317
FXUS02 KWBC 091902
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
302 PM EDT MON SEP 9 2024
VALID 12Z THU SEP 12 2024 - 12Z MON SEP 16 2024
...HEAVY RAINFALL AND POTENTIAL FLOODING FOR THE LOWER MISSISSIPPI
RIVER VALLEY ON THURSDAY AS TS FRANCINE MOVES INLAND...
..GENERAL OVERVIEW
THE MAIN THING THAT WILL MAKE WEATHER HEADLINES FOR THE END OF
THIS WEEK WILL BE TROPICAL STORM FRANCINE PROGRESSING INLAND ACROSS
LOUISIANA, ARKANSAS, AND MISSISSIPPI. THE RESULT WILL BE HEAVY
RAINFALL THAT COULD PRODUCE FLOODING AND GUSTY WINDS, AND THE RAIN
FROM THIS SYSTEM WILL LIKELY LIFT NORTHWARD TOWARDS THE OHIO VALLEY
AS IT WEAKENS FURTHER BY THE WEEKEND. ELSEWHERE, A PATTERN CHANGE
WILL BE ARRIVING TO THE WEST COAST AND INTERMOUNTAIN WEST AS AN
UPPER TROUGH AMPLIFIES OVER THE REGION AND HERALDS THE ARRIVAL OF
MUCH COOLER TEMPERATURES AND INCREASING RAINFALL CHANCES, COMPARED
TO RECENT CONDITIONS. MEANWHILE, WARM TEMPERATURES ARE EXPECTED
ACROSS MUCH OF THE NORTH-CENTRAL U.S. TO THE GREAT LAKES REGION
WITH AN UPPER LEVEL RIDGE BUILDING IN.
..GUIDANCE/PREDICTABILITY ASSESSMENT
LATEST GUIDANCE CONTINUES TO SHOW GOOD OVERALL AGREEMENT ON
TROPICAL STORM FRANCINE AS IT MAKES LANDFALL AND MOVES NORTHWARD
THROUGH THE LOWER MISSISSIPPI INTO THE MIDDLE MISSISSIPPI VALLEY
LATER THIS WEEK. THE CMC IS A BIT ON THE FASTER SIDE OF THE
GUIDANCE ENVELOPE, SO A BLEND OF THE ECMWF AND GFS SEEMED CLOSEST
TO THE OFFICIAL TRACK FROM THE NATIONAL HURRICANE CENTER FOR BOTH
THE PRESSURE/FRONTS PROGS AND THE QPF. OTHERWISE, LATER IN THE
PERIOD, THERE CONTINUES TO BE TIMING DIFFERENCES WITH THE NEXT
TROUGH INTO THE WEST COAST. THE 06Z GFS WAS SIGNIFICANTLY FASTER
THAN THE REST OF THE GUIDANCE, BUT THE NEW 12Z RUN TODAY DID TREND
SLOWER.
THE WPC PROGS WERE BASED ON A BLEND OF THE DETERMINISTIC MODELS,
WEIGHTED TOWARDS THE ECMWF AND GFS, FOR THE FIRST HALF OF THE
PERIOD. INCORPORATED UP TO 50 PERCENT OF THE ENSEMBLE MEANS FOR THE
LATTER HALF TO MITIGATE THE DIFFERENCES OUT WEST.
..WEATHER/HAZARDS HIGHLIGHTS
THE CORE OF THE HEAVIEST RAINFALL ASSOCIATED WITH A WEAKENING TS
FRANCINE IS FORECAST TO BE ACROSS EASTERN ARKANSAS AND INTO WESTERN
TENNESSEE AND NORTHERN MISSISSIPPI BY THURSDAY, AND THE QPF FROM
THE ECMWF/GFS BEST MATCHED THE NHC TRACK AND IS OVERALL MORE
PROGRESSIVE COMPARED TO EARLIER FORECASTS. THEREFORE, THE SLIGHT
RISK AREA ON DAY 4/THURSDAY HAS BEEN NUDGED NORTHWARD SOME FROM
OVERNIGHT CONTINUITY, AND ALSO EXTENDS EASTWARD OVER WESTERN
ALABAMA WHERE THE INFLOW BAND MAY RESIDE AND RESULT IN LOCALLY
HIGHER RAINFALL TOTALS AS WELL. GIVEN THE RECENT DRY CONDITIONS AND
REMAINING UNCERTAINTY IN THE SWATH OF HEAVIEST QPF, THERE IS NO
NEED FOR ANY MODERATE RISK AREAS FOR THE DAY 4 TIME PERIOD AT THIS
TIME, BUT MAY BE NEEDED FOR FUTURE UPDATES. FOR THE DAY 5/FRIDAY
TIME PERIOD, THE OVERALL QPF DECREASES AS THE LOW LIFTS FARTHER
NORTH AND STEADILY WEAKENS ACROSS THE OHIO VALLEY REGION, WHERE A
MARGINAL RISK IS PLANNED. THIS MARGINAL RISK ALSO INCLUDES THE
SOUTHERN APPALACHIANS WHERE MOIST UPSLOPE FLOW COULD PRODUCE EVEN
HIGHER RAINFALL TOTALS, AND SOUTHWARD TOWARDS THE FLORIDA PANHANDLE
ALONG THE FRONT/INFLOW.
OTHERWISE, THE TROUGH ARRIVING OVER THE WEST WILL RESULT IN
NUMEROUS SHOWERS ACROSS THE NORTHERN ROCKIES. THE HEAVIEST QPF FROM
THIS WILL LIKELY BE OVER NORTHWESTERN MONTANA, WITH SNOW FOR THE
HIGHEST MOUNTAIN RIDGES. CLOSED UPPER LOW PASSAGE AND INCREASED
ENSEMBLE ENHANCED QPF PROBABILITIES SEEMS TO ALSO SUPPORT A
MARGINAL RISK ERO AREA FOR DAY 4/THURSDAY. SHOWERS AND SOME STORMS
THEN REACH THE CENTRAL/NORTHERN PLAINS BY FRIDAY, WHILE THE GREAT
LAKES AND THE NORTHEAST U.S. REMAINS MOSTLY DRY.
TEMPERATURES WILL STILL FEEL MORE LIKE AUGUST ACROSS MUCH OF THE
NORTH-CENTRAL U.S. THROUGH THE END OF THE WEEK AND INTO THE WEEKEND
FOR BOTH HIGHS AND LOWS, WITH POTENTIAL +20 DEGREE ANOMALIES FOR
LOWS ACROSS NORTH DAKOTA AND NORTHERN MINNESOTA BY FRIDAY MORNING.
MEANWHILE ACROSS THE WEST COAST REGION AND THE INTERMOUNTAIN WEST,
A RETURN TO REFRESHINGLY COOLER TEMPERATURES IS EXPECTED BY MIDWEEK
IN RESPONSE TO THE BUILDING UPPER TROUGH, AND FEELING MORE LIKE
OCTOBER. WIDESPREAD HIGHS IN THE 60S CAN BE EXPECTED FOR THE LOWER
ELEVATIONS BEHIND THE COLD FRONT. IT WILL LIKELY BE COOLER THAN
AVERAGE ACROSS PORTIONS OF THE MID-SOUTH FOR DAYTIME HIGHS TO CLOSE
OUT THE WEEK OWING TO INCREASED CLOUDS AND RAINFALL.
SANTORELLI/HAMRICK
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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