419
FXUS02 KWBC 101849
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
248 PM EDT SUN SEP 10 2023
VALID 12Z WED SEP 13 2023 - 12Z SUN SEP 17 2023
18Z UPDATE: THE 12Z MODEL GUIDANCE SUITE IS IN GOOD OVERALL
AGREEMENT ACROSS THE CONTINENTAL U.S. GOING INTO THE MIDDLE OF THE
WEEK, WITH MORE NOTEWORTHY DIFFERENCES FIRST APPEARING ACROSS THE
NORTHEAST PACIFIC GOING INTO THURSDAY AND FRIDAY REGARDING THE
TIMING OF SHORTWAVE FEATURES ORIGINATING FROM THE GULF OF ALASKA.
GOING INTO NEXT WEEKEND, THE CMC BECOMES MUCH STRONGER WITH THE
TROUGH DEVELOPING OVER THE GREAT LAKES REGION, AND STRONGER WITH
THE UPPER RIDGE ACROSS THE NORTHERN ROCKIES INTO SOUTH-CENTRAL
CANADA, SO THIS SOLUTION WAS NOT FAVORED BEYOND FRIDAY. INCLUSION
OF THE ENSEMBLE MEANS IS UP TO 40% FOR NEXT WEEKEND, ALONG WITH
SOME PREVIOUS WPC CONTINUITY. REGARDING THE TRACK OF HURRICANE
LEE, THE 12Z GFS AND TO A SLIGHTLY LESSER DEGREE THE CMC, ARE
FASTER WITH THE PROGRESSION NORTHWARD, WITH THE MAJORITY OF THE
GUIDANCE INDICATING THE STORM WILL REMAIN EAST OF THE NEW ENGLAND
COAST, BUT FUTURE MODEL TRENDS WILL CONTINUE TO BE MONITORED
CLOSELY. THE TIMING AND MAGNITUDE OF THE EASTERN U.S. TROUGH,
ALONG WITH THE POSITION OF THE MID-UPPER LEVEL HIGH OVER THE
NORTHERN ATLANTIC, WILL BE THE KEY PLAYERS IN THE EVENTUAL PATH OF
HURRICANE LEE. THE NATIONAL HURRICANE CENTER HAS ADDITIONAL
INFORMATION ON THIS STORM. CHANGES TO THE EXCESSIVE RAINFALL
OUTLOOK ARE NOTED IN THE WEATHER/HAZARDS SECTION BELOW. /HAMRICK
----------------------
..OVERVIEW
TODAY'S GUIDANCE IS FAIRLY CONSISTENT WITH THE LARGE SCALE PATTERN
EVOLUTION, FEATURING MEAN TROUGHING OVER THE EASTERN HALF OF THE
LOWER 48 THROUGH THE PERIOD AND A FAIRLY STRONG UPPER RIDGE
BUILDING OVER THE WEST BY FRIDAY-SUNDAY. WITHIN THE EASTERN
TROUGH, A GREAT LAKES UPPER LOW WILL LIFT NORTHEASTWARD WHILE
MULTIPLE SHORTWAVES INITIALLY OVER THE WESTERN U.S. AND CANADA
SHOULD FEED INTO THE EASTERN TROUGH WHILE THE WESTERN RIDGE
STRENGTHENS. THE GREAT LAKES UPPER LOW AND ASSOCIATED SURFACE
SYSTEM SHOULD PRODUCE SOME MODERATE TO HEAVY RAINFALL OVER THE
NORTHEAST AROUND MIDWEEK. BY NEXT WEEKEND, THE TRACK AND TIMING
OF HURRICANE LEE WILL REQUIRE MONITORING FOR ANY POTENTIAL EFFECTS
ON NEW ENGLAND AND THE CANADIAN MARITIMES. SPECIFICS WILL BE
DEPENDENT ON FINER DETAILS WITHIN THE EASTERN NORTH AMERICA MEAN
TROUGH, WITH TYPICAL PREDICTABILITY DECLINING A WEEK OUT IN TIME.
