601
FXUS02 KWBC 090700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT SAT SEP 09 2023
VALID 12Z TUE SEP 12 2023 - 12Z SAT SEP 16 2023
..OVERVIEW
LATEST MODELS AND ENSEMBLES SHOW VARYING DEGREES OF UPPER
TROUGHING OVER EASTERN NORTH AMERICA DURING THE PERIOD. WITHIN
THIS MEAN TROUGH, THE MOST PROMINENT FEATURE WILL BE AN UPPER LOW
FORECAST TO CLOSE OFF OVER THE GREAT LAKES BY MIDWEEK AND THEN
LIFT NORTHEASTWARD AS DIFFUSE SHORTWAVE ENERGY ENTERING THE
WESTERN U.S./CANADA EVENTUALLY FEEDS INTO THE TROUGH. THE GREAT
LAKES LOW AND SURFACE SYSTEM SHOULD PRODUCE SOME MODERATE TO HEAVY
RAINFALL FROM THE GREAT LAKES/MID-ATLANTIC INTO THE NORTHEAST. BY
THE END OF NEXT WEEK INTO THE WEEKEND, THE TRACK AND TIMING OF
HURRICANE LEE WILL REQUIRE MONITORING FOR ANY POTENTIAL EFFECTS ON
NEW ENGLAND AND THE CANADIAN MARITIMES. OVER THE WEST, GUIDANCE
DISAGREES OVER THE STRENGTH/EVOLUTION OF INITIAL SHORTWAVE ENERGY
BUT SUGGESTS THAT UPPER RIDGING SHOULD ULTIMATELY STRENGTHEN OVER
THE REGION BY LATE NEXT WEEK INTO THE WEEKEND. THE COMBINATION OF
WESTERN SHORTWAVE ENERGY AND A PERSISTENT SOUTHERN PLAINS FRONT
WILL FAVOR MULTIPLE EPISODES OF RAIN AND THUNDERSTORMS OVER THE
SOUTHERN PLAINS THROUGH THE PERIOD, AND EXTENDING AS FAR WEST AS
ARIZONA TUESDAY-WEDNESDAY. COOLER THAN AVERAGE HIGHS WILL PREVAIL
OVER A MAJORITY OF THE CENTRAL/EASTERN U.S. WHILE MODERATELY ABOVE
NORMAL HIGHS SHOULD BECOME ESTABLISHED OVER THE FAR NORTHERN
PLAINS AND PACIFIC NORTHWEST BY LATE NEXT WEEK/WEEKEND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
THE PRIMARY FORECAST ISSUE FOR THE SYSTEM CLOSING OFF OVER THE
GREAT LAKES BY TUESDAY-WEDNESDAY AND LIFTING NORTHEASTWARD
THEREAFTER IS THAT LATEST GFS RUNS STRAY TO THE FAST SIDE AFTER
DAY 3 TUESDAY. EVEN THE GEFS MEAN LEANS MORE TO THE REMAINING
SLOWER MODELS/ENSEMBLE MEANS. ON THE OTHER HAND, MODELS AND
ENSEMBLES HAVE BEEN MORE VARIABLE AND DIVERSE WITH SPECIFICS OF
UPSTREAM WESTERN NORTH AMERICAN ENERGY THAT SHOULD BE FEEDING INTO
THE EASTERN MEAN TROUGH LATE IN THE PERIOD. PRECISE DAY-TO-DAY
DETAILS OF THE EASTERN TROUGH WILL LIKELY PLAY AN IMPORTANT ROLE
IN DETERMINING THE PATH OF HURRICANE LEE, WHICH GUIDANCE EXPECTS
TO BE TRACKING NORTHWARD ALONG 65-70W LONGITUDE BY NEXT SATURDAY.
THE GREATEST GUIDANCE SPREAD IS FOR TIMING, WITH GFS RUNS BEING
THE FAST EXTREME WHILE THE 12Z ECMWF/ECMWF MEAN ARE SLOWEST.
WHILE EAST-WEST DIFFERENCES IN TRACK ARE MUCH SMALLER RELATIVE TO
SPEED, PRECISE LONGITUDE WILL BE VERY IMPORTANT WITH RESPECT TO
ANY POTENTIAL EFFECTS ON NEW ENGLAND AND THE CANADIAN MARITIMES.
CONTINUE TO MONITOR NATIONAL HURRICANE CENTER PRODUCTS FOR THE
LATEST INFORMATION ON THE STRENGTH/TRACK OF LEE. MEANWHILE, THERE
IS A LOT OF SPREAD FOR DETAILS OF SMALLER SCALE/LOWER
PREDICTABILITY ENERGY THAT MAY BE OVER PARTS OF THE WEST INTO
MID-LATE WEEK, FAVORING AN INTERMEDIATE BLEND. OVERALL THE
CONSENSUS EVOLUTION IS TOWARD A STRONGER MEAN RIDGE PREVAILING
OVER MOST OF THE WEST LATE NEXT WEEK INTO THE WEEKEND.
