492
FXUS02 KWBC 270719
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
318 AM EDT SUN AUG 27 2023
VALID 12Z WED AUG 30 2023 - 12Z SUN SEP 03 2023
...WHAT IS CURRENTLY TROPICAL DEPRESSION TEN IS FORECAST TO IMPACT
PORTIONS OF FLORIDA NORTHEASTWARD INTO GEORGIA AND THE CAROLINAS
WEDNESDAY-THURSDAY WITH HEAVY RAIN AND HIGH WINDS...
..OVERVIEW
TROPICAL DEPRESSION TEN IS FORECAST TO MOVE NORTHWARD THROUGH THE
EASTERN GULF OF MEXICO DURING THE SHORT RANGE PERIOD AND
STRENGTHEN PER THE NATIONAL HURRICANE CENTER'S FORECAST, REACHING
NEAR THE FLORIDA BIG BEND BY EARLY WEDNESDAY POTENTIALLY AS A
HURRICANE. THE SYSTEM IS LIKELY TO TAKE A NORTHEASTERLY TRACK
AFTER THAT, SPREADING HEAVY RAINFALL FROM FLORIDA INTO GEORGIA AND
THE CAROLINAS. HOWEVER, THERE CONTINUES TO BE A FAIR AMOUNT OF
SPREAD IN MODEL GUIDANCE WITH THIS SYSTEM'S EXACT TRACK, SO
NOTABLE CHANGES TO THE FORECAST COULD OCCUR. ELSEWHERE, THE
NORTHWEST COULD SEE LINGERING HEAVY RAIN INTO WEDNESDAY, WHILE THE
NEXT SURGE OF MONSOONAL MOISTURE APPEARS TO MOVE UP ACROSS THE
DESERT SOUTHWEST INTO THE FOUR CORNERS STATES AROUND
THURSDAY-FRIDAY. INITIALLY COOLER WEATHER IS LIKELY ACROSS MUCH OF
THE COUNTRY AT LEAST COMPARED TO LAST WEEK, BUT TEMPERATURES ARE
FORECAST TO REBOUND TO ABOVE NORMAL IN THE CENTRAL U.S. ESPECIALLY
BY LATE WEEK INTO NEXT WEEKEND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE IS GENERALLY IN GOOD AGREEMENT WITH THE
SYNOPTIC-SCALE FLOW IN THE MIDLATITUDES ESPECIALLY EARLY IN THE
PERIOD. THE MIDWEEK PATTERN FEATURES AN UPPER TROUGH ATOP THE
NORTHEASTERN U.S. BEFORE LIFTING INTO THE CANADIAN MARITIMES LATE
WEEK, AS WELL AS AN UPPER LOW MOVING EASTWARD FROM THE NORTHWEST
ACROSS THE NORTH-CENTRAL U.S./SOUTH-CENTRAL CANADA BORDER. THE
LATTER FEATURE SHOWS A BIT MORE SPREAD, WITH THE 12Z ECMWF A BIT
SLOWER THAN OTHER 12/18Z GUIDANCE, WHILE ON THE WHOLE THE NEWER
00Z GUIDANCE ENDS UP WEAKER AND MORE PROGRESSIVE WITH THIS FEATURE
BY THURSDAY-FRIDAY. ADDITIONAL ENERGY LOOKS TO DIG THROUGH THE
EASTERN PACIFIC LATE IN THE WEEK AND APPEARS LIKELY TO FORM A
CLOSED LOW NEAR OR OVER THE WEST COAST. THERE IS SOME SPREAD WITH
THE POSITION OF THIS LOW MEANDERING LATE IN THE WEEK, BUT THE ONLY
REAL OUTLIER SEEMED TO BE THE 12Z CMC THAT EJECTS THE LOW INTO THE
NORTHWEST AND PHASES IT WITH THE NORTHERN STREAM BY NEXT SUNDAY.
THE 00Z CMC WAS MORE IN LINE WITH THE GUIDANCE CONSENSUS IN
KEEPING THE FEATURE HELD BACK WEST.