DURING MID-LATE WEEK THE SHORTWAVE ENERGY EMERGING FROM THE WEST
WILL HELP TO GENERATE AREAS OF RAIN AND THUNDERSTORMS OVER THE
ROCKIES, AND POTENTIAL FOR HEAVIER TOTALS OVER THE SOUTHERN PLAINS
WHERE A WAVY FRONT MAY ADD FOCUS. COOLER THAN AVERAGE HIGHS
PREVAILING OVER A MAJORITY OF THE CENTRAL/EASTERN U.S. MID-LATE
WEEK SHOULD MODERATE SOME BY THE WEEKEND WHILE ABOVE NORMAL HIGHS
SHOULD BECOME ESTABLISHED OVER THE PACIFIC NORTHWEST INTO NORTHERN
HIGH PLAINS LATE WEEK THROUGH THE WEEKEND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
GUIDANCE CONTINUES TO SHOW SOME TIMING/DETAIL DIFFERENCES FOR THE
UPPER LOW TRACKING NORTHEAST FROM THE GREAT LAKES AND ASSOCIATED
SURFACE SYSTEM. GFS RUNS CONTINUE TO BE ON THE FAST SIDE, WITH
THE GEFS MEAN A TAD SLOWER, AND MOST OTHER SOLUTIONS SOMEWHAT
SLOWER YET. HOWEVER THE SLOWER MAJORITY HAS NUDGED SLIGHTLY
FASTER OVER THE PAST DAY. PREFERENCE CONTINUES TO BE WITH THE
MAJORITY CLUSTER BUT PERHAPS WITH CONFIDENCE NOT QUITE AS HIGH
GIVEN THOSE TRENDS. BEHIND THIS SYSTEM, THERE IS REASONABLE
AGREEMENT THAT A COMBINATION OF SHORTWAVES OVER SOUTHERN CANADA
AND THE WESTERN U.S. SHOULD ULTIMATELY FEED INTO THE LONG-TERM
MEAN TROUGH OVER THE EASTERN HALF OF THE COUNTRY. THE SCALE OF
EACH FEATURE LIMITS PREDICTABILITY IN SPECIFICS BY 6-7 DAYS OUT IN
TIME, FAVORING A CONSERVATIVE BLEND APPROACH FOR NOW. OPERATIONAL
RUNS OFFER POTENTIAL FOR AN EMBEDDED UPPER LOW SOMEWHERE BETWEEN
THE UPPER MIDWEST/GREAT LAKES AND HUDSON BAY/JAMES BAY BY NEXT
SUNDAY. BY NEXT WEEKEND, THE SPECIFICS OF THE TWO EASTERN U.S.
TROUGH FEATURES AND NORTHWARD TIMING OF HURRICANE LEE AS IT TRACKS
WEST OF BERMUDA WILL DETERMINE WHAT EFFECTS LEE MAY HAVE ON NEW
ENGLAND AND THE CANADIAN MARITIMES. RECENT GFS RUNS HAVE
CONSISTENTLY BEEN ON THE FAST EXTREME OF THE SPREAD WHILE THE MUCH
SLOWER 12Z ECMWF IS ON THE WESTERN FRINGE OF THE ENVELOPE. A
BLEND OF THE 12Z CMC/ECENS/CMCENS AND 18Z GEFS PROVIDED THE BEST
EDITABLE STARTING POINT TO DEPICT A REASONABLE EXTRAPOLATION OF
THE OFFICIAL NATIONAL HURRICANE CENTER TRACK OF LEE. SOME
GUIDANCE INDICATES THAT LEE MAY TRANSITION TO AN EXTRATROPICAL
SYSTEM BY THE TIME ITS TRACK REACHES EAST OF NEW ENGLAND. MINUS
THE NEW 00Z CMC THAT STRAYS FROM OTHER GUIDANCE IN KEEPING MORE
SHORTWAVE ENERGY LINGERING OVER THE WEST, THERE IS GOOD CONSENSUS
AMONG MODELS/MEANS FOR THE STRONG RIDGE BUILDING OVER THE WEST BY
FRIDAY-SUNDAY.
FORECAST CONSIDERATIONS FOR THE INITIAL NORTHEAST SYSTEM FAVORED
AN EARLY BLEND OF THE 12Z ECMWF/UKMET/CMC AND 18Z GEFS MEAN. THEN
AS DETAIL UNCERTAINTIES INCREASED THE FORECAST INCREASED WEIGHT OF
THE 12Z ECENS/CMCENS ALONG WITH THE 18Z GEFS WHILE STEADILY
REDUCING ECMWF INPUT, SO THAT BY THE END OF THE PERIOD THE BLEND
PRIMARILY CONSISTED OF THE MEANS AND A SMALL 12Z CMC COMPONENT.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS (EROS) THAT COVER
WEDNESDAY THROUGH THURSDAY NIGHT REFLECT HEAVY RAINFALL/FLASH
FLOOD POTENTIAL OVER TWO GENERAL AREAS, ONE OVER THE NORTHEAST
(DAY 4) AND ANOTHER OVER PORTIONS OF THE ROCKIES AND SOUTHERN
PLAINS (BOTH DAYS). THE SYSTEM AFFECTING THE NORTHEAST ON
WEDNESDAY CONTINUES TO SHOW SOME TIMING UNCERTAINTY BUT THERE IS A
PERSISTENT SIGNAL FOR THE VIGOROUS DYNAMICS AND MOISTURE INPUT TO
PRODUCE AREAS OF HEAVY RAINFALL ACROSS A REGION THAT WILL HAVE WET
SOIL CONDITIONS DUE TO RECENT/FORECAST RAINFALL LEADING UP TO THE
OUTLOOK TIME FRAME. THIS SCENARIO FAVORS MAINTENANCE OF THE
SLIGHT RISK AREA WITH A MINOR EASTWARD ADJUSTMENT TO ACCOUNT FOR
GUIDANCE TRENDS. MEANWHILE THE MARGINAL RISK AREA WAS EXTENDED A
LITTLE SOUTHWARD ALONG THE MID-ATLANTIC COAST IN LIGHT OF SIGNALS
FOR SOME ACTIVITY ALONG THE TRAILING COLD FRONT DURING THE DAY
WEDNESDAY. FOR THE 12Z UPDATE, THE EXISTING DAY 4 SLIGHT IS
PLANNED TO BE TRIMMED BACK SOME ON THE WESTERN EDGE GIVEN A
SLIGHTLY FASTER TREND IN THE GUIDANCE, AND THE MARGINAL RISK
CONFINED TO DELAWARE AND POINTS NORTH.
