561
FXUS02 KWBC 271900
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 PM EDT SUN AUG 27 2023
VALID 12Z WED AUG 30 2023 - 12Z SUN SEP 03 2023
...TROPICAL STORM IDALIA IS FORECAST TO IMPACT PORTIONS OF FLORIDA
NORTHEASTWARD INTO GEORGIA AND THE CAROLINAS WEDNESDAY-THURSDAY
WITH HEAVY RAIN AND HIGH WINDS...
..OVERVIEW
THIS MORNING THE NATIONAL HURRICANE CENTER UPGRADED TROPICAL
DEPRESSION TEN TO TROPICAL STORM IDALIA. THE NHC FORECAST FROM THE
15Z ADVISORY SHOWS IDALIA MOVING NORTHWARD THROUGH THE EASTERN
GULF OF MEXICO DURING THE SHORT RANGE PERIOD AND STRENGTHENING,
REACHING NEAR THE FLORIDA BIG BEND BY EARLY WEDNESDAY AS A
HURRICANE. THE SYSTEM WILL LIKELY TAKE A NORTHEASTERLY TRACK AFTER
THAT, SPREADING HEAVY RAINFALL FROM FLORIDA INTO GEORGIA AND THE
CAROLINAS, AND THEN CONTINUE EASTWARD OVER THE ATLANTIC AWAY FROM
NORTH CAROLINA. RECENT MODEL SPREAD FOR THIS SYSTEM'S EXACT TRACK
OFFERS POTENTIAL FOR NOTABLE FORECAST CHANGES BUT AT LEAST THE
NEWEST MODEL RUNS ARE STARTING TO CLUSTER A LITTLE BETTER FOR THE
TRACK. THE UPPER PATTERN FARTHER WEST SHOULD BECOME MORE AMPLIFIED
BY LATE WEEK AND NEXT WEEKEND AS AMPLIFYING WEST COAST TROUGH
ENERGY CLOSES OFF AN UPPER LOW AND SOUTHWEST U.S. INTO CENTRAL
PLAINS RIDGING REBUILDS MORE OVER THE CENTRAL U.S./ROCKIES. A
SYSTEM INITIALLY EJECTING FROM THE NORTHWEST MAY PRODUCE LINGERING
HEAVY RAIN NEAR THE CANADIAN BORDER INTO WEDNESDAY, WHILE THE
UPPER LOW DEVELOPING NEAR THE WEST COAST MAY THEN LEAD TO SOME
RAINFALL TO ITS EAST. 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 WILL LIKELY REBOUND TO ABOVE NORMAL IN THE CENTRAL
U.S. ESPECIALLY BY LATE WEEK INTO NEXT WEEKEND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THROUGH LATE THIS WEEK THE GUIDANCE AGREES FAIRLY WELL FOR MOST
ASPECTS OF THE LARGE SCALE PATTERN EVOLUTION AND SYNOPTIC
FEATURES. THE 12Z CMC IS SLOW TO EJECT INITIAL ENERGY OUT OF THE
NORTHWEST DUE TO GREATER EMPHASIS ON A TRAILING FEATURE VERSUS A
MORE CONSOLIDATED EASTERN UPPER LOW IN MOST OTHER SOLUTIONS.
NON-CMC RUNS EXHIBIT TYPICAL TIMING SPREAD FOR THIS FEATURE THAT
SHOULD OPEN UP AS IT CONTINUES ACROSS SOUTHERN CANADA. REGARDING
THE AMPLIFYING UPPER TROUGH/EMBEDDED LOW ALONG THE WEST COAST,
LATEST DEEPER TRENDS IN THE ENSEMBLE MEANS OFFER IMPROVED
CONFIDENCE IN THE OPERATIONAL MODEL CONSENSUS WHILE THE LATEST
GEFS/ECMWF MEANS SEEM TO FAVOR LEANING AWAY FROM SOLUTIONS THAT
ARE ON THE EASTERN PART OF THE SPREAD BY THE END OF THE PERIOD
(12Z CMC AND TO SOME DEGREE 06Z GFS). BY NEXT WEEKEND THERE IS
GREATER SPREAD ACROSS CANADA WHERE FLOW IS FORECAST TO BECOME
INCREASINGLY PROGRESSIVE ON AVERAGE, BUT LATEST GFS RUNS HOLD ONTO
MORE AMPLIFIED RIDGING FROM THE NORTHERN PLAINS INTO CANADA. GEFS
MEAN RUNS HINT AT SOME RIDGING BUT ARE FLATTER OVER CANADA.
