732
FXUS02 KWBC 111859
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 PM EDT WED SEP 11 2024
VALID 12Z SAT SEP 14 2024 - 12Z WED SEP 18 2024
..GENERAL OVERVIEW
THE REMNANT LOW ASSOCIATED WITH WHAT IS NOW HURRICANE FRANCINE WILL
CONTINUE TO DISSIPATE ACROSS THE OHIO VALLEY OVER THE WEEKEND,
THOUGH WITH SOME LINGERING POTENTIAL FOR LOCALLY HEAVY RAINFALL TO
THE SOUTHEAST. MEANWHILE, A CLOSED UPPER HIGH EXPECTED TO DRIFT
OVER THE GREAT LAKES/NORTHEAST WILL PROMOTE ABOVE AVERAGE
TEMPERATURES NEAR ITS PATH INTO EARLY NEXT WEEK. CONSOLIDATING
ENERGY ALOFT SOUTH OF THE UPPER RIDGE MAY INTERACT WITH AN INITIAL
FRONTAL WAVE OFF THE SOUTHEAST COAST TO DEVELOP A SURFACE SYSTEM
(POSSIBLY BECOMING SUBTROPICAL/TROPICAL FOR A TIME) THAT TRACKS
BACK ONTO LAND AND PRODUCES SOME ENHANCED RAINFALL. ACROSS THE
WESTERN U.S., AN AMPLIFIED MEAN TROUGH WILL LIKELY SET UP NEAR THE
WEST COAST WITH AN EMBEDDED LOW POSSIBLY REACHING NEAR CALIFORNIA
BY EARLY MONDAY AND THEN EJECTING NORTHEASTWARD AS ADDITIONAL
ENERGY DIGS INTO THE TROUGH BY WEDNESDAY. THE EJECTING UPPER LOW
MAY PRODUCE AN AREA OF SIGNIFICANT PRECIPITATION DEPENDING ON ITS
EXACT TRACK.
..GUIDANCE/PREDICTABILITY ASSESSMENT
THE UPDATED FORECAST BASED ON 00Z/06Z GUIDANCE STARTED WITH A BLEND
OF THE 00Z ECMWF/UKMET/CMC AND 06Z GEFS MEAN FOR ABOUT THE FIRST
HALF OF THE PERIOD AND THEN QUICKLY TRANSITIONED TO A HALF MODEL
(MORE ECMWF THAN CMC) AND HALF ENSEMBLE MEAN (06Z GEFS AND 00Z
ECENS/CMCENS) MIX BY TUESDAY-WEDNESDAY. PRIMARY CONSIDERATIONS FOR
REPLACING THE GFS WITH THE GEFS MEAN IN THE FORECAST WERE THAT THE
00Z/06Z GFS RUNS WERE A COMBINED DEEP/NORTHWARD EXTREME WITH THE
SYSTEM THAT MAY DEVELOP OVER THE WESTERN ATLANTIC AND THEN TRACK
INTO THE CAROLINAS AND BEYOND, WHILE GFS RUNS ALSO STRAYED SOMEWHAT
TO THE FAST SIDE WITH THE UPPER TROUGH/LOW EJECTING FROM WEST NEXT
WEEK.
THE EVOLUTION OVER THE WESTERN ATLANTIC INTO EASTERN U.S. STILL
HAS BELOW AVERAGE CONFIDENCE IN SPECIFICS GIVEN DEPENDENCE ON
SMALLER SCALE ENERGY UNDERNEATH THE NORTHEAST UPPER RIDGE,
POTENTIAL CONVECTIVE INFLUENCES WHILE OFFSHORE, AND HOW MUCH TIME
IT SPENDS OVER WATER. THE NEW 12Z CMC IS GRADUALLY RAISING THE
PROBABILITY OF SOME VARIATION OF THE GFS/ECMWF SCENARIO, THOUGH
THE UKMET/ICON HAVE VERY DIFFERENT IDEAS AND THE MACHINE LEARNING
(ML) MODELS ARE ALSO MIXED REGARDING HOW MUCH SURFACE DEVELOPMENT
MAY OCCUR. CYCLONE PHASE GUIDANCE SUGGESTS POTENTIAL FOR THIS
FEATURE TO BECOME SUBTROPICAL/TROPICAL FOR A TIME, SO THE
COORDINATED WPC/NHC DEPICTION DEPICTS SUCH AN IDEA AFTER SUNDAY.
NEW 12Z MODEL TRENDS GENERALLY SHOW FASTER PROGRESSION FROM THE
WESTERN ATLANTIC WESTWARD/NORTHWESTWARD.
WITHIN THE WESTERN MEAN TROUGH, MOST OF THE ML MODELS PROVIDED
ADDED SUPPORT ALONG WITH THE ENSEMBLE MEANS AND ECMWF/CMC FOR
SLOWER MONDAY-WEDNESDAY SYSTEM EJECTION VERSUS THE 00Z/06Z GFS,
WITH THE NEW 12Z GFS ADJUSTING SOMEWHAT SLOWER. THE ML MODELS ALSO
REFLECT MOST DYNAMICAL GUIDANCE IN DIGGING ADDITIONAL ENERGY INTO
THE TROUGH BY NEXT WEDNESDAY, THOUGH WITH TYPICAL UNCERTAINTY FOR
THE TIME FRAME REGARDING EXISTENCE/LOCATION OF A POSSIBLE CLOSED
LOW.
