517
FXUS02 KWBC 140659
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
259 AM EDT THU SEP 14 2023
VALID 12Z SUN SEP 17 2023 - 12Z THU SEP 21 2023
...HURRICANE LEE FORECAST TO TRACK AWAY FROM NEW ENGLAND AFTER
EARLY SUNDAY...
..OVERVIEW
CONTINUE TO FOLLOW NATIONAL HURRICANE CENTER ADVISORIES FOR THE
LATEST INFORMATION ON HURRICANE LEE, WHOSE EXTRATROPICAL
REFLECTION SHOULD BE TO THE EAST OF MAINE AS OF EARLY SUNDAY AND
TRACK AWAY FROM NEW ENGLAND THEREAFTER, LEADING TO A DRIER TREND
AND LIGHTER WINDS ACROSS THE REGION. BEHIND LEE, AN UPPER TROUGH
REACHING THE EASTERN U.S. BY SUNDAY-MONDAY WILL GRADUALLY LIFT OUT
INTO MIDWEEK. THE FRONT ASSOCIATED WITH THIS TROUGH MAY PRODUCE
SOME AREAS OF RAIN WITH VARYING INTENSITY DURING THE WEEKEND INTO
EARLY NEXT WEEK. A PERSISTENT WAVY FRONT SHOULD MAINTAIN A WET
PATTERN OVER THE FLORIDA PENINSULA FOR MOST OF THE PERIOD WHILE
TEXAS AND THE GULF COAST WILL LIKELY TREND DRIER AFTER THE
WEEKEND. OVER THE WEST, GUIDANCE CONTINUES TO ADVERTISE A
SIGNIFICANT PATTERN CHANGE AFTER INITIAL RIDGING WEAKENS AND
PROGRESSES EASTWARD, BUT OVER THE PAST COUPLE DAYS THERE HAS BEEN
SOME VARIABILITY IN HOW NEXT WEEK'S EXPECTED UPPER TROUGH EVOLVES.
THIS TROUGH AND LEADING COLD FRONT MAY BRING SOME MOISTURE INTO
THE PACIFIC NORTHWEST AND NORTHERN ROCKIES. ABOVE NORMAL
TEMPERATURES OVER THE INTERIOR NORTHWEST AND NORTHERN HIGH PLAINS
ON SUNDAY WILL SHIFT EASTWARD ACROSS THE NORTHERN TIER WHILE MOST
OF THE WEST SEES A PRONOUNCED COOLING TREND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
LEADING INTO THE START OF THE FORECAST PERIOD EARLY SUNDAY, MODELS
HAVE BEEN SHOWING SOME MODEST EAST-WEST VARIABILITY FOR THE TRACK
OF LEE IN RECENT RUNS BUT CURRENTLY THEY CLUSTER WELL IN SHOWING A
POSITION TO THE EAST OF MAINE AT THAT TIME. LEE WILL THEN
PROGRESS AWAY FROM NEW ENGLAND DURING THE DAY SUNDAY. FOR THE
TRAILING UPPER TROUGH REACHING THE EAST BY SUNDAY-MONDAY, RECENT
MODEL RUNS HAVE GENERALLY TRENDED TOWARD A FAIRLY OPEN DEPICTION
AS OPPOSED TO SOME EARLIER SOLUTIONS THAT HAD AN EMBEDDED UPPER
LOW OVER OR JUST NORTH OF THE GREAT LAKES. DETAILS ARE STILL
SOMEWHAT UNCERTAIN WITH SOME ROOM STILL EXISTING FOR A
WEAK/TRANSIENT EMBEDDED LOW DEPENDING ON EXACTLY HOW THE SHORTWAVE
ENERGY EVOLVES. AS THE TROUGH STARTS LIFTING AWAY, THE 12Z CMC
SHOWED A COMPACT UPPER LOW DROPPING INTO THE UPPER MIDWEST EARLY
NEXT WEEK. OTHER GUIDANCE DID NOT HAVE THIS FEATURE, SO THE CMC
WAS EXCLUDED FROM THE FORECAST. THE NEW 00Z CMC HAS AT LEAST
WEAKENED IT TO A FARTHER NORTH SHORTWAVE BUT IT STILL COMPARES
POORLY TO OTHER GUIDANCE.
MODELS AND ENSEMBLE MEANS ARE CONSISTENT IN THE GENERAL IDEA OF AN
UPPER TROUGH AMPLIFYING OVER THE WEST NEXT WEEK BUT THE
OPERATIONAL MODELS HAVE BEEN VARYING QUITE A BIT FOR THE DETAILS.
24-36 HOURS AGO THE MODELS WERE SUGGESTING THAT THE TROUGH
REACHING WESTERN NORTH AMERICA BY MONDAY WOULD BE WHAT ULTIMATELY
SETTLES OVER THE WEST. HOWEVER RUNS STARTING WITH THE 12Z/13
CYCLE HEAVE BEEN SHOWING A TRAILING COMPACT SHORTWAVE THAT ENDS UP
BEING THE FEATURE THAT DIGS INTO THE REGION AS AN UPSTREAM PACIFIC
RIDGE SHARPENS, ULTIMATELY LEADING TO A FAIRLY DEEP UPPER LOW OVER
OR NEAR THE NORTHERN GREAT BASIN BY NEXT THURSDAY. GFS/ECMWF/CMC
RUNS ALL SHOW THIS CLOSED LOW, SO IT SEEMS REASONABLE TO TREND THE
FORECAST TO THAT GENERAL IDEA BUT FOR NOW IN WEAKER FORM GIVEN THE
DETAIL INCONSISTENCY OVER RECENT DAYS. TELECONNECTIONS RELATIVE
TO STRONG POSITIVE HEIGHT ANOMALIES OVER OR NEAR EASTERN HUDSON
BAY/JAMES BAY BY THE END OF THE FORECAST PERIOD SUGGEST THAT
NEGATIVE ANOMALIES ASSOCIATED WITH THE WESTERN TROUGH SHOULD BE
FAIRLY MODEST. THIS COULD BE ACHIEVED EITHER BY THE UPPER LOW
BEING WEAKER OR BY BEING DEEP BUT PROGRESSIVE ENOUGH TO YIELD
MODEST ANOMALIES FROM A MULTI-DAY MEAN PERSPECTIVE.
