961
FXUS02 KWBC 262205
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
605 PM EDT SAT AUG 26 2023
VALID 12Z TUE AUG 29 2023 - 12Z SAT SEP 02 2023
...A TROPICAL SYSTEM IS FORECAST TO IMPACT PORTIONS OF THE
SOUTHEASTERN U.S. WITH HEAVY RAIN LIKELY FROM FLORIDA PANHANDLE
NORTHEASTWARD THROUGH MIDWEEK NEXT WEEK...
..OVERVIEW
A TROPICAL DISTURBANCE CURRENTLY OVER THE NORTHWESTERN CARIBBEAN
SEA IS FORECAST TO DEVELOP INTO A TROPICAL CYCLONE PER THE
NATIONAL HURRICANE CENTER AS THE SYSTEM TRACKS NORTHWARD THROUGH
THE EASTERN GULF OF MEXICO AND INTO FLORIDA AROUND MIDWEEK, LIKELY
WITH A TURN NORTHEASTWARD INTO THE WESTERN ATLANTIC AFTER THAT
WITH POSSIBLY HEAVY RAIN SPREADING INTO COASTAL GEORGIA AND THE
CAROLINAS. HOWEVER, THERE CONTINUES TO BE A FAIR AMOUNT OF SPREAD
IN MODEL GUIDANCE WITH THIS SYSTEM'S TRACK, SO NOTABLE CHANGES TO
THE FORECAST COULD OCCUR. ELSEWHERE, AN UPPER LOW COULD SPREAD
LOCALLY HEAVY RAINFALL INTO THE NORTHWEST TUESDAY-WEDNESDAY, WHILE
THE NEXT SURGE OF MONSOONAL MOISTURE APPEARS TO MOVE UP ACROSS THE
DESERT SOUTHWEST INTO THE FOUR CORNERS STATES LATE NEXT WEEK.
COOLER WEATHER IS LIKELY ACROSS MUCH OF THE COUNTRY AT LEAST
COMPARED TO THE PAST WEEK, BUT TEMPERATURES ARE FORECAST TO
REBOUND TO ABOVE NORMAL IN THE CENTRAL U.S. ESPECIALLY BY THE
LATTER PART OF NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
MODEL GUIDANCE IS GENERALLY IN GOOD AGREEMENT WITH THE
SYNOPTIC-SCALE FLOW ESPECIALLY EARLY IN THE PERIOD, FEATURING AN
UPPER TROUGH DIGGING INTO THE GREAT LAKES TUESDAY-WEDNESDAY BEFORE
LIFTING AND RETREATING INTO THE CANADIAN MARITIMES LATE WEEK, AS
WELL AS AN UPPER LOW MOVING EASTWARD ACROSS THE PACIFIC NORTHWEST
INTO THE NORTHERN PLAINS THROUGH THE MEDIUM-RANGE PERIOD. MODEL
GUIDANCE WAS WITHIN REASON FOR THESE FEATURES, WITH A MULTI-MODEL
DETERMINISTIC BLEND WORKING FINE. BY NEXT WEEKEND, THE
DETERMINISTIC GUIDANCE BEGINS TO DIVERGE NEAR THE WEST COAST WHERE
AN UPPER TROUGH COULD DEVELOP INTO A CLOSED LOW. THE ENSEMBLE
MEANS APPEAR MUCH MORE AGREEABLE WITH THIS FEATURE. THUS, A LARGE
PORTION OF TODAY'S DAY 7 WPC FORECASTS WAS BASED ON A COMPOSITE
BLEND OF THE 06Z GEFS, 00Z EC MEAN, AND THE 00Z CMC MEAN.
THE LARGEST UNCERTAINTY CONTINUES TO BE RELATED TO WHAT COMES OUT
OF THE TROPICAL SYSTEM THAT IS FORECAST TO DEVELOP OVER THE
EASTERN GULF OF MEXICO EARLY NEXT WEEK. SINCE YESTERDAY, THE GFS
HAS BEEN OFFERING AN ALTERNATIVE SCENARIO, TAKING THIS SYSTEM ON A
MORE NORTHERLY TRACK INTO THE INTERIOR SOUTHEASTERN U.S. VERSUS A
TRACK TOWARD THE NORTHEAST ACROSS NORTHERN FLORIDA AND JUST OFF
THE CAROLINA COASTS AS DEPICTED BY THE ECMWF. HOWEVER, RECENT
ECMWF RUNS, AS WELL AS RUNS FROM THE CMC AND ICON, HAVE SHOWN A
WESTWARD TREND IN THE FORECAST TRACK OF THIS SYSTEM OVER THE
EASTERN GULF OF MEXICO, WHICH IS IN CLOSER AGREEMENT WITH THE GFS
SCENARIO. THEREAFTER, THE ECMWF CONTINUES TO TAKE THE SYSTEM ON A
MORE NORTHEASTERLY COURSE TOWARD THE CAROLINA COASTS VERSUS THE
SLOWER AND MORE INLAND TRACK DEPICTED BY THE GFS. IT APPEARS THAT
MODELS CONTINUE TO HAVE NOTABLE DISAGREEMENTS ON HOW TO JUGGLE
BETWEEN AN UPPER LOW CURRENTLY NEAR THE CENTRAL GULF COAST, THE
LIFTING EAST COAST TROUGH, AND THEIR MUTUAL INTERACTION WITH THE
TROPICAL SYSTEM. THE WPC MEDIUM-RANGE FORECAST PACKAGE FOR DAYS 3
- 5 WAS BASED ON A COMPOSITE BLEND OF 40% FROM THE 00Z ECMWF/EC
MEAN, 40% FROM THE 06Z GFS/GEFS, 20% FROM THE 00Z CMC/CMC MEAN.
