784
FXUS02 KWBC 110700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT MON SEP 11 2023
VALID 12Z THU SEP 14 2023 - 12Z MON SEP 18 2023
..OVERVIEW
THE MODELS AND ENSEMBLES AGREE FAIRLY WELL ON THE IDEA OF A STRONG
UPPER RIDGE BUILDING OVER THE WEST LATE THIS WEEK INTO THE START
OF NEXT WEEK, WHILE DEPARTING WESTERN U.S. SHORTWAVE ENERGY AND
SOUTHERN CANADA ENERGY COMBINE TO YIELD AN AMPLIFYING UPPER TROUGH
OVER THE EASTERN HALF OF THE COUNTRY. AHEAD OF THIS DEVELOPING
TROUGH, A LEADING UPPER LOW/TROUGH WILL EJECT ACROSS SOUTHEASTERN
CANADA AND THE NORTHEASTERN U.S. LATE THIS WEEK. PRECISE UPPER
FLOW DETAILS NEAR THE EAST COAST AND SPECIFICS OF RIDGING FARTHER
EAST OVER THE ATLANTIC WILL BE IMPORTANT IN DETERMINING THE EXACT
TRACK AND TIMING OF HURRICANE LEE AND CORRESPONDING EFFECTS ON NEW
ENGLAND AND THE CANADIAN MARITIMES. THE INITIAL SHORTWAVE ENERGY
OVER THE WEST INTO THE PLAINS ALONG WITH EASTERLY LOW LEVEL FLOW
OVER PARTS OF THE SOUTHERN PLAINS WILL HELP TO GENERATE AREAS OF
RAIN AND THUNDERSTORMS THAT COULD BE LOCALLY HEAVY OVER THE
ROCKIES AND INTO THE SOUTHERN PLAINS. COOLER THAN AVERAGE HIGHS
PREVAILING OVER A MAJORITY OF THE CENTRAL/EASTERN U.S. LATE THIS
WEEK SHOULD MODERATE SOME BY THE WEEKEND WHILE ABOVE NORMAL HIGHS
SHOULD BECOME ESTABLISHED OVER THE PACIFIC NORTHWEST INTO NORTHERN
HIGH PLAINS LATE THIS WEEK INTO THE START OF NEXT WEEK.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
AS HAS BEEN THE CASE DURING AT LEAST THE PAST COUPLE DAYS, THE
GFS/GEFS LEAN A BIT FASTER THAN MOST OTHER GUIDANCE FOR THE SYSTEM
LIFTING THROUGH NEW ENGLAND. AFTER THIS SYSTEM LIFTS OUT, THE
FOCUS WILL TURN TO HURRICANE LEE WHICH MAY BECOME EXTRATROPICAL BY
THE TIME ITS NORTHWARD TRACK REACHES OFFSHORE NEW ENGLAND AND THE
CANADIAN MARITIMES BY THE WEEKEND. RELATIVE TO THE FULL RANGE OF
GUIDANCE, THE 18Z/00Z GFS RUNS LEAN SOMEWHAT ON THE WESTERN SIDE
OF THE ENVELOPE. IN PRINCIPLE THE 12Z ECMWF/ECENS/UKMET AND 00Z
UKMET ARE CLOSER TO THE MORE LIKELY SCENARIO WHILE THE LATEST CMC
RUNS ARE CLOSE LONGITUDE-WISE BUT A LITTLE FASTER. THEN THE
MODELS AND ENSEMBLE MEMBERS SHOW RAPIDLY INCREASING DETAIL SPREAD
WITHIN THE OVERALL TROUGH AMPLIFYING OVER THE EAST-CENTRAL U.S.
FROM THE WEEKEND INTO EARLY NEXT WEEK. POSSIBILITIES RANGE
BETWEEN AN OPEN TROUGH AND ONE OR MORE EMBEDDED LOWS ANYWHERE
BETWEEN THE MIDWEST/GREAT LAKES AND CANADA. POOR CLUSTERING AND
RUN-TO-RUN VARIABILITY CURRENTLY FAVOR A CONSERVATIVE APPROACH
MORE LIKE THE OPEN TROUGH OF THE ENSEMBLE MEANS, WITH THE 12Z
ECMWF CLOSEST TO THAT IDEA.
BASED ON FORECAST PREFERENCES OVER THE COURSE OF THE PERIOD, THE
UPDATED FORECAST WAS WEIGHTED MORE TOWARD THE 12Z ECMWF/UKMET/CMC
WITH JUST A LITTLE 18Z GEFS INPUT FOLLOWED BY A TRANSITION TOWARD
60-70 PERCENT GEFS/ECENS/CMCENS WITH THE REST 12Z ECMWF BY DAYS
6-7 SUNDAY-MONDAY. NOTE THAT THE MOST LIKELY TRACK FOR LEE (EAST
OF LATEST GFS RUNS) CONTINUES TO SUGGEST THAT NBM QPF MAY BE TOO
HIGH OVER NEW ENGLAND AND THUS IT IS BEING ADJUSTED LOWER.