..WEATHER/HAZARDS HIGHLIGHTS
THE REMNANT LOW FROM FRANCINE DISSIPATE BY SATURDAY AND EVOLVE
INTO A SURFACE TROUGH, WHILE THE MAJORITY OF THE REMAINING
RAINFALL SHOULD BE LOCATED TO THE EAST AND SOUTHEAST OF THE LOW.
THERE WILL LIKELY BE A REMAINING INFLOW BAND ACROSS CENTRAL
TENNESSEE AND EXTENDING INTO NORTHERN ALABAMA, WHERE LOCALLY HIGHER
HIGHER RAINFALL TOTALS COULD BE OBSERVED. ORTHOGONAL LOW LEVEL
FLOW ACROSS THE SOUTHERN APPALACHIANS COULD RESULT IN SOME HIGHER
RAINFALL TOTALS HERE AS WELL, AND BOTH OF THESE REGIONS ARE
ENCOMPASSED IN A BROAD MARGINAL RISK AREA FOR THE DAY 4/SATURDAY
EXCESSIVE RAINFALL OUTLOOK. BY DAY 5/SUNDAY, A LINGERING BAND OF
ENHANCED RAINFALL POTENTIAL MAY EXIST OVER PARTS OF THE TENNESSEE
VALLEY/SOUTH WITH A MARGINAL RISK AREA IN THE ERO. IN ADDITION,
WHILE THE CURRENT DETERMINISTIC FORECAST FOR THE SYSTEM THAT MAY
DEVELOP OFF THE SOUTHEAST COAST IS NOT AS FAST, TRENDS IN THE 12Z
MODELS OFFER ENOUGH SUPPORT FROM PROBABILISTIC PERSPECTIVE TO
INTRODUCE A MARGINAL RISK AREA IN THE DAY 5 ERO NEAR THE COAST OF
THE CAROLINAS. BEYOND EARLY MONDAY, SOME AREAS OF HEAVY RAINFALL MY
EXTEND FARTHER WEST/NORTHWEST INTO THE APPALACHIANS, WITH NORTHWARD
EXTENT BEING SENSITIVE TO STRENGTH OF RIDGING AT THE SURFACE AND
ALOFT.
SHOWERS AND THUNDERSTORMS ARE LIKELY THIS WEEKEND FOR PARTS OF THE
SOUTHWEST WITH AN INCREASED MOISTURE CONNECTION AHEAD OF A
POSSIBLE TROPICAL DISTURBANCE LIFTING NORTH OVER THE SOUTHERN BAJA
PENINSULA, AND A WEAK SHORTWAVE PASSING THROUGH. TO ACCOUNT FOR
THE POSSIBILITY OF ENHANCED RAINFALL, MARGINAL RISK AREAS ARE
MAINTAINED FOR SOUTHERN PORTIONS OF ARIZONA ON SATURDAY, AND
EXTENDING FARTHER NORTH INTO ARIZONA AND WESTERN NEW MEXICO ON
SUNDAY. ELSEWHERE ACROSS THE WEST, INCREASING SHOWERS AND STORMS
ARE LIKELY BY EARLY NEXT WEEK AS UPPER TROUGHING AMPLIFIES NEAR THE
WEST COAST AND EJECTS NORTHEASTWARD. BASED ON THE CURRENT FORECAST
OF THE EJECTING EMBEDDED LOW, THE NORTHERN ROCKIES MAY BE A FOCUS
FOR HEAVY RAINFALL AROUND TUESDAY. THE SOUTHERN PLAINS MAY ALSO SEE
SCATTERED SHOWERS/STORMS AS TROPICAL MOISTURE GETS WRAPPED IN
AHEAD OF THE WESTERN TROUGH.
NEAR TO SLIGHTLY BELOW NORMAL HIGH TEMPERATURES OVER THE WEST ON
SATURDAY WILL TREND COLDER WITH TIME HEADING INTO THE START OF NEXT
WEEK AND THEN PERSIST THROUGH WEDNESDAY AS MEAN TROUGHING ALOFT
BECOMES ESTABLISHED. A FAIRLY BROAD AREA OF HIGHS 10 TO PERHAPS 20
DEGREES BELOW NORMAL WILL BE POSSIBLE BY NEXT MONDAY-WEDNESDAY FROM
CALIFORNIA AND PARTS OF OREGON INTO THE INTERMOUNTAIN WEST AND
NORTHERN ROCKIES. MEANWHILE, EXPECT WARM CONDITIONS TO CONTINUE
FROM MOST OF THE PLAINS INTO THE GREAT LAKES AND PARTS OF THE
NORTHEAST WHERE UPPER LEVEL RIDGING WILL TEND TO GOVERN THE OVERALL
WEATHER PATTERN.
RAUSCH/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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