885
FXUS02 KWBC 290658
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
258 AM EDT SUN SEP 29 2024
VALID 12Z WED OCT 02 2024 - 12Z SUN OCT 06 2024
..OVERVIEW
LATEST MODELS CONTINUE TO SHOW PROGRESSIVE MEAN FLOW ACROSS THE
NORTHERN LOWER 48 FROM MIDWEEK THROUGH NEXT WEEKEND, SUPPORTING A
SERIES OF FRONTS DURING THE PERIOD. THE FRONT EXPECTED TO REACH THE
NORTHWEST AROUND FRIDAY MAY PRODUCE ORGANIZED LIGHT TO LOCALLY
MODERATE RAIN, BUT OTHERWISE ANY PRECIPITATION ALONG THE FRONTS
SHOULD BE FAIRLY LIGHT AND SCATTERED. FARTHER SOUTH, PERSISTENT
UPPER RIDGING OVER AND NEAR THE SOUTHWEST WILL KEEP THAT PART OF
THE COUNTRY UNSEASONABLY HOT WITH SOME OF THE HEAT REACHING THE
PLAINS. THERE IS A PERSISTENT SIGNAL FOR INCREASING RAINFALL ALONG
THE GULF COAST, ESPECIALLY CENTRAL/WESTERN AREAS, AS A WEAKNESS
DEVELOPS BETWEEN THE SOUTHWEST RIDGE AND ANOTHER RIDGE BUILDING
OVER FLORIDA AND PARTS OF THE ATLANTIC/CARIBBEAN. IN ADDITION SOME
MODELS CONTINUE TO SHOW POTENTIAL FOR TROPICAL DEVELOPMENT FROM THE
NORTHWESTERN CARIBBEAN INTO THE GULF OF MEXICO, FURTHER ENHANCING
RAINFALL IF A SYSTEM NEARS THE GULF COAST. HOWEVER THERE IS STILL A
LOT OF UNCERTAINTY OVER THE COMBINED DETAILS OF FLOW ALOFT AND
POSSIBLE TROPICAL DEVELOPMENT.
..GUIDANCE/PREDICTABILITY ASSESSMENT
RECENT GUIDANCE HAS BEEN EXHIBITING DECENT MAJORITY CLUSTERING FOR
THE CHARACTER OF NORTHERN STREAM FLOW IN PRINCIPLE, BUT WITH ENOUGH
STRAY SOLUTIONS FROM ONE MODEL OR ANOTHER PLUS WIDE ENSEMBLE SPREAD
TO TEMPER CONFIDENCE. A POSITIVE TREND OVER THE PAST DAY HAS BEEN
THE ADJUSTMENT OF THE CMC TOWARD WHAT HAS BEEN THE MOST COMMON
DYNAMICAL/MACHINE LEARNING (ML) MODEL THEME WHICH BY FRIDAY WOULD
HAVE A TROUGH OVER THE EASTERN PACIFIC, RIDGE BUILDING INTO WESTERN
NORTH AMERICA, AND A BROAD TROUGH TO THE EAST. 12Z/18Z GFS RUNS
HAD STRAYED FROM THIS PATTERN DUE TO SENDING OUT SOME LEADING
PACIFIC SHORTWAVE ENERGY, ULTIMATELY RESULTING IN A VERY OUT-OF-
PHASE SOLUTION FOR DOWNSTREAM FLOW ALOFT AND SURFACE FRONTAL
POSITIONS BY NEXT WEEKEND. THE NEW 00Z GFS HAS SNAPPED BACK CLOSER
TO CONSENSUS. UKMET RUNS STILL DIFFER FROM THE MAJORITY THOUGH.
THERE ARE STILL DIFFERENCES FOR TROUGH AMPLITUDE/TIMING AS IT COMES
INTO THE NORTHWEST SATURDAY (INCLUDING THE 00Z ECMWF TRENDING
NOTICEABLY FLATTER), AND AS IT AMPLIFIES SOMEWHAT INTO THE CENTRAL
U.S. BY NEXT SUNDAY, CONTINUING TO FAVOR A BLEND APPROACH. MOST
GUIDANCE EXPECTS A TRAILING UPPER RIDGE TO REACH THE WEST COAST AT
THAT TIME.
THE FORECAST CONTINUES TO BE CHALLENGING ACROSS THE SOUTHERN TIER
AND INTO THE TROPICS. GFS/CMC RUNS REMAIN THE MOST AGGRESSIVE IN
THEIR DEVELOPMENT OF A CARIBBEAN WAVE AND TRACKING THE SYSTEM AT
LEAST AS FAR AS THE NORTHEASTERN GULF OF MEXICO. THE 12Z ECMWF HAD
REFLECTED A SIMILAR IDEA BUT IN SLOWER FORM (REACHING ONLY THE
CENTRAL GULF BY SUNDAY), WHILE THE NEW 00Z RUN TAKES A LOT LONGER
TO SHOW ANY DEFINITION AT THE SURFACE. ECMWF AIFS GUIDANCE HAS
INDICATED A NORTHEASTWARD NUDGE VERSUS THE 12Z ECMWF RUN, WHILE
REMAINING DYNAMICAL AND ML GUIDANCE SHOWS A WIDE ARRAY OF
POSSIBILITIES. MEANWHILE THERE ARE STILL IMPORTANT DETAIL
DIFFERENCES FOR THE POTENTIAL PLAINS/MISSISSIPPI VALLEY WEAKNESS
ALOFT THAT WOULD SUPPORT A WETTER PATTERN ALONG THE GULF COAST
(MOST LIKELY CENTRAL/EASTERN PORTIONS) REGARDLESS OF THE DEGREE OF
TROPICAL DEVELOPMENT.
