258
FXUS02 KWBC 130700
PMDEPD
EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
300 AM EDT WED SEP 13 2023
VALID 12Z SAT SEP 16 2023 - 12Z WED SEP 20 2023
...HURRICANE LEE LIKELY TO BRING RAIN AND STRONG WINDS TO NEW
ENGLAND...
..OVERVIEW
LATEST SOLUTIONS CONTINUE TO SHOW TYPICAL SPREAD AND RUN-TO-RUN
VARIABILITY FOR HURRICANE LEE, SLOWLY NARROWING IN ON A TRACK THAT
SHOULD REACH SOMEWHERE BETWEEN EASTERN MAINE AND NOVA SCOTIA BY
LATE SATURDAY. BY THAT TIME THE 03Z NATIONAL HURRICANE CENTER
ADVISORY SHOWS LEE TRANSITIONING TO EXTRATROPICAL STATUS NEAR THE
FAR WESTERN COAST OF NOVA SCOTIA. THIS TRACK WOULD BRING STRONG
WINDS TO NEW ENGLAND AND SIGNIFICANT RAINFALL TO MAINE. BEHIND
LEE, AN UPPER TROUGH AMPLIFYING INTO THE EASTERN U.S. DURING
SATURDAY-MONDAY SHOULD WEAKEN TOWARD 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. MEANWHILE THE
GUIDANCE IS ADVERTISING A SIGNIFICANT PATTERN CHANGE OVER THE
WESTERN U.S. DURING THE PERIOD, AS A STRONG UPPER RIDGE OVER THE
REGION AS OF SATURDAY WEAKENS AND GIVES WAY TO AN AMPLIFYING
TROUGH THAT MAY ALSO INCORPORATE A WEAK UPPER LOW NEARING
CALIFORNIA. THIS TROUGH AND LEADING COLD FRONT MAY BRING SOME
MOISTURE INTO THE PACIFIC NORTHWEST AND NORTHERN ROCKIES NEXT
WEEK. ABOVE NORMAL TEMPERATURES OVER THE PACIFIC NORTHWEST AND
NORTHERN HIGH PLAINS THIS WEEKEND WILL THEN SHIFT OVER MORE OF THE
NORTHERN PLAINS WHILE MUCH OF THE WEST SEES A PRONOUNCED COOLING
TREND.
..GUIDANCE EVALUATION/PREDICTABILITY ASSESSMENT
FOR HURRICANE LEE, MODEL RUNS CONTINUE THEIR TYPICAL RUN-TO-RUN
WOBBLING FOR TRACK AND TIMING WITHIN A GRADUALLY NARROWING
ENVELOPE. FOR THE UPDATED FORECAST BASED ON 12Z/18Z GUIDANCE, THE
12Z/18Z GFS AND GEFS MEAN WERE GENERALLY CLOSEST TO THE TRACK
DEPICTED BY THE 03Z NHC ADVISORY. THE 12Z ECMWF/ECMWF MEAN WERE A
LITTLE FARTHER WEST BUT THE ECMWF REPRESENTED AN EASTWARD
ADJUSTMENT FROM ITS 00Z/12 RUN THAT WAS A WESTERN EXTREME, AND THE
NEW 00Z ECMWF HAS COME IN CLOSE TO THE 12Z RUN. THE NEW 00Z
GFS/UKMET HAVE ADJUSTED A LITTLE WESTWARD. THE UPPER TROUGH
AMPLIFYING INTO THE EAST DURING DURING SATURDAY-MONDAY STILL
EXHIBITS SOME EMBEDDED LOW-PREDICTABILITY DETAIL DIFFERENCES
WITHIN A MORE AGREEABLE MEAN EVOLUTION. THE MAJORITY OF GUIDANCE
IS SUGGESTING THAT AN EMBEDDED UPPER LOW MAY TRACK OVER OR A
LITTLE NORTH OF THE UPPER GREAT LAKES, LEANING AWAY FROM SOME
EARLIER GUIDANCE RUNS THAT INDICATED POTENTIAL FOR A FARTHER SOUTH
TRACK. GUIDANCE AGREES ON WEAKENING THE TROUGH AFTER MONDAY BUT
WITH CONTINUED UNCERTAINTY FOR DETAILS.
THE FORECAST OVER THE WEST BY THE LATTER HALF OF THE PERIOD HAS
EXHIBITED A WIDE RANGE OF CONTINUITY OR TRENDS OVER THE PAST
COUPLE DAYS DEPENDING ON THE GUIDANCE SOURCE. OVERALL THE
ENSEMBLE MEANS HAVE BEEN STEADIER WITH THE CURRENT FORECAST OF
UPPER TROUGHING BY MIDWEEK BUT LATEST RUNS ARE SHOWING A STRONGER
NORTHERN STREAM COMPONENT OF FLOW AND GEFS MEANS HAVE BEEN CLOSER
TO CURRENT IDEAS. GFS RUNS HAVE VARIED BUT SHOWED MORE SIGNAL FOR
WESTERN TROUGHING THAN ECMWF RUNS WHICH HAVE ONLY VERY RECENTLY
SWITCHED TO AN AMPLIFIED WESTERN TROUGH VERSUS FLAT MEAN FLOW
CROSSING CANADA. WHILE THE GFS/GEFS APPEAR TO HAVE LED THE TREND,
THE 18Z GFS IN PARTICULAR LOOKS EXCESSIVELY DEEP/SOUTH WITH ITS
UPPER LOW. RESULTING NEGATIVE HEIGHT ANOMALIES SEEM EXTREME
RELATIVE TO TELECONNECTIONS BASED ON THE STRONG POSITIVE ANOMALIES
FORECAST OVER OR NEAR EASTERN HUDSON BAY IN THE D+8 MULTI-DAY
MEANS. THE 12Z GFS AND NEW 00Z RUN AT LEAST SEEM MORE REASONABLE
WITH A MORE NORTHWARD AND TRANSIENT UPPER LOW. THE 00Z GFS COULD
STILL BE TOO DEEP AND OVERDOING HEIGHT FALLS REACHING THE PLAINS.
