575
FXUS02 KWBC 291937
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
337 PM 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 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
THE PERIOD BEGINS MID-NEXT WEEK WITH A SOUTHERN STREAM
SHORTWAVE/ASSOCIATED COASTAL LOW MOVING AWAY OVER THE OPEN ATLANTIC
FROM THE NORTHEASTERN U.S., A NORTHERN STREAM UPPER-TROUGH AND
ACCOMPANYING COLD FRONT TO THE WEST MOVING THROUGH THE NORTHEASTERN
U.S., AND GENERALLY MORE ZONAL FLOW WITH WESTERN EXTENT AS THE
NORTHERN STREAM REMAINS IN PLACE TO THE NORTH ALONG THE
U.S./CANADIAN BORDER. THE GFS IS A BIT STRONGER WITH THE COASTAL
LOW, BUT WITH THE SEPARATION BETWEEN THE LOW AND THE UPPER-
TROUGH/FRONT TO THE WEST, PRECIPITATION CHANCES WILL BE TRENDING
DOWN BY MID-WEEK. THE NORTHERN STREAM REMAINS PROGRESSIVE TO THE
WEST, WITH EMBEDDED SHORTWAVE ENERGY IN THE FLOW THAT NOT
SURPRISINGLY HAS SOME TIMING DIFFERENCES AMONGST THE GUIDANCE.
HOWEVER, PRIOR FRONTAL PASSAGES SHOULD LEAD TO DRY CONDITIONS WITH
LIMITED CONCERN OVER POTENTIAL PRECIPITATION CHANCES THAT MIGHT
HAVE A GREATER DISPARITY IN EXPECTED LOCATION GIVEN THE GUIDANCE
DIFFERENCES.
THE PATTERN REMAINS A BIT UNCERTAIN BY LATE NEXT WEEK/WEEKEND WITH
DIFFERENCES IN THE DETERMINISTIC GUIDANCE WITH RESPECT TO AN
UPPER-TROUGH OVER THE NORTHEASTERN PACIFIC APPROACHING SOUTHWESTERN
CANADA AND TRAVERSING EASTWARD IN VICINITY OF THE NORTHERN BORDER.
THE GFS HAS TRENDED CLOSER TO THE ECMWF WITH RESPECT TO THE PATH
OF THE UPPER-TROUGH, DEPICTING A LOWER- AMPLITUDE WAVE REMAINING
ALONG THE NORTHERN BORDER RATHER THAN DIGGING SOUTHWARD INTO THE
WESTERN U.S., THOUGH STILL DEPICTING THE POTENTIAL FOR SOME ENERGY
TO SPLIT FROM THE NORTHERN STREAM OVER NORTHERN CALIFORNIA.
HOWEVER, THE 00Z ECMWF TRENDED WEAKER WITH THE UPPER-TROUGH
COMPARED TO THE PAST FEW RUNS, AND ALSO BRINGS IT EVENTUALLY MORE
SOUTHWARD THAN THE OTHER GUIDANCE WITH EASTWARD EXTENT, WHICH KEEPS
THE DEGREE OF UNCERTAINTY IN THE FORECAST. THE DIFFERENCES ARE
ALSO APPARENT IN THE SURFACE REFLECTION, WITH A STRONGER SURFACE
LOW TAKING A MORE NORTHERLY PATH IN THE GFS COMPARED TO THE ECMWF.
ONE POSITIVE CHANGE IS THAT BOTH THE CMC AND CMC ENSEMBLE MEAN HAVE
GENERALLY COME IN PHASE WITH THE OTHER GUIDANCE DESPITE SOME
TIMING DIFFERENCES WITH THE PROGRESSION OF THE UPPER-TROUGH. AT THE
SURFACE, THE CMC IS MORE SIMILAR TO THE GFS WITH A STRONGER
SURFACE LOW TAKING A MORE NORTHERLY TRACK IN CANADA COMPARED TO THE
ECMWF. THE ECENS MEAN WAS MOST SIMILAR TO THE CLUSTER OF
DETERMINISTIC SOLUTIONS, OFFERING A BASE SOLUTION WITH THE
DETERMINISTIC GUIDANCE PROVIDING MORE SPECIFIC DETAILS. SIMILAR TO
THE ACTIVITY TO THE EAST, EVEN WITH THE NOTED DIFFERENCES,
PRECIPITATION CHANCES LOOK TO REMAIN RATHER LIMITED REGARDLESS OF
THE EVENTUAL SYSTEM PATH/TIMING GIVEN THE MORE NORTHERLY TRACK
AMONGST THE GUIDANCE OVERALL. PORTIONS OF THE PACIFIC NORTHWEST MAY
GET SOME LIGHT TO MODERATE PRECIPITATION. HOWEVER, THERE IS AN
EFFECT ON FORECAST TEMPERATURES, WITH AN UPWARD TREND OVER THE
NORTHWEST AND NORTH-CENTRAL U.S. LESS PROGRESSION OF COOLER AIR TO
THE NORTH SOUTHWARD.
