430
FXUS02 KWBC 280707
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
306 AM EDT MON AUG 28 2023
VALID 12Z THU AUG 31 2023 - 12Z MON SEP 04 2023
...TROPICAL STORM IDALIA WILL BE PULLING OFFSHORE ON THURSDAY WITH
PERHAPS SOME LINGERING RAIN AND WINDS IN THE CAROLINAS...
..OVERVIEW
AT THE START OF THE MEDIUM RANGE PERIOD THURSDAY, IDALIA IS
FORECAST TO BE IN THE WESTERN ATLANTIC AND PULLING EASTWARD AFTER
TRACKING ACROSS FLORIDA TO THE CAROLINAS MIDWEEK. SOME LINGERING
RAIN AND WIND COULD AFFECT THE CAROLINAS THROUGH THURSDAY ON THE
BACKSIDE OF THE TROPICAL SYSTEM. SCATTERED STORMS MAY ALSO PERSIST
INTO THE WEEKEND FROM THE CENTRAL GULF COAST INTO FLORIDA WITH A
MEANDERING FRONTAL SYSTEM. FARTHER WEST, THE PATTERN LOOKS TO
AMPLIFY BY LATE WEEK AS AN UPPER LOW FORMS NEAR THE WEST COAST,
LEADING TO SOME RAINFALL FOR THE PACIFIC NORTHWEST AND NORTHERN
CALIFORNIA, WHILE FARTHER SOUTH THE NEXT SURGE OF MONSOONAL
MOISTURE APPEARS TO MOVE UP ACROSS THE DESERT SOUTHWEST INTO THE
FOUR CORNERS STATES AROUND THURSDAY-FRIDAY. AN UPPER RIDGE EAST OF
THE LOW WILL HELP INCREASE TEMPERATURES TO WELL ABOVE NORMAL
ACROSS THE CENTRAL U.S. LATE WEEK INTO EARLY NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE FOR IDALIA HAS BEEN WAFFLING OVER THE PAST FEW
DAYS, WITH SOME WIGGLES IN THE TRACK BUT ESPECIALLY DIFFERENCES
WITH THE TIMING OF THE SYSTEM. ON THE WHOLE THE NATIONAL HURRICANE
CENTER'S FORECAST AND MODEL OUTPUTS HAVE BEEN TRENDING FASTER WITH
THE TROPICAL CYCLONE BY THE THURSDAY TIMEFRAME. THE 12Z ECMWF WAS
PARTICULARLY FAST THOUGH, SO FAVORED OTHER GUIDANCE LIKE THE GFS
THERE TO BETTER COMPLY WITH THE NHC TRACK. THE TIMING AND TRACK OF
IDALIA WILL DEPEND ON INTERACTION WITH A FRONTAL SYSTEM TO THE
WEST AND TROUGHING ALOFT POTENTIALLY HELPING ACCELERATE IT
NORTHEASTWARD.
THE MIDLATITUDE SYNOPTIC-SCALE FLOW IS IN REASONABLY GOOD
AGREEMENT WITH THE BULK OF GUIDANCE INTO LATE WEEK, WITH A PATTERN
FEATURING TROUGHING THAT IS QUICK TO EXIT THE NORTHEAST, A
SHORTWAVE/SMALL CLOSED LOW COMING ACROSS THE NORTH-CENTRAL
U.S./SOUTH-CENTRAL CANADA, AND POTENT ENERGY DROPPING SOUTH AND
FORMING A CLOSED UPPER LOW NEAR/ATOP THE WEST COAST. THE MAIN
OUTLIER IN THE 12/18Z MODEL CYCLE WAS THE 12Z CMC, WHICH WAS
SLOWER WITH THE LATTER TWO FEATURES EVEN BEGINNING THURSDAY AND
NEVER CAUGHT UP TO THE POSITIONS OF OTHER GUIDANCE, AND THE 12Z
CMC MEAN WAS EAST WITH THE WEST COAST TROUGH. THE NEWER 00Z
DETERMINISTIC CMC FORTUNATELY SEEMS TO BE IN BETTER ALIGNMENT WITH
CONSENSUS WITH THESE FEATURES, THOUGH ENDS UP SEEMING SLOW/WEST AS
THE PERIOD PROGRESSES. BY EARLY NEXT WEEK, THE TIMING OF THE UPPER
LOW SHIFTING EASTWARD AND POTENTIALLY OPENING UP INTO A TROUGH
BECOMES SOMEWHAT QUESTIONABLE AMONG THE 12/18Z GUIDANCE SUITE AS
WELL AS THE NEWER 00Z SUITE--THE LATTER GENERALLY HAD A FASTER
TREND COMPARED TO THE PREVIOUS CYCLE, BUT THIS REMAINS UNCERTAIN.
