GFS RUNS ALL SHOWED SOME SEMBLANCE
OF THIS, WITH THE 12 AND 18Z RUNS VERY QUICK TO MAKE THIS LOOP.
INTERESTINGLY THE 00Z CMC INDICATES THIS POTENTIAL AS WELL, THOUGH
IS SLOWER TO DO SO, BUT IS NOT TOO FAR OFF FROM THE 00Z GFS, BOTH
SHOWING THE SYSTEM APPROACHING THE FLORIDA ATLANTIC COAST BY
AROUND NEXT TUESDAY. SOME GEFS AND CMC ENSEMBLE MEMBERS ARE
SHOWING A SIMILAR TRACK WHILE ESSENTIALLY NO 12Z EC ENSEMBLE
MEMBERS ARE. THE NATIONAL HURRICANE CENTER TRACK TAKES IDALIA EAST
AWAY FROM THE U.S. LATE WEEK AND THEN DECELERATING WITH TIME, BUT
IN THE EASTWARD DIRECTION. THERE IS A FAIR AMOUNT OF UNCERTAINTY
IN ITS TRACK SINCE IT ENDS UP IN AN AREA OF WEAK FLOW WITH LITTLE
TO STEER IT. SO THIS WILL CONTINUE TO BE MONITORED.
..WEATHER/HAZARDS HIGHLIGHTS
RAIN IS LIKELY ACROSS THE NORTHWEST WITH AN AMPLIFYING UPPER
TROUGH/EMBEDDED CLOSED LOW EXPECTED TO SETTLE ALONG THE WEST COAST
LATE WEEK. THE AMPLE FORCING FOR ASCENT THAT THIS WILL PROVIDE AS
WELL AS SOME INSTABILITY IN PLACE COULD ALLOW FOR SOME LOCALIZED
FLOODING ISSUES IN INTERIOR PARTS OF THE NORTHWEST, IN PORTIONS OF
OREGON, NORTHERN CALIFORNIA, AND NEVADA. MARGINAL RISKS OF
EXCESSIVE RAINFALL ARE IN PLACE FOR THIS THREAT FOR FRIDAY AND
SATURDAY, WITH BURN SCARS AND URBAN AREAS PARTICULARLY VULNERABLE.
FARTHER EAST, CONVECTION IS LIKELY ON FRIDAY ACROSS ARIZONA AND
UTAH IN PARTICULAR AND NORTHWARD INTO THE NORTHERN GREAT
BASIN/ROCKIES WITH A SURGE OF MONSOONAL MOISTURE FROM MEAN
SOUTHERLY FLOW EAST OF THE TROUGH. PLENTY OF INSTABILITY IN PLACE
AND ABOVE AVERAGE MOISTURE LEVELS INTO THE DESERT SOUTHWEST
SUPPORT MARGINAL RISKS IN THE ERO BOTH FRIDAY AND SATURDAY WITH
THIS ACTIVITY, LIKELY PEAKING ON FRIDAY. AS THE TROUGH SHIFTS
INLAND, COVERAGE OF PRECIPITATION COULD FOCUS ACROSS THE
INTERMOUNTAIN WEST EARLY NEXT WEEK AND EVENTUALLY INTO THE
NORTHERN PLAINS.
ADDITIONALLY, A STALLED FRONT ACROSS THE GULF COAST WILL PROVIDE A
FOCUS FOR MOISTURE TO POOL, AND SCATTERED THUNDERSTORMS ARE LIKELY
FROM THE CENTRAL GULF COAST INTO FLORIDA THROUGH MUCH OF THE WEEK
AND THE WEEKEND. THERE MAY BE A NONZERO CHANCE OF FLASH FLOODING
WITH THIS ACTIVITY THAT LOOKS TO PEAK FRIDAY, ESPECIALLY IF HIGH
RAIN RATES OCCUR IN SENSITIVE URBAN AREAS.
COOLER THAN AVERAGE HIGH TEMPERATURES ARE FORECAST FOR THE
NORTHWEST AND SHIFTING/EXPANDING INTO CALIFORNIA TO THE GREAT
BASIN GIVEN THE TROUGHING ALOFT. SLIGHTLY BELOW NORMAL
TEMPERATURES ARE FORECAST ACROSS THE EAST AS WELL FOR ON FRIDAY
BEFORE WARMING TO NEAR NORMAL BY THE WEEKEND. BUT IN BETWEEN, A
STRENGTHENING UPPER RIDGE WILL ALLOW TEMPERATURES TO RISE
CONSIDERABLY IN THE CENTRAL U.S. BY LATE WEEK AND THROUGH THE
HOLIDAY WEEKEND. HIGHS WELL INTO THE 90S WILL EXPAND ALL THE WAY
INTO THE NORTHERN PLAINS AND EAST ACROSS THE MISSISSIPPI VALLEY,
WHILE HIGHS EXCEEDING 100 ARE LIKELY FOR THE SOUTHERN HALF OF THE
PLAINS. THESE FORECAST HIGH TEMPERATURES ARE AROUND 10-20 DEGREES
ABOVE AVERAGE, PERHAPS LOCALLY HIGHER IN THE UPPER MIDWEST. SOME
RECORD WARM MAXIMUM AND MINIMUM TEMPERATURES COULD BE SET THERE AS
WELL. AS THE UPPER RIDGE SHIFTS EAST WITH TIME, TEMPERATURES IN
THE EAST LOOK TO WARM TO ABOUT 10-15 DEGREES ABOVE NORMAL BY
MONDAY-TUESDAY.
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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