Prognostic Meteorological Discussion
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FXUS02 KWBC 220636
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
235 AM EDT TUE AUG 22 2017

VALID 12Z FRI AUG 25 2017 - 12Z TUE AUG 29 2017


...GUIDANCE EVALUATION/PREFERENCES...

MODELS CONTINUE TO SHOW GOOD AGREEMENT WITH AN UPPER TROUGH AND
EMBEDDED CIRCULATION STARTING IN ALBERTA FRI 25 AUG AND
PROGRESSING TO THE CANADIAN PRAIRIES SAT.  PART OF THE SHORTWAVE
ENERGY DIVES SOUTHEAST ON THE BACK SIDE OF THE HIGH AMPLITUDE
WESTERN ANTICYCLONE INTO UPPER MS VALLEY.

THE 1Z GFS HAS MOVED THIS UPPER TROUGH EAST FASTER THAN THE
MAJORITY OF THE SUITE OF MODELS/ENSEMBLES AND THE 18Z GFS HAS
SLOWED FORWARD MOTION CONSIDERABLY TOWARDS THE 12Z GEFS MEAN AND
12Z ECMWF ENSEMBLE MEAN.
THE ECMWF CONTINUES TO SHOW RUN TO RUN VARIABILITY WITH ONE RUN
PROGRESSING WITH AN OPEN WAVE AND ALTERNATE RUNS SHOWING AN
EMBEDDED CLOSED LOW DEVELOPING.  GIVEN THE FLIP FLOPPING IN THE
OPERATIONAL RUNS...LOW WEIGHT OF EACH OF THE LAST 2 RUNS WAS
USED...PLUS MORE WEIGHTING TO THE 12Z ECMWF ENSEMBLE
MEAN...PARTICULARLY GIVEN THE EXCELLENT OVERLAP WITH THE 12Z GEFS
MEAN.

THE GFS/ECMWF AND THEIR ENSEMBLES/MEANS CONTINUE TO SHOW
DEVELOPMENT OF A GREAT BASIN TO SOUTHWEST US DEEP LAYER
ANTICYCLONE...SO THE REGION WILL BE WATCHED FOR DEVELOPMENT OF
ANOTHER HEAT WAVE.

THE MODELS SHOW THE REMNANTS OF TROPICAL STORM HARVEY WILL CROSS
THE BAY OF CAMPECHE AND MOVE GENERALLY WEST/NORTHWEST INTO MEXICO
OR SOUTHERN TEXAS. THE 18Z-00Z GFS AND 12Z ECMWF/00Z UKMET MOVE
THE CIRCULATION INTO TEXAS ALTHOUGH THE TRACKS DIVERGE.  THE
NATIONAL HURRICANE CENTER IS ISSUING TROPICAL WEATHER OUTLOOKS
COVERING THIS POTENTIALLY REDEVELOPING SYSTEM.

THE UPPER LEVEL TROUGH INITIALLY OVER THE NORTHEAST IS SLOW TO
MOVE OFFSHORE THIS WEEKEND.
TYPICAL TIMING/PHASING DIFFERENCES EXIST WITH THE UPPER
TROUGH...WITH A MULTI-MODEL CONSENSUS USED TO ADDRESS DIFFERENCES.
SECONDARY WAVES ROUNDING THE BASE OF THE TROUGH EXCITE WAVE
DEVELOPMENT OVER FLORIDA THIS WEEKEND...WITH THE WAVES MOVING
ALONG THE QUASI STATIONARY FRONT OFF THE SOUTHEAST COAST EARLY
NEXT WEEK. THE WAVE POSITIONS ARE SPREAD QUITE A DISTANCE ALONG
THE FRONTAL BOUNDARY...SO THERE IS BETTER CONFIDENCE ON THE
FRONTAL AXIS THAN WHERE ANY PARTICULAR WAVE OF LOW PRESSURE IS.

FOR ALL SYSTEMS...THE WPC FORECAST USES A BLEND OF THE OPERATIONAL
18Z GFS/12Z ECMWF WITH THE 12Z GEFS/ECMWF ENSEMBLE MEANS WITH MORE
ENSEMBLE MEAN WEIGHTING THROUGH THE FORECAST...GIVEN LESS RUN TO
RUN VARIABILITY IN THE MEANS AND AGREEMENT AMONG THE MEANS.

...SENSIBLE WEATHER HIGHLIGHTS...

THE REMNANTS OF TROPICAL STORM HARVEY HAVE A 70 PERCENT CHANCE OF
BECOMING A TROPICAL CYCLONE IN THE LATEST OUTLOOK FROM THE
NATIONAL HURRICANE CENTER(NHC).  WITH TRENDS BRINGING THE
CIRCULATION IN THE VICINITY OF THE TEXAS COAST...MULTIPLE MODELS
NOW PRODUCE A HEAVY RAIN EVENT IN THE VICINITY OF THE TEXAS
COAST...INCLUDING THE 12Z ECMWF/00Z GFS/00Z UKMET.  MANUAL QPF
USED THE MORE CONSERVATIVE GEFS MEAN QPF UNTIL BETTER AGREEMENT
DEVELOPS...AS THE ECMWF/GFS/UKMET HEAVY RAIN AREAS DID NOT OVERLAP
WELL.
SEE THE FORECASTS/TROPICAL WEATHER OUTLOOKS FROM THE NHC ON THE
CHANCES OF HARVEY REDEVELOPING. MOISTURE INTERACTING WITH THE
LINGERING FRONT ALONG THE GULF COAST COULD PRODUCE OTHER RAINS OF
CONCERN FOR FLASH FLOOD/RIVER FLOOD POTENTIAL.

EXPECT SHOWERS AND THUNDERSTORMS TO FOCUS ACROSS THE SOUTHEAST AND
THE GULF COASTS ALONG A LINGERING STATIONARY FRONTAL BOUNDARY.
FLORIDA SHOULD ALSO SEE AN INCREASE IN RAINFALL AS A TROPICAL WAVE
INTERACTS WITH THIS FRONTAL BOUNDARY...AND WAVES OF LOW PRESSURE
DEVELOP ALONG THE FRONT. TO THE NORTH...SHOWERS AND THUNDERSTORMS
WILL DEVELOP ACROSS THE CENTRAL U.S. AND THE NORTHERN PLAINS/UPPER
MIDWEST AHEAD OF UPPER TROUGHING.  SHOWERS AND THUNDERSTORMS
SHOULD ALSO DEVELOP ACROSS THE SOUTHERN ROCKIES LATE IN THE WEEK
WITH RETURN FLOW REACHING THE SOUTHERN PLAINS BY THE WEEKEND.

REGARDING TEMPERATURES ACROSS THE COUNTRY...WITH THE UPPER TROUGH
ACROSS THE GREAT LAKES/OHIO VALLEY/NORTHEAST...TEMPERATURES WILL
BE SEVERAL DEGREES COOLER THAN NORMAL LATER THIS WEEK AND THROUGH
THE WEEKEND. STRONG RIDGING OUT WEST WILL ALSO LEAD TO WARMER THAN
AVERAGE TEMPERATURES ACROSS MUCH OF THE WESTERN U.S. AND THE GREAT
BASIN/SOUTHWEST...EXPANDING INTO THE NORTHERN ROCKIES EARLY NEXT
WEEK.

PETERSEN/SANTORELLI



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