FORECAST CONSIDERATIONS LED TO STARTING WITH AN OPERATIONAL MODEL
BLEND ON DAY 3 TUESDAY, FOLLOWED BY REPLACING THE GFS COMPONENT
WITH THE 18Z GEFS MEAN FROM DAY 4 WEDNESDAY ONWARD DUE TO THE GFS
BEING FAST WITH THE GREAT LAKES SYSTEM. DECREASING CONFIDENCE IN
OTHER DETAILS LED TO 60 PERCENT TOTAL WEIGHT OF THE 18Z GEFS AND
12Z ECENS/CMCENS MEANS BY DAYS 6-7 FRIDAY-SATURDAY, WITH LINGERING
INPUT FROM THE 12Z CMC/ECMWF. WITH SOME MANUAL ADJUSTMENT, THE
FORECAST REFLECTS AN INTERMEDIATE TIMING FOR LEE BY THE END OF THE
PERIOD.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS (EROS) THAT COVER TUESDAY
THROUGH WEDNESDAY NIGHT REFLECT HEAVY RAINFALL/FLASH FLOOD
POTENTIAL OVER TWO GENERAL AREAS, ONE OVER CENTRAL/NORTHERN
PORTIONS OF THE EAST AND THE OTHER OVER/WEST OF THE SOUTHERN
PLAINS. DURING THE DAY 4 TIME FRAME, THE SYSTEM BEGINNING TO
EVOLVE OVER THE GREAT LAKES MAY GENERATE SOME LOCALLY ENHANCED
RAINFALL FROM THE GREAT LAKES/APPALACHIANS INTO THE MID-ATLANTIC.
GUIDANCE IS NOT FULLY AGREEABLE ON THE EXACT FORECAST SCENARIO
QPF-WISE BUT THIS AREA WILL HAVE WET ANTECEDENT CONDITIONS FROM
RECENT AND NEAR-FUTURE RAINFALL. BY DAY 5 EXPECT A MORE
PRONOUNCED POTENTIAL FOR HEAVIER RAINFALL OVER PORTIONS OF THE
NORTHEAST, WHERE THE SLOWER NON-GFS MAJORITY CLUSTER FOR THE
SYSTEM HEADING INTO THE REGION MERITS A SLIGHT RISK. FROM THE
SOUTHERN PLAINS WESTWARD, SHORTWAVE ENERGY WILL INTERACT WITH
ANOMALOUS MOISTURE TO YIELD AREAS OF LOCALLY HEAVY RAIN AND
THUNDERSTORMS. A FRONT PERSISTING OVER THE SOUTHERN PLAINS WILL
PROVIDE ANOTHER RAINFALL FOCUS. THE CORRESPONDING MARGINAL RISK
AREA ON DAY 4 EXTENDS FROM PARTS OF TEXAS INTO ARIZONA, WHILE THE
DAY 5 MARGINAL RISK REFLECTS A NORTHWARD EXPANSION OF ACTIVITY
INTO COLORADO AND PARTS OF KANSAS/OKLAHOMA BUT A DRIER TREND OVER
ARIZONA. WITHIN THESE MARGINAL RISK AREAS, POTENTIAL EXISTS FOR
AN EVENTUAL EMBEDDED SLIGHT RISK DEPENDING ON GUIDANCE
AGREEMENT/CONTINUITY AND MORE CLARITY ON SOIL CONDITIONS, IN LIGHT
OF SOME AREAS BEING FAIRLY DRY TO START BUT EXPECTED TO RECEIVE
SOME RAINFALL SOON BEFORE THE DAYS 4-5 PERIOD.
SOME MODERATE TO HEAVY RAINFALL MAY CONTINUE INTO EASTERN NEW
ENGLAND THROUGH LATE NEXT WEEK. 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. THE RAIN AND THUNDERSTORMS OVER THE SOUTHERN
PLAINS AND ROCKIES SHOULD CONTINUE THROUGH THE END OF THE WEEK,
THOUGH WITH A SOMEWHAT LIGHTER/MORE SCATTERED TREND OVER THE
ROCKIES.
BELOW NORMAL HIGH TEMPERATURES OVER THE CENTRAL U.S. AND
MIDWEST/GREAT LAKES AS OF TUESDAY SHOULD GRADUALLY SPREAD FARTHER
EASTWARD, THOUGH WITH A MODERATING TREND BY FRIDAY-SATURDAY. BEST
POTENTIAL FOR HIGHS 10-15F BELOW NORMAL WILL BE DURING
TUESDAY-THURSDAY OVER THE CENTRAL/SOUTH-CENTRAL PLAINS AS WELL AS
THE GREAT LAKES/OHIO VALLEY. SLIGHTLY ABOVE NORMAL TEMPERATURES
MAY HOLD ON OVER FAR SOUTHERN TEXAS A LITTLE BEYOND MIDWEEK, WHILE
THE PACIFIC NORTHWEST EASTWARD TO THE FAR NORTHERN PLAINS MAY
TREND SOMEWHAT ABOVE NORMAL BY THURSDAY-SATURDAY.
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, 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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