FORECAST CONSIDERATIONS LED TO THE STARTING BLEND CONSISTING OF A
COMPOSITE OF 12Z/18Z MODELS EARLY IN THE PERIOD WITH SOME SLIGHT
ADJUSTMENTS FOR THE DEPICTION OF LEE, FOLLOWED BY A TRANSITION
TOWARD MORE ENSEMBLE INPUT. DAY 7 WEDNESDAY INCORPORATED 60
PERCENT TOTAL 18Z GEFS/12Z ECENS MEANS AND LINGERING COMPONENTS OF
THE 12Z ECMWF/GFS/CMC.
..WEATHER/HAZARDS HIGHLIGHTS
THE DAY 4 EXCESSIVE RAINFALL OUTLOOK COVERING THE
SATURDAY-SATURDAY NIGHT PERIOD DEPICTS A SLIGHT RISK AREA OVER
APPROXIMATELY THE EASTERN HALF OF MAINE ALONG WITH A BAND OF
MARGINAL RISK TO THE WEST, REFLECTING THE POTENTIAL FOR HEAVY
RAINFALL IN ASSOCIATION WITH LEE. SOIL CONDITIONS IN MAINE ARE ON
THE WETTER SIDE OF NORMAL OVER MOST AREAS (NEAR AVERAGE OVER PARTS
OF THE SOUTH), SO EXPECT SOME SENSITIVITY TO HEAVY RAINFALL.
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. THE DAY
4 OUTLOOK CONTINUES TO BE CLEAR OF ANY OTHER RISK AREAS DUE TO
UNCERTAINTY IN RAINFALL AMOUNTS/RATES, BUT WITH ONGOING REGIONS OF
INTEREST. THESE INCLUDE SOUTHERN TEXAS AND THE GULF COAST (SOME
LINGERING MOISTURE AND INSTABILITY BUT INCONSISTENT GUIDANCE
SIGNALS FOR RAINFALL) ALONG WITH THE TENNESSEE VALLEY AND
SURROUNDING AREAS (WIDE RANGE OF GUIDANCE QPFS AND MIXED SIGNALS
FOR MOISTURE ANOMALIES OVER THE REGION), PLUS THE FLORIDA
PENINSULA WHERE A WAVY FRONT WILL BE DRAPED ACROSS THE AREA SHOULD
PROMOTE A WET PATTERN (BUT WITH DRIER THAN AVERAGE SOIL CONDITIONS
TO START WITH AND WIDE GUIDANCE SPREAD FOR QPF). NO RISK AREAS
ARE INDICATED ON THE DAY 5 ERO (SUNDAY-SUNDAY NIGHT) BUT WILL
CONTINUE TO MONITOR THE FLORIDA PENINSULA AND PERHAPS THE SOUTHERN
APPALACHIANS AS A COLD FRONT REACHES THE AREA.
EXPECT THE FLORIDA PENINSULA TO REMAIN IN A FAIRLY WET PATTERN
BEYOND SUNDAY AS THE WAVY FRONT PERSISTS OVER THE AREA, BUT WITH
NO CLEAR SIGNAL YET FOR ORGANIZED HEAVY RAINFALL. RECENT TRENDS
FOR A DEEPER UPPER TROUGH TO REACH THE WEST NEXT WEEK HAVE RAISED
THE POTENTIAL PRECIPITATION TOTALS, WITH BEST EMPHASIS MOST LIKELY
TO BE FROM THE PACIFIC NORTHWEST INTO THE NORTHERN ROCKIES. SOME
HIGH ELEVATIONS IN THE ROCKIES COULD SEE SOME SNOW IF THE UPPER
TROUGH IS DEEP ENOUGH. AHEAD OF THIS TROUGH, AREAS OF RAIN COULD
DEVELOP OVER PARTS OF THE PLAINS BY AROUND THE MIDDLE OF NEXT WEEK.
EXPECT AN AREA OF HIGH TEMPERATURES AT LEAST 10-15F ABOVE NORMAL
OVER THE NORTHWEST U.S./NORTHERN HIGH PLAINS DURING THE WEEKEND,
WITH A BROADER AREA OF ABOVE NORMAL LOWS OVER THE WEST. THEN AN
UPPER TROUGH AMPLIFYING INTO THE WEST SHOULD BRING A PRONOUNCED
COOLER TREND WITH MUCH OF THE WESTERN U.S. SEEING HIGHS OF 5-15F
BELOW NORMAL BY NEXT TUESDAY-WEDNESDAY, WHILE THE INITIAL WARMTH
EVENTUALLY MODERATES BY MIDWEEK AS IT PROGRESSES ACROSS THE
NORTHERN PLAINS. THE SOUTH-CENTRAL PLAINS WILL START OUT THE
WEEKEND WITH HIGHS SEVERAL DEGREES BELOW NORMAL FOLLOWED BY A
WARMING TREND OVER MUCH OF THE SOUTHERN PLAINS. THE EAST SHOULD
SEE NEAR TO SOMEWHAT BELOW NORMAL HIGHS FROM THE WEEKEND INTO
EARLY NEXT WEEK AND THEN NEAR NORMAL TEMPERATURES BY THE MIDDLE OF
NEXT WEEK.
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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