..WEATHER/HAZARDS HIGHLIGHTS
DURING THE DAY 4 EXCESSIVE RAINFALL OUTLOOK PERIOD OF
THURSDAY-THURSDAY NIGHT THERE SHOULD BE A BROAD AREA OF RAIN AND
THUNDERSTORM POTENTIAL FROM THE CENTRAL HALF OF THE ROCKIES INTO
THE SOUTHERN PLAINS, WITH ONE AREA OF SHORTWAVE ENERGY OVER THE
GREAT BASIN AND SOME WEAK IMPULSES FLOWING INTO THE PLAINS.
EASTERLY LOW LEVEL FLOW DIRECTED OVER THE SOUTHERN HIGH PLAINS
INTO A POTENTIAL SURFACE TROUGH MAY ALSO SUPPORT CONVECTION. A
MARGINAL RISK AREA ENCOMPASSES THIS OVERALL REGION THAT COULD SEE
LOCALLY HEAVY RAINFALL. THERE ARE TWO EMBEDDED SLIGHT RISK AREAS
THAT REFLECT THE BEST COMBINATION OF GUIDANCE SIGNALS/PARAMETERS.
ONE IS OVER THE SOUTHWESTERN OKLAHOMA/NORTHERN TEXAS, REFLECTING
SOME INDICATION THAT ENHANCED CONVECTION COULD EXTEND
NORTHWESTWARD VERSUS PRIOR ERO ISSUANCE. ESPECIALLY THE WESTERN
PART OF THIS SLIGHT RISK AREA IS EXPECTED TO HAVE MEANINGFUL
RAINFALL BEFORE DAY 4. MEANWHILE A SEPARATE SLIGHT RISK AREA
CENTERED MOSTLY OVER WYOMING WILL BE INTRODUCED, WITH GUIDANCE
STARTING TO SHOW A MORE PRONOUNCED SIGNAL FOR A SLOW MOVING GREAT
BASIN SHORTWAVE AND ANOMALOUS MOISTURE COMBINING TO SUPPORT
LOCALIZED HEAVY RAINFALL POTENTIAL. THE DAY 5 EXCESSIVE RAINFALL
OUTLOOK THAT COVERS FRIDAY-FRIDAY NIGHT DEPICTS A MARGINAL RISK
AREA FROM THE CENTRAL HIGH PLAINS INTO PARTS OF TEXAS. CONVECTION
SHOULD CONTINUE INTO THIS TIME FRAME BUT MOST GUIDANCE SHOWS SOME
DECREASE IN MOISTURE ANOMALIES AND THE BEST FOCUS FOR RAINFALL
BECOMES LESS DEFINED.
SOME RAIN/THUNDERSTORMS SHOULD CONTINUE OVER PARTS OF THE SOUTHERN
ROCKIES/PLAINS AND PERHAPS THE SOUTH BEYOND EARLY SATURDAY BUT
LIKELY WITH LESS COVERAGE AND LIGHTER TOTALS. MEANWHILE A FRONT
SETTLING OVER THE FLORIDA PENINSULA MAY PRODUCE ONE OR MORE DAYS
OF RAIN OVER THAT REGION. OVER NEW ENGLAND, EXPECT ANY LINGERING
RAIN FROM THE MIDWEEK SYSTEM TO DEPART ON THURSDAY. CONTINUE TO
MONITOR NATIONAL HURRICANE CENTER PRODUCTS REGARDING THE TRACK OF
HURRICANE LEE AND POTENTIAL EFFECTS ON NEW ENGLAND AND THE
CANADIAN MARITIMES BY LATE WEEK AND THE WEEKEND. UNCERTAINTY WITH
DETAILS FOR THE UPPER TROUGH EVOLVING OVER THE EASTERN HALF OF THE
COUNTRY DURING THE WEEKEND AND EARLY NEXT WEEK KEEPS CONFIDENCE
LOW FOR WHAT THE SURFACE PATTERN MAY LOOK LIKE ALONG WITH ANY
AREAS OF RAINFALL, THOUGH INTO FRIDAY OR SO A FRONT REACHING THE
UPPER MIDWEST COULD PRODUCE SOME AREAS OF MOSTLY LIGHT RAIN.
BELOW NORMAL HIGH TEMPERATURES WILL PREVAIL OVER MOST OF THE
ROCKIES, CENTRAL/SOUTH-CENTRAL PLAINS, AND MUCH OF THE EAST LATE
THIS WEEK. EXPECT COOLEST READINGS UP TO 10-15F BELOW NORMAL OVER
PARTS OF THE ROCKIES AND SOUTH-CENTRAL PLAINS WHERE
CLOUDS/RAINFALL SHOULD BE MOST PERSISTENT. THE GREAT LAKES/OHIO
VALLEY/APPALACHIANS SHOULD SEE HIGHS 5-10F OR SO BELOW NORMAL
BEHIND THE COLD FRONT PUSHING OFF THE EAST COAST ON THURSDAY.
TEMPERATURES SHOULD MODERATE SATURDAY-MONDAY, REACHING NEAR NORMAL
BY EARLY NEXT WEEK. CONTINUED HOT WEATHER WITH HIGHS UP TO 5-10F
ABOVE NORMAL WILL BE POSSIBLE OVER SOUTHERN TEXAS THROUGH 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 WHILE THE PACIFIC
NORTHWEST MAY START TO SEE A COOLER TREND BY NEXT MONDAY AS A
PACIFIC FRONT APPROACHES.
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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