ADDITIONALLY, WHILE UPPER RIDGING IN THE CENTRAL U.S. IS QUITE
AGREEABLE AMONG MODELS, THE NORTH-CENTRAL U.S. MAY BECOME A
TROUBLE SPOT AS ADDITIONAL TROUGHING MAKES ITS WAY INTO
SOUTH-CENTRAL CANADA AND MODELS VARY AS TO HOW FAR SOUTH IT MAY
COME AND POTENTIALLY SUPPRESS THE NORTHERN SIDE OF THE RIDGE. THUS
THE WPC FORECAST BLEND FAVORED A DETERMINISTIC BLEND FAVORING THE
GFS AND ECMWF EARLY IN THE PERIOD, BUT SHIFTED TOWARD INCLUDING
AND INCREASING GEFS AND EC ENSEMBLE MEANS TO ABOUT HALF BY THE END
OF THE PERIOD GIVEN INCREASING SPREAD.
..WEATHER/HAZARDS HIGHLIGHTS
THE PRIMARY WIND AND FLOODING CONCERNS FROM TROPICAL STORM IDALIA
HAVE NOW SHIFTED INTO THE SHORT RANGE PERIOD, BUT THE CAROLINAS IN
PARTICULAR COULD STILL SEE HEAVY RAINFALL AND HIGH WINDS THURSDAY.
THERE HAS BEEN A FASTER TREND IN THE STORM TRACK OVER THE PAST
DAY, WITH MORE OF THE RAIN LIKELY FALLING IN THE SHORT RANGE. THUS
THE ERO RISK AREAS WERE SHRUNK ONCE AGAIN FOR THURSDAY/DAY 4, AS
EASTERN NORTH CAROLINA SEEMS TO BE THE ONLY AREA WITH ENOUGH
AGREEMENT TO SEE HEAVY RAIN PERHAPS REMAINING INTO THURSDAY FOR A
SLIGHT RISK, WHILE A MARGINAL RISK COVERS AREAS THAT COULD SEE
HEAVY RAIN OR POSSIBLY NO RAIN AT ALL DEPENDENT ON IDALIA'S
EVENTUAL TRACK AND TIMING. ADDITIONALLY, FRONTS 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
THURSDAY-FRIDAY, ESPECIALLY IF HIGH RAIN RATES OCCUR IN SENSITIVE
URBAN AREAS.
SOME LIGHT TO MODERATE RAIN IS POSSIBLE OVER THE PACIFIC NORTHWEST
AND NORTHERN CALIFORNIA WITH AN AMPLIFYING UPPER TROUGH/EMBEDDED
CLOSED LOW EXPECTED TO SETTLE ALONG THE WEST COAST THROUGH LATE
WEEK. AS THE TROUGH SHIFTS INLAND, COVERAGE OF PRECIPITATION COULD
INCREASE EASTWARD INTO THE INTERMOUNTAIN WEST EARLY NEXT WEEK.
FARTHER SOUTH, CONVECTION APPEARS LIKELY TO INCREASE BY THURSDAY
AND FRIDAY ACROSS ARIZONA AND UTAH IN PARTICULAR WITH A SURGE OF
MONSOONAL MOISTURE FROM MEAN SOUTHERLY FLOW. PLENTY OF INSTABILITY
IN PLACE AND ABOVE AVERAGE MOISTURE LEVELS INTO THE DESERT
SOUTHWEST SUPPORT MARGINAL RISKS IN THE ERO BOTH THURSDAY AND
FRIDAY WITH THIS ACTIVITY.
COOLER THAN AVERAGE CONDITIONS, AT LEAST IN TERMS OF HIGH
TEMPERATURES, ARE FORECAST FOR THE NORTHWEST AND
SHIFTING/EXPANDING INTO CALIFORNIA TO THE GREAT BASIN GIVEN THE
TROUGHING ALOFT. BELOW NORMAL TEMPERATURES ARE FORECAST ACROSS THE
EAST AS WELL FOR THURSDAY-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 NORTH INTO THE NORTH-CENTRAL PLAINS AND EAST ACROSS
THE MISSISSIPPI VALLEY, WHILE HIGHS EXCEEDING 100 ARE LIKELY FOR
THE SOUTHERN HALF OF THE PLAINS. THESE TEMPERATURES WOULD BE
AROUND 10-20 DEGREES ABOVE AVERAGE.
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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