MOST ATTENTION WILL LIKELY FOCUS ON THE POTENTIAL FOR TROPICAL
DEVELOPMENT IN THE CARIBBEAN/GULF OF MEXICO, WITH THE NATIONAL
HURRICANE CENTER INDICATING A 50% CHANCE OF DEVELOPMENT OVER THE
NEXT 7 DAYS. BOTH THE DETERMINISTIC AND ENSEMBLE GUIDANCE INDICATE
A DISTURBANCE IN THE GULF BUT TO VARYING DEGREES WITH RESPECT TO
TIMING, PATH, AND STRENGTH. THE GFS IN PARTICULAR HAS BEEN MORE
AGGRESSIVE IN THE PRIOR FEW RUNS WITH A STRONGER, FASTER SYSTEM
FOLLOWING A MORE WESTERN TRACK. FOR NOW, THE WPC/NHC COORDINATED
FORECAST BRINGS AN AREA OF LOW PRESSURE NORTHWARD FROM THE WESTERN
CARIBBEAN AND INTO THE EASTERN GULF OF MEXICO BY NEXT WEEKEND,
EVENTUALLY APPROACHING THE GULF COAST OF FLORIDA. THIS IS A
COMPROMISE SOLUTION BETWEEN THE 00Z ECMWF/CMC AND 06Z GFS AS WELL
AS THE ENSEMBLE MEMBERS THAT TEND TO CLUSTER AROUND THIS SOLUTION.
THE 12Z GFS DID REVERT TO A MORE AGGRESSIVE FASTER AND WESTERN
SOLUTION. TRENDED WETTER WITH THE QPF FORECAST IN VICINITY OF THE
GULF COAST AND OVER FLORIDA GIVEN THE CONSISTENT EXPECTATION FOR AT
LEAST INCREASING TROPICAL MOISTURE IN THE VICINITY. THE NBM HAD
TRENDED SIGNIFICANTLY UPWARD, ESPECIALLY OVER FLORIDA, COMPARED TO
THE PRIOR FORECAST. A CONTRIBUTION FROM THE ENSEMBLE MEANS WAS USED
TO TEMPER AMOUNTS A BIT AND INCREASE COVERAGE GIVEN THE
UNCERTAINTY TIED TO TROPICAL DEVELOPMENT, BUT WOULD EXPECT A NEED
TO STEADILY INCREASE TOTALS AS CONFIDENCE INCREASES.
OPTED FOR A COMPOSITE BLEND FOR THE UPDATED WPC FORECAST EARLY IN
THE PERIOD AS THE DETERMINISTIC GUIDANCE (00Z ECMWF/CMC/UKMET AND
06Z GFS) IS IN RELATIVELY GOOD AGREEMENT. AN INCREASINGLY GREATER
CONTRIBUTION FROM THE ECENS MEAN WAS INCLUDED DURING THE MID- TO
LATE FORECAST PERIOD AS THIS SOLUTION OFFERED THE BEST COMPROMISE
BETWEEN INCREASING DIFFERENCES IN THE 00Z CMC/06Z GFS AND 00Z ECMWF
IN THE LATE PERIOD, WITH THE DETERMINISTIC GUIDANCE USED TO
PROVIDE A BIT GREATER DETAIL.
..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, INCLUDING
FLORIDA, 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 IN THE GREAT LAKES VICNITY FRIDAY AND THE NORTHEAST AGAIN
SATURDAY. SOME ORGANIZED LIGHT TO LOCALLY MODERATE RAIN MAY BE
POSSIBLE OVER THE NORTHWEST WITH A FRONT ARRIVING AROUND FRIDAY,
THOUGH THESE CHANCES HAVE BEEN TRENDING DOWNWARD. RAIN CHANCES MAY
INCREASE WITH NORTHERN EXTENT ALONG THE SOUTHEASTERN ATLANTIC COAST
AS TROPICAL MOISTURE FLOWS NORTH AHEAD OF A COLD FRONT NEXT
WEEKEND.
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. TEMPERATURES LOOK TO TREND ABOVE
AVERAGE IN THE NORTHWEST AND NORTH-CENTRAL U.S. LATE NEXT WEEK AND
INTO THE WEEKEND AS COOLER AIR REMAINS NORTH OVER CANADA.
TEMPERATURES OVER THE EAST WILL VARY WITH FRONTAL PROGRESSION BUT
LIKELY AVERAGE OUT TO NEAR OR MODERATELY ABOVE NORMAL.
PUTNAM/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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