Prognostic Meteorological Discussion
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FXUS02 KWBC 210650
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
249 AM EDT THU SEP 21 2017

VALID 12Z SUN SEP 24 2017 - 12Z THU SEP 28 2017

...OVERVIEW/GUIDANCE EVALUATION/PREFERENCES...

MODELS/ENSEMBLES ARE FAIRLY AGREEABLE AND CONSISTENT WITH THE
LARGE SCALE FLOW EVOLUTION, SHOWING A GRADUAL EROSION OF THE
INITIALLY DEEP WESTERN TROUGH/STRONG EASTERN RIDGE PATTERN.  BY
THE LATTER HALF OF THE PERIOD A RIDGE WILL BEGIN TO BUILD INTO THE
WEST WHILE A DOWNSTREAM TROUGH SHOULD EXTEND SOUTHWESTWARD FROM
THE UPPER MS VALLEY/GREAT LAKES, LEADING INTO WHAT SHOULD BECOME
AN EASTERN U.S. TROUGH BEYOND THE END OF THE FORECAST PERIOD.

FOR ABOUT THE FIRST HALF OF THE PERIOD THE 12Z-18Z GFS/12Z ECMWF
AND THEIR ENSEMBLE MEANS AGREE FAIRLY WELL WITH MOST FORECAST
DETAILS OVER THE LOWER 48, LEADING TO THOSE SOLUTIONS (MORE WEIGHT
TO THE OPERATIONAL RUNS) SERVING AS THE STARTING POINT FOR THE
UPDATED FORECAST BLEND.  SOME ENSEMBLES DIFFER FROM THIS CONSENSUS
FOR SOME ASPECTS OF EJECTING WESTERN TROUGH ENERGY AND CANADIAN
ENERGY THAT COMBINE TO SUPPORT ONE OR MORE NORTHERN PLAINS INTO
SOUTHERN CANADA SURFACE WAVES.  HOWEVER THE MORE STRONGLY
DEVELOPED WAVES IN RECENT UKMET AND CMC RUNS ARE EITHER NEAR THE
FRINGE OR OUTSIDE OF THE ENSEMBLE SURFACE LOW ENVELOPE AND THUS
LOWER PROBABILITY SOLUTIONS.  TOWARD MIDWEEK 12Z-18Z GFS RUNS
BEGIN TO BRING EASTERN PACIFIC HEIGHT FALLS MORE AGGRESSIVELY INTO
THE WEST COAST RIDGE VERSUS MOST OTHER GUIDANCE.  THIS IS SOMEWHAT
MORE PRONOUNCED IN THE 18Z GFS, WHILE FARTHER EAST THE 12Z GFS
STRAYS TOWARD A NEUTRAL TILT TROUGH HEADING INTO THE GREAT
LAKES/NORTHEAST BY DAY 7 THU IN CONTRAST TO THE CONSENSUS FORECAST
OF A SLOWER POSITIVELY TILTED TROUGH.  OVERALL THE NEW 00Z GFS
SEEMS TO PROVIDE MEANINGFUL IMPROVEMENT OVER THE 12Z/18Z RUNS.
THE 12Z ECMWF COMPARES FAVORABLY TO THE ENSEMBLE MEANS OVER MOST
OF THE EASTERN PACIFIC/NORTH AMERICA DOMAIN TO THE END OF THE
FORECAST PERIOD.  THE ONE EXCEPTION WOULD BE FOR LINGERING
SOUTHWEST INTO SOUTHERN ROCKIES TROUGHING, WITH THE ECMWF TRENDING
A LITTLE FASTER THAN CONSENSUS.  THE 18Z GFS LOOKED A LITTLE FAST
AS WELL, WITH THE NEW 00Z VERSION ADJUSTING CLOSER TO THE MEANS.
THE LATTER HALF OF THE FORECAST FAVORED THE 18Z GEFS/12Z ECMWF
MEANS ALONG WITH MINORITY INPUT FROM THE 12Z ECMWF.  THIS
CONSENSUS, ALONG WITH THE 18Z/00Z GFS RUNS THAT ARE SIMILAR IN
PRINCIPLE, REPRESENTS A SLIGHTLY SLOWER TREND FOR THE GREAT LAKES
INTO SOUTHEAST CANADA WAVE AND TRAILING COLD FRONT.