THE LARGEST UNCERTAINTY CONTINUES TO BE RELATED TO THE TRACK AND
TIMING OF THE TROPICAL CYCLONE (CURRENTLY T.D. TEN), WITH SPREAD
IN SOLUTIONS EVIDENT IN THE ENSEMBLE MEMBERS AND DETERMINISTIC
MODELS. IN GENERAL OVER THE PAST COUPLE OF DAYS, THE GFS RUNS HAVE
BEEN ON THE WESTERN SIDE OF THE ENVELOPE AFTER LANDFALL, CMC RUNS
HAVE BEEN FAST, AND THE ECMWF HAS BEEN GENERALLY FAVORED AS IT HAS
BEEN PERSISTENT--THOUGH WITH A GRADUAL WESTWARD TREND OVER THE
PAST FEW RUNS FOR BETTER ODDS FOR A SYSTEM ONSHORE NEAR THE
WESTERN ATLANTIC COAST RATHER THAN OFFSHORE. THERE WAS SOME
CONVERGENCE OF MODELS IN THE 12/18Z CYCLE, ESPECIALLY AS THE 18Z
GFS HOPPED TO THE EAST TO BE IN BETTER ALIGNMENT WITH THE ECMWF.
THE 00Z GFS DID SHIFT BACK WEST A BIT THOUGH, WHILE THE 00Z ECMWF
CONTINUES A SLIGHT TREND WEST AND MORE ONSHORE. MODEL SPREAD IS
ALSO EVIDENT IN THE TIMING OF THE SYSTEM. MOST 12/18Z MODEL
GUIDANCE WAS SLOWER THAN THE NATIONAL HURRICANE CENTER'S TRACK
FROM THE 03Z ADVISORY, BUT THEIR ADVISORY WAS VERY SIMILAR TO
THEIR PREVIOUS ONE. SO THE TIMING IS ANOTHER ASPECT THAT WILL
CONTINUE TO BE MONITORED. IT APPEARS THAT THESE DIFFERENCES ARE
RELATED IN PART TO DISAGREEMENTS ON HOW TO JUGGLE BETWEEN AN UPPER
LOW/TROUGH OVER THE GULF AND GULF COAST STATES AND THE LIFTING
EAST COAST TROUGH, AND THEIR MUTUAL INTERACTION WITH THE TROPICAL
SYSTEM.
THE WPC FORECAST WAS BASED ON A MOSTLY DETERMINISTIC MODEL BLEND
EARLY IN THE FORECAST PERIOD. GRADUALLY INCREASED THE PROPORTION
OF THE ENSEMBLE MEANS AS THE PERIOD PROGRESSED TO AROUND HALF BY
DAYS 6-7, AND PHASED OUT THE CMC GIVEN ITS DIFFERENCES ACROSS THE
WEST. THE FORECAST FOR THE TROPICAL CYCLONE FOLLOWED THE NHC
ADVISORY.
..WEATHER/HAZARDS HIGHLIGHTS
CURRENT TROPICAL DEPRESSION TEN (WHICH IS LIKELY TO STRENGTHEN) IS
FORECAST TO MOVE INTO FLORIDA AROUND THE BIG BEND AREA BY
WEDNESDAY, WITH THE FORECAST TRACK OF THE SYSTEM NORTHEASTWARD
ACROSS THE ATLANTIC COASTLINES OF GEORGIA AND THE CAROLINAS.