SHORTWAVE ENERGY EMERGING FROM THE WEST WILL PROMOTE AT LEAST A
COUPLE DAYS OF CONVECTION OVER THE ROCKIES, WITH THE BEST SIGNALS
CURRENTLY APPEARING OVER COLORADO AND NEW MEXICO ON DAY 4 AND
WYOMING/COLORADO BY DAY 5. MEANWHILE LEADING ENERGY INTERACTING
WITH A WAVY FRONT SHOULD FOCUS AN AREA OF POTENTIALLY HEAVY
CONVECTION OVER THE SOUTHERN PLAINS WEDNESDAY-THURSDAY. EXPECTED
RAINFALL TOTALS AND CONVECTIVE ORGANIZATION SUPPORT MAINTAINING A
SLIGHT RISK AREA OVER THE SOUTH-CENTRAL PLAINS ON DAY 4 AND
CONTINUING SOUTHEASTWARD INTO DAY 5. THE SURROUNDING MARGINAL
RISK AREA ALSO ENCOMPASSES THE ROCKIES ACTIVITY WHICH HAS A
SOMEWHAT MORE DIFFUSE SIGNAL IN THE GUIDANCE. FOR THE 12Z UPDATE,
THE EXISTING DAY 4 SLIGHT IS PLANNED TO BE TRIMMED BACK ON THE
NORTHERN EDGE TO ROUGHLY THE KS/OK BORDER, AND NO MAJOR CHANGES
FOR THE DAY 5 ERO. THE MARGINAL RISK AREA WAS EXPANDED SOME TO
INCLUDE MORE OF THE FOUR CORNERS REGION.
AFTER THE WEDNESDAY SYSTEM DEPARTS FROM NEW ENGLAND, RAINFALL AND
OTHER SENSIBLE WEATHER DETAILS FOR NEW ENGLAND BY NEXT WEEKEND
WILL DEPEND ON THE TRACK AND TIMING OF HURRICANE LEE. CONTINUE TO
MONITOR NATIONAL HURRICANE CENTER PRODUCTS FOR THE LATEST
INFORMATION ON LEE OVER THE COMING DAYS. RAIN AND THUNDERSTORMS
OVER THE ROCKIES SHOULD GRADUALLY TREND LIGHTER WITH MORE
SOUTHWARD SUPPRESSION LATE WEEK INTO THE WEEKEND AS UPPER RIDGING
BUILDS OVER THE WEST, WHILE SOME SHOWERS/STORMS MAY EXTEND ACROSS
THE SOUTH.
EXPECT BELOW NORMAL HIGH TEMPERATURES OVER A MAJORITY OF THE
CENTRAL AND EASTERN U.S., WITH SOME MODERATION LIKELY BY THE
WEEKEND. COOLEST READINGS RELATIVE TO AVERAGE SHOULD BE DURING
WEDNESDAY-THURSDAY WHEN PARTS OF THE CENTRAL/SOUTH-CENTRAL PLAINS
MAY BE UP TO 15-20F BELOW NORMAL DUE TO CLOUDS/RAINFALL AND THE
GREAT LAKES/OHIO VALLEY MAY BE 10-15F BELOW NORMAL UNDER THE UPPER
LOW/TROUGH THAT EVENTUALLY LIFTS AWAY. CONTINUED HOT WEATHER WITH
HIGHS UP TO 5-10F ABOVE NORMAL WILL BE POSSIBLE OVER SOUTHERN
TEXAS MID-LATE WEEK BEFORE A COOLER TREND. THE UPPER RIDGE
BUILDING INTO THE WEST WILL SUPPORT HIGHS 10F OR MORE ABOVE NORMAL
OVER THE PACIFIC NORTHWEST FROM LATE WEEK THROUGH THE WEEKEND,
WITH A BROADER AREA OF MORNING LOWS 5-15F ABOVE NORMAL ACROSS THE
WEST. SOME OF THIS WARMTH MAY EXTEND INTO THE NORTHERN HIGH
PLAINS.
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, EXCESSIVE RAINFALL OUTLOOK,
WINTER WEATHER OUTLOOK PROBABILITIES, HEAT INDICES AND KEY
MESSAGES ARE AT:
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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