230
FXUS02 KWBC 241854
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
254 PM EDT TUE SEP 24 2024
VALID 12Z FRI SEP 27 2024 - 12Z TUE OCT 01 2024
...HELENE TO BRING IMPACTFUL HEAVY RAIN, HIGH WIND, AND COASTAL
FLOODING THREAT TO THE CENTRAL TO EASTERN GULF COAST/SOUTHEAST
AHEAD/WITH LANDFALL AS A MAJOR HURRICANE ON THURSDAY...
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE MAIN FORECAST STORY IS THE THREAT AND IMPACTS ASSOCIATED WITH
TROPICAL STORM HELENE. RECENT GUIDANCE HAS COME INTO BETTER
AGREEMENT IN DEPICTING A SPLIT FLOW PATTERN WITH AN UPPER- TROUGH
LIFTING NORTHWARD OVER THE NORTHEAST AS A CUT-OFF UPPER-LOW DROPS
SOUTHWARD OVER THE MIDDLE MISSISSIPPI VALLEY/OZARKS
REGION. THIS UPPER-LEVEL LOW THEN LINGERS IN THE GENERAL VICINITY
INTO THE WEEKEND AS UPPER-RIDGING MOVES IN TO THE NORTH OVER THE
GREAT LAKES/SOUTHEASTERN CANADA. AT THE SAME TIME, THE NHC FORECAST
FOR HELENE BRINGS THE SYSTEM NORTHWARD ACROSS THE FLORIDA
PANHANDLE REGION AS A MAJOR HURRICANE AND INTO THE SOUTHEAST AS IT
UNDERGOES EXTRATROPICAL TRANSITION THURSDAY INTO FRIDAY. THE 06Z
GFS/00Z ECMWF MADE FOR A NICE COMPROMISE BETWEEN THE MORE SOUTHWEST
UKMET AND MORE NORTHEAST CANADIAN MODEL WITH THE COLD LOW WHICH
HELENE OCCLUDES BACK INTO, SO THE PRESSURES, FRONTS, QPF, AND WINDS
WERE WEIGHTED TOWARDS THOSE PIECES OF GUIDANCE. THE REMAINDER OF
THE GRIDS STARTED WITH THE 13Z NBM WHICH WAS WEIGHTED MORE TOWARDS
THE 06Z GFS/00Z ECMWF SOLUTIONS.
..WEATHER/HAZARDS HIGHLIGHTS
HELEN MOVES NORTHWARD INTO THE GULF OF MEXICO THREATENING MARITIME
INTERESTS AND THE CENTRAL TO EASTERN GULF COAST/SOUTHEAST MID-LATE
WEEK, WITH THE SYSTEM SET TO MOVE INLAND THURSDAY INTO FRIDAY.
CONFIDENCE IS GROWING IN A LARGE SYSTEM, DUE TO ITS INTERACTION
WITH A COLD LOW TO ITS NORTHWEST AND WEST, THAT WILL BRING
SIGNIFICANT RAINFALL/WINDS/SURF/COASTAL FLOODING FOR THE CENTRAL TO
EASTERN GULF COAST/FLORIDA, WITH IMPACTFUL WEATHER POSSIBLE
THROUGH THE SOUTHEAST, AND WRAPPING BACK INTO THE MID-SOUTH LATE
WEEK INTO THE WEEKEND. THE DAY 4/FRIDAY ERO PERIOD FEATURES ITS
ANOMALOUS MOISTURE FEED INTO THE SOUTHERN APPALACHIANS AS WELL AS
OVER THE OZARK REGION AS HELENE OCCLUDES UNDER THE COLD LOW, WITH
BOTH REGIONS COVERED WITH SLIGHT RISKS OUTLOOK THREAT AREAS. A
SIMILAR AREA DEPICTED FOR THE DAY 5/SATURDAY ERO SHOWS A SCALING
DOWN ERO VERSION AS HELENE BROADENS AND WEAKENS. HOWEVER, AMPLE
LINGERING DEEP TROPICAL MOISTURE WILL REMAIN TO FUEL THE ELONGATED
MARGINAL THREAT AREA. MORE CERTAIN LOCAL FOCUS CLOSER TO OCCURRENCE
WOULD LIKELY LEAD TO SLIGHT RISK ISSUANCE UPGRADES, ESPECIALLY
GIVEN EARLIER PERIOD HEAVY RAINFALL WILL CREATE SATURATED SOILS. A
MODERATE RISK WAS CONSIDERED FOR PORTIONS OF THE APPALACHIANS FOR
THE DAY 4 ERO, BUT THE MAIN DRAWBACKS SHIFTS IN THE MODEL GUIDANCE
WITH HELENE'S TRACK AND CONCERN THAT HELENE ACCELERATES FASTER THAN
THE GUIDANCE CURRENTLY ADVERTISES. LEFT THE OVERNIGHT ERO LEVELS
AS THEY WERE.
THE WASHINGTON COASTAL RANGES AND CASCADES WILL SEE ENHANCED RAIN
LATE WEEK WITH PACIFIC SYSTEM PASSAGES EARLY TO MID NEXT WEEK, BUT
THE REMAINDER OF THE WEST AND MOST OF THE PLAINS WILL BE DRY.
WARMER THAN AVERAGE TEMPERATURES ARE EXPECTED TO TRANSLATE FROM THE
NORTHERN PLAINS EAST INTO THE MIDWEST LATER THIS WEEK INTO THE
WEEKEND FOLLOWING UPPER-LEVEL RIDGING, WITH ANOMALIES DECREASING BY
EARLY NEXT WEEK. COOLER THAN AVERAGE TEMPERATURES ARE EXPECTED
CENTERED AROUND THE OZARKS REGION UNDER THE INFLUENCE OF POST-
TROPICAL CYCLONE HELENE, ALSO RETURNING CLOSER TO AVERAGE BY EARLY
NEXT WEEK AS THE UPPER- LOW DISSIPATES.
ROTH/SCHICHTEL
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 (QPF), EXCESSIVE RAINFALL
OUTLOOK (ERO), WINTER WEATHER OUTLOOK (WWO) PROBABILITIES, HEAT
INDICES, AND KEY MESSAGES CAN BE ACCESSED FROM:
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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