CONSIDERATIONS BASED ON 12Z/18Z GUIDANCE LED TO STARTING WITH AN
OPERATIONAL MODEL COMPOSITE ON WEDNESDAY BUT THEN QUICKLY REPLACING
THE GFS WITH THE GEFS MEAN DUE TO DEVELOPING DIFFERENCES FOR
NORTHERN STREAM FLOW. THE BLEND REMOVED THE UKMET LATE IN ITS RUN
DUE TO ITS NORTHERN STREAM DETAILS, FOLLOWED BY THE CMC FOR ITS
FARTHER EAST GULF SYSTEM. THIS ULTIMATELY LED TO NEXT WEEKEND'S
FORECAST CONSISTING OF SOME 12Z ECMWF AND THE REST 18Z GEFS/12Z
ECMWF MEANS. THE BLEND WAS ADJUSTED TO REFLECT YESTERDAY'S WPC/NHC
COORDINATED TRACK FOR THE POSSIBLE GULF SYSTEM THROUGH SATURDAY,
BEST REPRESENTED BY A SLIGHTLY SLOWER VERSION OF THE 18Z GFS.
NORTHEASTWARD EXTRAPOLATION INTO SUNDAY CONTINUED TO BE A LITTLE
SOUTH OF THE 18Z GFS.
..WEATHER/HAZARDS HIGHLIGHTS
REGARDLESS OF HOW MUCH TROPICAL DEVELOPMENT OCCURS OVER THE GULF OF
MEXICO, THE FORECAST PATTERN EVOLUTION SHOULD SUPPORT AN INCREASE
OF MOISTURE/RAINFALL ALONG THE GULF COAST WITH TIME. BY DAY
5/THURSDAY SOME SOLUTIONS ARE SUGGESTING THAT PARTS OF THE CENTRAL
GULF COAST COULD START TO SEE SOME LOCALLY HEAVY ACTIVITY. HOWEVER
FLASH FLOOD GUIDANCE VALUES ARE QUITE HIGH OVER THE REGION AND
GUIDANCE IS VERY DISPERSED REGARDING IF/WHERE HEAVY RAINFALL MAY
OCCUR, SO THE DAY 5 ERO DEPICTS NO RISK AREA FOR NOW. FROM LATE
WEEK INTO THE WEEKEND THE BEST POTENTIAL FOR SIGNIFICANT RAINFALL
WILL BE ALONG THE EASTERN HALF OF THE GULF COAST, BUT WITH
CONSIDERABLE UNCERTAINTY IN THE SPECIFICS AS REFLECTED BY GUIDANCE
SPREAD FOR POSSIBLE TROPICAL DEVELOPMENT AND TIMING/TRACK.
EXPECT LIGHTER RAINFALL ELSEWHERE ALONG PORTIONS OF THE FRONTS
SUPPORTED BY NORTHERN STREAM DYNAMICS. SUCH RAINFALL AREAS INCLUDE
THE NORTHEAST ON WEDNESDAY AND PERHAPS A LITTLE LIGHT/SCATTERED
RAIN WITH A GREAT LAKES FRONT BY FRIDAY. SOME ORGANIZED LIGHT TO
LOCALLY MODERATE RAIN WILL BE POSSIBLE OVER THE NORTHWEST WITH A
FRONT ARRIVING AROUND FRIDAY, FOLLOWED BY INCREASING RAIN CHANCES
OVER THE EAST-CENTRAL U.S. AS THE FRONT REACHES THAT PART OF THE
COUNTRY BY NEXT SUNDAY.
THE COMBINATION OF PERSISTENT UPPER RIDGING OVER OR NEAR THE
SOUTHWEST AND WARM FLOW AHEAD OF A COUPLE FRONTS AS THEY APPROACH
THE PLAINS WILL PRODUCE WELL ABOVE AVERAGE TEMPERATURES FROM THE
SOUTHERN HALF OR SO OF THE WEST INTO THE CENTRAL U.S. THROUGH THE
END OF THE WEEK. EXPECT HIGHS 10-15F ABOVE AVERAGE WITH LOCALLY
HIGHER ANOMALIES. RECORD HIGHS SHOULD BE MOST NUMEROUS FROM THE
SOUTHWEST/CALIFORNIA INTO THE CENTRAL HIGH PLAINS ON WEDNESDAY,
FOLLOWED BY A GRADUAL DECREASE TO PERHAPS A FEW LOCATIONS BY
SATURDAY. NEXT WEEKEND'S COLD FRONT SHOULD FINALLY BEGIN TO
SUPPRESS THE VERY WARM TO HOT WEATHER BY SUNDAY, THOUGH PERHAPS NOT
QUITE YET FOR THE SOUTHWEST. THE NORTHWEST SHOULD SEE MORE MODERATE
READINGS ON EITHER SIDE OF NORMAL. TEMPERATURES OVER THE EAST WILL
VARY WITH FRONTAL PROGRESSION BUT LIKELY AVERAGE OUT TO NEAR OR
MODERATELY ABOVE NORMAL.
RAUSCH
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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