562
FXUS02 KWBC 282039
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
438 PM EDT MON AUG 28 2023
VALID 12Z THU AUG 31 2023 - 12Z MON SEP 04 2023
...TROPICAL CYCLONE IDALIA WILL BE PULLING OFFSHORE THURSDAY WITH
PERHAPS SOME LINGERING RAIN AND WINDS IN THE CAROLINAS...
..OVERVIEW
TROPICAL CYCLONE IDALIA IS FORECAST TO TRACK NEAR THE CAROLINAS
EARLY IN THE MEDIUM RANGE PERIOD, THEN GRADUALLY PULL AWAY FROM
THE COAST LATER THIS WEEK. SOME LINGERING RAIN AND WIND ON THE
BACKSIDE OF THE SYSTEM COULD AFFECT THE CAROLINAS ON THURSDAY.
SCATTERED STORMS MAY ALSO PERSIST INTO THE WEEKEND ALONG THE GULF
COAST AS A FRONTAL BOUNDARY STALLS ACROSS THE REGION. 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 SOLUTIONS FOR IDALIA REMAIN HIGHLY UNCERTAIN, ESPECIALLY
WITH THE TIMING OF THE SYSTEM. THE ECMWF CONTINUES TO BE THE
FASTEST SOLUTION, WHILE THE GFS IS CONSIDERABLY SLOWER. FOR THIS
FORECAST PACKAGE, A DETERMINISTIC BLEND OF THE GFS/ECMWF/CMC/UKMET
WERE USED TO CREATE A FORECAST THAT IS IN LINE WITH THE NHC
FORECAST 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.
IN TERMS OF THE UPPER LEVEL PATTERN, THE MODEL GUIDANCE IS IN
RELATIVELY GOOD AGREEMENT THROUGHOUT THE MEDIUM RANGE PERIOD. AN
UPPER LOW WILL SLIDE DOWN THE WEST COAST WHILE A RIDGE STRENGTHENS
OVER THE CENTRAL U.S. AND TROUGHING DEVELOPS OVER THE NORTHEAST.
THE CMC HAS COME INTO BETTER AGREEMENT WITH OTHER GUIDANCE ON THE
PATTERN, BUT THE DETERMINISTIC SOLUTION IS STILL A BIT WEST OF THE
CONSENSUS. FOR THIS REASON, THE CMC WAS WEIGHTED SLIGHTLY LESS
THAN THE GFS/ECMWF/UKMET IN THE WPC MODEL BLEND. THE BLEND IS
PURELY DETERMINISTIC THROUGH THE FIRST HALF OF THE PERIOD, AND THE
GEFS AND EC ENSEMBLE MEANS WERE ADDED DURING THE SECOND HALF TO
SMOOTH OUT SUBTLE DIFFERENCES IN THE DETERMINISTIC SOLUTIONS AS
THEY GO OUT IN TIME.
..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.
MINOR CHANGES WERE MADE TO THE DAY 4 ERO WITH ONLY A SLIGHT
EXPANSION AND NORTHWARD SHIFT WITH THE SLIGHT RISK AREA OVER
EASTERN NORTH CAROLINA. THE FORECAST RAINFALL FOR THURSDAY AND
THURSDAY NIGHT HAS A SIGNIFICANT AMOUNT OF UNCERTAINTY, WHICH WILL
ULTIMATELY DEPEND ON THE TRACK AND SPEED. SMALL SHIFTS IN THE
FORECAST COULD HAVE A SIZABLE AFFECT ON THE HEAVY RAINFALL
POTENTIAL.
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 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 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 FORECAST HIGH TEMPERATURES ARE AROUND 10-20 DEGREES
ABOVE AVERAGE.
DOLAN/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
HAZARDS:
- HEAVY RAIN ACROSS PORTIONS OF THE MID-ATLANTIC, THU, AUG 31.
- FLOODING POSSIBLE ACROSS PORTIONS OF FLORIDA AND SOUTHERN
GEORGIA.
- FLOODING LIKELY ACROSS PORTIONS OF LOWER MICHIGAN.
- HAZARDOUS HEAT ACROSS PORTIONS OF SOUTHERN TEXAS, MON, SEP 4.
- HIGH WINDS ACROSS PORTIONS OF COASTAL AREAS OF NORTH AND SOUTH
CAROLINA, THU, AUG 31.
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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