NEW 12Z MODELS ARRIVING SO FAR ARE FINALLY IMPROVING THE TRACK
CLUSTERING FOR IDALIA FOR THE TIME BEING, CLOSER TO THE OFFICIAL
NATIONAL HURRICANE CENTER FORECAST. NOW THE GREATER SPREAD APPEARS
TO BE FOR TIMING VERSUS TRACK. MUCH FASTER TREND OF THE NEW 12Z
ECMWF VERSUS THE PRIOR RUN HIGHLIGHTS THIS UNCERTAINTY. LOOKING
BACK AT RECENT RUNS FOR TRACK, THE GFS HAD BEEN ON THE WESTERN
SIDE OF THE ENVELOPE AFTER LANDFALL, CMC RUNS HAVE BEEN FAST (00Z
UKMET QUITE FAST AS WELL), 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. IT APPEARS
THAT THESE DIFFERENCES ARE RELATED IN PART TO DISAGREEMENTS ON
EXACTLY HOW AN UPPER LOW/TROUGH OVER THE GULF AND GULF COAST
STATES AND A SEPARATE UPPER TROUGH LIFTING AWAY FROM THE
NORTHEAST/MID-ATLANTIC MAY INTERACT WITH THE TROPICAL SYSTEM.
THE WPC FORECAST STARTED WITH A DETERMINISTIC MODEL BLEND EARLY IN
THE FORECAST PERIOD. THE FORECAST GRADUALLY INCREASED THE
PROPORTION OF THE ENSEMBLE MEANS TO 40 PERCENT BY LATE IN THE
PERIOD WITH LINGERING INPUT FROM THE 00Z ECMWF/00Z-06Z GFS/00Z CMC
(IN ORDER OF MORE TO LESS WEIGHT).
..WEATHER/HAZARDS HIGHLIGHTS
TROPICAL STORM IDALIA 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. PLANNED UPDATES FOR THE DAYS 4-5 EROS ARE FAIRLY MODEST,
MAINLY ACCOUNTING FOR LATEST GUIDANCE TRENDS TRIMMING THE
POTENTIAL WESTERN EXTENT OF RAINFALL (ESPECIALLY BY DAY 5) WHILE
INCLUDING A LITTLE MORE OF THE WESTERN COAST OF THE NORTHERN
FLORIDA PENINSULA IN THE SLIGHT RISK AREA ON DAY 4 TO ACCOUNT FOR
THE POTENTIAL OF LOCALLY HEAVY RAINFALL IN SOUTHWESTERLY FLOW, AS
SOME GUIDANCE IS SIGNALING.
IN THE NORTHWEST, HEAVY RAIN COULD CONTINUE INTO WEDNESDAY ACROSS
PARTS OF THE NORTHERN ROCKIES CLOSE TO THE CANADIAN BORDER AS THE
STRONG UPPER LOW OVERHEAD PROVIDES GOOD FORCING FOR RAINFALL WHILE
SOME INSTABILITY MAY LINGER. SOME LIGHT TO MODERATE RAIN IS
POSSIBLE OVER NORTHERN CALIFORNIA AND THE PACIFIC NORTHWEST WITH
AN AMPLIFYING UPPER TROUGH/EMBEDDED CLOSED LOW EXPECTED TO SETTLE
ALONG THE WEST COAST BY LATE WEEK THROUGH THE WEEKEND. 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 SUPPORT
A MARGINAL RISK IN THE ERO FOR DAY 5/THURSDAY WITH THIS ACTIVITY.
MOST OF THE CURRENT 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 COOLER THAN AVERAGE 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. THESE HIGHS WOULD BE
AROUND 10-15 DEGREES ABOVE AVERAGE AND PERHAPS LOCALLY HIGHER OVER
THE CENTRAL PLAINS.
RAUSCH/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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