REGARDING TROPICAL SYSTEMS OVER THE WESTERN ATLANTIC, T.S. JOSE
SHOULD GRADUALLY WEAKEN AS IT MEANDERS SOUTH OF NEW ENGLAND AND
POSSIBLY DISSIPATE BY AROUND THE MIDDLE OF THE PERIOD.  HURRICANE
MARIA IS MOST LIKELY TO TRACK NORTHWARD A LITTLE WEST OF 70W
LONGITUDE FOR MOST OF THE PERIOD.  MARIA SHOULD BEGIN TO RECURVE
NORTHEASTWARD AROUND OR JUST AFTER THE END OF THE FORECAST PERIOD
AS THE UPSTREAM TROUGH ALOFT APPROACHES.  AS HAS BEEN THE CASE IN
RECENT DAYS, MODELS/ENSEMBLE MEMBERS SHOW INCREASING TRACK
DIVERGENCE MID-LATE PERIOD WITH SIGNIFICANT RUN TO RUN VARIABILITY
FOR LONGITUDE OF THE TRACK IN PARTICULAR.  OF NOTE IN LATEST
GUIDANCE THE 00Z CMC HAS TRENDED CLOSER TO OTHER GUIDANCE AFTER
THE 12Z RUN HAD BEEN ON THE WEST SIDE OF THE ENVELOPE ALONG THE
EAST COAST.  THE INITIAL BLEND WAS ADJUSTED TO CONFORM TO 03Z NHC
ADVISORIES FOR THE ABOVE SYSTEMS ALONG WITH BEST EXTRAPOLATION FOR
MARIA TO DAY 7.


...SENSIBLE WEATHER/HIGHLIGHTS...

THE LIKELY HEAVY RAINFALL EVENT OVER THE CENTRAL U.S. SHOULD HAVE
ITS INITIAL FOCUS FROM THE CENTRAL PLAINS INTO THE UPPER MIDWEST
IN ASSOCIATION WITH PERSISTENT SOUTH-SOUTHWESTERLY FLOW ALOFT, ONE
OR MORE SURFACE WAVES, AND ANOMALOUSLY HIGH MOISTURE.  THEN THE
SOUTHERN PLAINS SHOULD SEE THE HIGHEST RAINFALL POTENTIAL AS THE
SOUTHERN PART OF THE WESTERN TROUGH ALOFT MERELY DRIFTS ALONG
WHILE THE NORTHERN PORTION BECOMES MORE PROGRESSIVE.  ALONG WITH
THE DYNAMICS/MOISTURE, A PERIOD OF LOW LEVEL UPSLOPE FLOW MAY HELP
TO ENHANCE THE SOUTHERN PLAINS ACTIVITY.  SOME EMBEDDED CONVECTION
MAY BE STRONG TO SEVERE.  CONSULT SPC OUTLOOKS FOR LATEST DETAILS
ON SEVERE POTENTIAL AS DETAILS COME INTO BETTER FOCUS.  ON THE
WESTERN FRINGE OVER THE OVERALL PRECIP AREA, THE CENTRAL ROCKIES
WILL TREND DRIER WITH TIME BUT THE SOUTHERN ROCKIES WILL SEE MORE
PERSISTENT ACTIVITY.  LOCATIONS ALONG THE EAST COAST SHOULD
CONTINUE TO MONITOR FORECASTS FOR HURRICANE MARIA, WITH IMPACTS
SENSITIVE TO THE EXACT TRACK.  AT THE VERY LEAST ROUGH SEAS AND
DANGEROUS RIP CURRENTS APPEAR LIKELY WHILE A TRACK IN THE WESTERN
HALF OF THE SPREAD WOULD BRING SOME RAIN/STRONGER WINDS TO
LOCATIONS FROM THE EASTERN CAROLINAS NORTHWARD.

EXPECT THE MOST EXTREME TEMPERATURE ANOMALIES SUN-TUE WITH HIGHS
AT LEAST 10-20F BELOW NORMAL FROM THE INTERIOR WEST INTO THE
NORTHERN-CENTRAL PLAINS AND A SIMILAR MAGNITUDE ABOVE NORMAL FROM
THE MIDWEST INTO THE NORTHEAST.  A FEW DAILY RECORD HIGHS/WARM
LOWS MAY BE POSSIBLE IN THE WARM SECTOR EARLY IN THE PERIOD.
LATER IN THE PERIOD THE BELOW NORMAL HIGHS SHOULD BE MOST
PREVALENT OVER THE ROCKIES AND SOUTHERN PLAINS WITH MORE MODEST
COOL ANOMALIES OVER THE REMAINDER OF THE CENTRAL U.S..  THE UPPER
RIDGE BUILDING TOWARD THE WESTERN U.S. WILL SUPPORT A TREND TO
ABOVE NORMAL TEMPS OVER THE WEST COAST STATES.

RAUSCH

$$





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