THIS BLEND YIELDED A FORECAST TRACK FOR THE TROPICAL SYSTEM
SLIGHTLY MORE TO THE WEST ACROSS THE EASTERN GULF OF MEXICO THAN
IN PREVIOUS FORECASTS, FOLLOWED BY A SIMILAR FORECAST TRACK NEAR
THE CAROLINA COASTS THEREAFTER. FORECASTS FOR THE REMAINDER OF
THE COUNTRY ARE SIMILAR TO WPC CONTINUITY.
..WEATHER/HAZARDS HIGHLIGHTS
THE TROPICAL CYCLONE THAT IS LIKELY TO FORM IS FORECAST TO MOVE
THROUGH THE EASTERN GULF OF MEXICO AND INTO FLORIDA
TUESDAY-WEDNESDAY, WITH THE MOST LIKELY TRACK OF THE SYSTEM
NORTHEASTWARD ACROSS FLORIDA AND INTO THE WESTERN ATLANTIC BY
THURSDAY NEAR THE GEORGIA/CAROLINA COAST. SLIGHT RISKS OF
EXCESSIVE RAINFALL ARE IN PLACE FOR THESE AREAS FOR THE DAYS 4-5
EROS AS A STARTING POINT. THE FORWARD SPEED OF, AND HOW FAR INLAND
THE SYSTEM EVENTUALLY TRACKS, WILL DETERMINE THE FLASH FLOOD
POTENTIAL OVER THE AFFECTED LAND AREAS. OBVIOUSLY, A SLOWER AND
MORE INLAND TRACK, AS DEPICTED BY THE RECENT MODEL TREND, WOULD
RESULT IN HEAVIER RAINFALL AMOUNTS. BUT REGARDLESS THE HEAVY
TROPICAL RAINFALL WITH HIGH RAIN RATES COULD CAUSE FLASH FLOODING
EVEN IF THE SYSTEM IS MOVING QUICKLY. IN ADDITION, TROPICAL
DOWNPOURS AND SQUALLS FROM THE OUTER RAINBANDS OF THE SYSTEM MAY
POSE SOME FLASH FLOODING THREAT. THESE ARE CERTAINLY SUBJECT TO
CHANGE AS FORECASTS FOR THE TROPICAL SYSTEM ARE REFINED. A COLD
FRONT LOCATED WEST OF THE TROPICAL LOW COULD HELP ELONGATE THE
FOOTPRINT OF THE HEAVY RAINFALL.
FARTHER NORTH INTO THE MID-ATLANTIC AND NORTHEAST, SOME RAIN AND
STORMS ARE POSSIBLE TUESDAY-WEDNESDAY WITH A COUPLE OF FRONTAL
SYSTEMS, BEFORE A SURFACE HIGH SNEAKS IN FOR A DRYING TREND.
FRONTS ACROSS THE GULF COAST WILL PROVIDE A FOCUS FOR MOISTURE TO
POOL, AND SCATTERED THUNDERSTORMS ARE LIKELY THERE THROUGH MUCH OF
THE WEEK.
IN THE NORTHWEST, THE STRONG UPPER LOW COMING THROUGH ON
TUESDAY-WEDNESDAY WILL PROVIDE GOOD FORCING FOR RAINFALL THERE.
THERE DOES LOOK TO BE SOME INSTABILITY IN PLACE, SO LOCALIZED RAIN
AMOUNTS/RATES MAY BE HIGH ENOUGH TO CAUSE A FEW INSTANCES OF FLASH
FLOODING, WHILE RAIN TOTALS ARE LIKELY TO EXCEED AT LEAST LOW (1
YEAR) AVERAGE RECURRENCE INTERVALS. MARGINAL RISKS ARE IN PLACE IN
THE EROS WITH THIS ACTIVITY. AFTER A RELATIVE LULL IN THE MONSOON
AROUND MIDWEEK, INCREASING CONVECTION APPEARS LIKELY BY LATE WEEK
INTO ARIZONA AND THE SURROUNDING STATES.
FINALLY SOME RELIEF OF THE HEAT IS IN STORE FOR THE GULF COAST
REGION BY TUESDAY AS CONSIDERABLY LOWER HEAT INDICES ARE FORECAST
AS A FRONT OR TWO COME THROUGH. THE DESERT SOUTHWEST COULD SEE
ANOTHER HOT DAY ON TUESDAY THOUGH, WITH HIGHS IN THE 110S, WHICH
SHOULD ABATE SOME BY WEDNESDAY AND BEYOND. MULTIPLE FRONTS
CROSSING THE EAST WILL KEEP TEMPERATURES SOMEWHAT COOLER THAN
AVERAGE FOR MUCH OF NEXT WEEK. THE SAME WILL BE TRUE FOR THE
NORTHWEST WITH THE UPPER LOW OVERHEAD. THE CENTRAL U.S. WILL
LIKELY START OFF SOMEWHAT COOLER THAN AVERAGE BUT WITH A WARMING
TREND IN STORE THROUGH THE WEEK. TEMPERATURES ARE FORECAST TO RISE
INTO THE 90S INTO THE NORTH-CENTRAL PLAINS AND MIDWEST, AROUND
10-15 DEGREES ABOVE AVERAGE.
KONG/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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