THE ABOVE CONSIDERATIONS LED TO STARTING THE UPDATED FORECAST WITH
A COMPOSITE OF THE 12Z ECMWF/UKMET AND 12Z/18Z GFS EARLY IN THE
PERIOD AND THEN SHIFTING TOWARD 50-60 PERCENT TOTAL INPUT FROM THE
18Z GEFS/12Z ECENS MEANS BY DAYS 6-7 WEDNESDAY-THURSDAY.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAYS 4-5 EXCESSIVE RAINFALL OUTLOOKS COVERING SUNDAY THROUGH
MONDAY NIGHT SHOW NO RISK AREAS FOR ORGANIZED FLASH FLOOD
POTENTIAL. THE FLORIDA PENINSULA CONTINUES TO BE ONE REGION THAT
MERITS MONITORING THOUGH, WITH A FAIRLY WET PATTERN EXPECTED DUE
TO PERSISTENCE OF A WAVY FRONT. HOWEVER MOISTURE ANOMALIES ARE
ONLY MODERATELY ABOVE NORMAL, FLASH FLOOD GUIDANCE VALUES ARE
HIGH, AND MODELS DO NOT SHOW MUCH CLUSTERING FOR ANY RELATIVELY
HEAVIER RAIN THAT MAY FALL. SHORTER TERM GUIDANCE MAY ULTIMATELY
HELP TO REFINE ANY AREAS THAT COULD AT LEAST MERIT A MARGINAL
RISK. ON SUNDAY THERE WILL BE SOME LINGERING MOISTURE AND
INSTABILITY OVER SOUTHERN TEXAS AND THE GULF COAST REGION BUT NO
COHERENT SIGNAL FOR HEAVY RAINFALL, WHILE A COLD FRONT PUSHING
INTO THE APPALACHIANS/SOUTHEAST MAY PRODUCE SOME LOCALLY ENHANCED
RAINFALL BUT AVAILABLE MOISTURE LOOKS TO BE ONLY NEAR CLIMATOLOGY.
THE FLORIDA PENINSULA SHOULD CONTINUE TO SEE PERIODS OF RAIN
BEYOND MONDAY DUE TO THE WAVY FRONT, WITH HEAVIER AMOUNTS STILL
TENDING TO BE LOCALIZED. THE SOUTHEAST COAST MAY BE NEAR THE EDGE
OF RAINFALL THAT PERSISTS OVER THE WESTERN ATLANTIC. UPPER
TROUGHING EXPECTED TO AMPLIFY OVER THE WEST NEXT WEEK MAY BRING
SOME PRECIPITATION TO AT LEAST THE PACIFIC NORTHWEST AND NORTHERN
ROCKIES. DIFFERENCES IN GUIDANCE DETAILS OVER RECENT DAYS TEMPERS
CONFIDENCE IN SPECIFICS, THOUGH CURRENT SOLUTIONS WOULD SUGGEST
FAIRLY LIGHT TOTALS IN THE PACIFIC NORTHWEST WITH SOMEWHAT HIGHER
TOTALS POSSIBLE IN PARTS OF THE ROCKIES. HIGH ELEVATIONS IN THE
ROCKIES COULD SEE SOME SNOW IF THE UPPER TROUGH AND POSSIBLE
EMBEDDED LOW ARE DEEP ENOUGH. AHEAD OF THE WESTERN TROUGH/LOW,
AREAS OF RAIN WILL LIKELY DEVELOP OVER THE PLAINS BY
TUESDAY-THURSDAY. SOME RAINFALL COULD BE LOCALLY HEAVY BUT IT
WILL TAKE ADDITIONAL TIME TO DETERMINE WHICH AREAS MAY SEE THE
RELATIVELY HIGHER TOTALS.
AN AREA OF HIGH TEMPERATURES UP TO 10-15F OR SO ABOVE NORMAL OVER
THE INTERIOR NORTHWEST INTO THE HIGH PLAINS ON SUNDAY (WITH
BROADER COVERAGE OF WARM LOWS OVER THE WEST) WILL PROGRESS
EASTWARD WITH TIME, MODERATING SOME BY THE TIME THIS WARMTH
REACHES THE UPPER MIDWEST/WESTERN GREAT LAKES BY NEXT
WEDNESDAY-THURSDAY. THE UPPER TROUGH FORECAST TO AMPLIFY OVER THE
WEST NEXT WEEK SHOULD PROMOTE A COOLING TREND OVER THE REGION,
WITH A FAIRLY BROAD AREA OF HIGHS 5-15F BELOW NORMAL BY NEXT
THURSDAY. OTHERWISE EXPECT MOST TEMPERATURE ANOMALIES TO BE IN
THE SINGLE DIGITS, INCLUDING THE EAST LEANING A LITTLE BELOW
NORMAL SUNDAY-MONDAY AND THE SOUTHERN PLAINS/CENTRAL GULF COAST
TENDING TO BE SLIGHTLY ABOVE NORMAL AFTER SUNDAY.
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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