SLIGHT RISKS OF EXCESSIVE RAINFALL ARE IN PLACE FOR THESE AREAS
FOR THE DAYS 4-5 EROS ON WEDNESDAY AND THURSDAY. THE FORWARD SPEED
OF THE SYSTEM AND HOW FAR INLAND IT TRACKS WILL DETERMINE THE
FLASH FLOOD POTENTIAL OVER THE AFFECTED LAND AREAS. BUT
REGARDLESS, THE HEAVY TROPICAL RAINFALL WITH HIGH RAIN RATES COULD
CAUSE FLASH FLOODING EVEN IF THE SYSTEM IS MOVING QUICKLY. A COLD
FRONT LOCATED WEST OF THE TROPICAL CYCLONE COULD HELP ELONGATE THE
FOOTPRINT OF THE HEAVY RAINFALL. WEDNESDAY (DAY 4 IN TERMS OF ERO
TIMING) MAY SEE THE HEAVIEST RAIN TOTALS FROM THE TROPICAL
CYCLONE. BUT WITH CONTINUED SPREAD IN MODEL GUIDANCE AS TO THE
EXACT TRACK, PLUS THE CURRENTLY FORECAST HEAVIEST TOTALS OVER
EASTERN GEORGIA AND SOUTHERN SOUTH CAROLINA NOT REALLY OVERLAPPING
HEAVY RAIN FORECASTS IN THE SHORT RANGE, HELD OFF ON ANY MODERATE
RISK AT THIS POINT. BUT THE ERO RISK LEVEL ON WEDNESDAY IS LIKELY
TO BE MORE ENHANCED THAN SOME SLIGHT RISKS. THE POSITIONS OF THE
RISK AREAS ARE ALSO SUBJECT TO CHANGE AS FORECASTS FOR THE
TROPICAL SYSTEM ARE REFINED. IN ADDITION, TROPICAL DOWNPOURS AND
SQUALLS FROM THE OUTER RAINBANDS OF THE SYSTEM MAY POSE SOME FLASH
FLOODING THREAT FARTHER SOUTH INTO FLORIDA FOR EXAMPLE.
ADDITIONALLY, FRONTS ACROSS THE GULF COAST WILL PROVIDE A FOCUS
FOR MOISTURE TO POOL, AND SCATTERED THUNDERSTORMS ARE LIKELY THERE
THROUGH MUCH OF THE WEEK INTO NEXT WEEKEND.
IN THE NORTHWEST, HEAVY RAIN COULD CONTINUE INTO WEDNESDAY ACROSS
PARTS OF THE NORTHERN ROCKIES AS THE STRONG UPPER LOW OVERHEAD
PROVIDES GOOD FORCING FOR RAINFALL WHILE SOME INSTABILITY MAY
LINGER. LIGHT RAIN IS ALSO POSSIBLE INTO THE PACIFIC NORTHWEST
WITH ADDITIONAL HEIGHT FALLS THROUGH THE LATTER HALF OF THE WEEK.
FARTHER SOUTH, AFTER A RELATIVE LULL IN THE MONSOON AROUND
MIDWEEK, INCREASING CONVECTION APPEARS LIKELY BY LATE WEEK INTO
ARIZONA AND THE SURROUNDING STATES. PLENTY OF INSTABILITY IN PLACE
AND ABOVE AVERAGE MOISTURE LEVELS AT LEAST INTO THE DESERT
SOUTHWEST SEEMS SUFFICIENT TO HAVE A MARGINAL RISK IN THE ERO FOR
DAY 5/THURSDAY WITH THIS ACTIVITY.
MOST OF THE EXCESSIVE HEAT CONCERNS LOOK TO ABATE BY WEDNESDAY.
THE WESTERN GULF COAST MAY SEE HIGHS A FEW DEGREES ABOVE AVERAGE,
BUT HEAT INDICES WILL BE CONSIDERABLY LOWER AFTER A FRONT OR TWO
COME THROUGH. THE DESERT SOUTHWEST COULD SEE HIGHS IN THE LOW 110S
FOR ONE MORE DAY ON WEDNESDAY BEFORE A MODERATING TREND LATER IN
THE WEEK. MEANWHILE THE NORTHWEST AND THE EAST SHOULD GENERALLY BE
LOWER THAN AVERAGE IN TERMS OF TEMPERATURES WEDNESDAY-FRIDAY,
WARMING A BIT TO AROUND NORMAL IN THE EAST BY NEXT WEEKEND, WHILE
BELOW AVERAGE HIGHS DRIFT SOUTH A BIT FROM THE NORTHWEST INTO
CALIFORNIA AND THE CENTRAL GREAT BASIN. HOWEVER, THE CENTRAL U.S.
IS LIKELY TO SEE HEAT BUILDING GRADUALLY THROUGH LATE THIS WEEK
AND INTO NEXT WEEKEND. HIGHS WELL INTO THE 90S WILL EXPAND INTO
THE NORTH-CENTRAL PLAINS AND MIDWEST, WHILE HIGHS EXCEEDING 100
ARE LIKELY FOR THE SOUTHERN HALF OF THE PLAINS, AROUND 10-15
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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