066
FXUS02 KWBC 260727
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
326 AM EDT SAT AUG 26 2023
VALID 12Z TUE AUG 29 2023 - 12Z SAT SEP 02 2023
...THREAT OF A TROPICAL SYSTEM INCREASING FOR THE EASTERN GULF OF
MEXICO REGION MIDWEEK WITH HEAVY RAIN LIKELY ACROSS FLORIDA
NORTHEASTWARD...
..OVERVIEW
A TROPICAL DISTURBANCE CURRENTLY OVER THE NORTHWESTERN CARIBBEAN
SEA HAS A HIGH CHANCE OF FORMING INTO A TROPICAL CYCLONE PER THE
NATIONAL HURRICANE CENTER. THE SYSTEM IS FORECAST TO TRACK
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 IS 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 COULD INCREASE 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
TROUGHING ACROSS THE EAST THROUGH TUESDAY-WEDNESDAY BEFORE
SHIFTING AWAY LATE WEEK, AS WELL AS AN UPPER LOW MOVING INTO THE
PACIFIC NORTHWEST EASTWARD. MODEL GUIDANCE WAS WITHIN REASON FOR
THESE FEATURES, WITH A MULTI-MODEL DETERMINISTIC BLEND WORKING
FINE. THEN ADDITIONAL ENERGY UPSTREAM SPILLING INTO THE WEST COAST
BY LATE WEEK DOES SHOW SOME MORE NOTABLE MODEL DIFFERENCES. MOST
MODELS FROM THE 12/18Z CYCLE INDICATED AN UPPER LOW CLOSING OFF
ATOP THE WEST COAST, BUT THE 12Z CMC TOOK MORE PHASED TROUGHING
INLAND MORE QUICKLY. THIS SEEMED LIKE AN OUTLIER AND WAS NOT
FAVORED, BUT THE NEWER 00Z ECMWF SHOWS SOME HINTS OF THAT (THOUGH
IT DOES TAKE SOME ENERGY INTO THE PACIFIC FARTHER SOUTH OF OTHER
GUIDANCE). THESE TYPES OF SOLUTIONS CANNOT BE FULLY DISCOUNTED BUT
AT THE TIME OF THE FORECAST CREATION, FAVORED A BLEND OF THE 12Z
ECMWF/18Z GFS AS WELL AS THE GEFS AND EC ENSEMBLE MEANS FOR THE
LATTER PART OF THE WEEK FOR THE MIDLATITUDE FLOW.
THE MOST CONCERNING MODEL DIAGNOSTICS FEATURE REMAINS THE TROPICAL
CYCLONE THAT IS LIKELY TO FORM. NON-NCEP GUIDANCE HAS BEEN
REASONABLY PERSISTENT WITH A LOW TRACK NEARING THE BIG BEND OF
FLORIDA WEDNESDAY AND TURNING NORTHEAST INTO THE WESTERN ATLANTIC.
THE MAIN DIFFERENCES HAVE BEEN WITH THE SPEED OF THE SYSTEM (E.G.
CMC RUNS SHOW IT MOVING FASTER THAN THE ECMWF AND UKMET). HOWEVER,
GFS RUNS HAVE BEEN INCREDIBLY DIFFERENT. THE GFS HAS FINALLY
LATCHED ON TO HAVING A TROPICAL SYSTEM, BUT THE PAST FEW RUNS HAVE
ALL TRACKED IT WELL TO THE WEST SOMEWHERE ACROSS THE INTERIOR
SOUTHEAST/TENNESSEE VALLEY INSTEAD. WITH A COLD FRONT JUST TO THE
WEST AND A TROUGH AXIS, GFS-TYPE TRACKS DO NOT SEEM AS LIKELY, AND
THE FORECAST CONTINUES TO FAVOR THE NON-NCEP GUIDANCE INSTEAD,
ESPECIALLY THE ECMWF. THUS SIGNIFICANT CHANGES WERE NEEDED TO THE
QPF COMPARED TO THE NBM, AS THE NBM SHOWED AMPLE QPF IN THE
INTERIOR REGIONS LIKELY FROM GFS INFLUENCES. HEAVILY RELIED ON THE
ECMWF FOR THE OVERNIGHT QPF.
..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 THE SYSTEM
HOPEFULLY WILL PUT SOME LIMIT ON THE FLASH FLOOD POTENTIAL, 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 FROM THE OUTSKIRT 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.
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page