Preliminary Forecasts
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190
FXUS02 KWNH 280624
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
224 AM EDT SUN AUG 28 2016

VALID 12Z WED AUG 31 2016 - 12Z SUN SEP 04 2016

...OVERVIEW AND GUIDANCE/UNCERTAINTY ASSESSMENT...

MOST GUIDANCE EXPECTS THE UPR TROUGH SETTLING OVER THE WEST COAST
STATES TO BROADEN SOMEWHAT WITH TIME AS A CNTRL U.S. RIDGE AND ERN
CANADA/NERN U.S. TROUGH MAKE STEADY EWD PROGRESS.  TELECONNECTIONS
RELATIVE TO THE ERN PAC MEAN RIDGE SUPPORT THE WRN TROUGH AS WELL
AS THE HGT RISES FCST OVER THE EAST BY NEXT WEEKEND.

EVOLUTION OVER THE GULF OF MEXICO AND THE SOUTHEAST/FLORIDA
CONTINUES TO BE THE MOST UNCERTAIN PART OF THE FCST.  GUIDANCE AS
A WHOLE APPEARS TO HAVE TRENDED SOMEWHAT BETTER DEFINED WITH GULF
OF MEXICO LOW PRESSURE COMPARED TO 12-24 HRS AGO.  THE FULL
MODEL/ENSEMBLE ENVELOPE HAS REMAINED QUITE BROAD FOR TRACK BUT
WITH CONTINUED GREATER FOCUS ON THE ERN HALF OF THE GULF.  NEW 00Z
GUIDANCE INCLUDING THE GFS/GEFS MEAN/CMC/UKMET SHOW SOMEWHAT
BETTER CLUSTERING FOR TRACK INTO DAY 4 THU COMPARED TO MOST PRIOR
CYCLES.  CONSULT THE NATIONAL HURRICANE CENTER TROPICAL WEATHER
OUTLOOK FOR LATEST INFO REGARDING THIS FEATURE THAT IS CURRENTLY
BTWN THE BAHAMAS AND CUBA.

FOR THE WEST COAST MEAN TROUGH WHOSE AXIS MAY START TO DRIFT
INLAND DURING NEXT WEEKEND... HANDLING THE INDIVIDUAL EMBEDDED
COMPONENTS IS THE PRIMARY FCST PROBLEM.  OVERALL THE MODELS/MEANS
HAVE TRENDED A LITTLE LESS AMPLIFIED WITH THE TROUGH THUS ALLOWING
HIGHER POTENTIAL FOR LEADING ENERGY TO EJECT DOWNSTREAM.  THE 12Z
ECMWF DETAILS ARE A QUESTION MARK BUT THE 18Z/12Z GFS RUNS APPEAR
PARTICULARLY AGGRESSIVE IN BRINGING HGT FALLS ACROSS SRN CANADA
AND NRN TIER CONUS FROM LATE THU ONWARD.  ON THE OTHER HAND GIVEN
RECENT TRENDS THE 18Z GEFS MEAN MAY BE UNDERDONE IN REFLECTING
SHRTWV ENERGY THAT MAY EJECT ACROSS SRN CANADA.  THROUGH DAY 7 SUN
THE ENSEMBLE MEANS ARE FAIRLY SIMILAR WITH THE REMAINING TROUGH
THAT BEGINS TO MOVE INLAND.

MODELS/ENSEMBLES STILL EXHIBIT SOME DEGREE OF SPREAD FOR DETAILS
OF THE NERN TROUGH THAT ULTIMATELY PASSES INTO THE ATLC.  IMPACT
OF THESE DIFFS ON THE SFC PATTERN APPEARS MODEST.

A BLEND INCLUDING THE 18Z GFS/GEFS MEAN AND 12Z ECMWF/ECMWF MEAN
REFLECTS THE MOST COMMON ASPECTS OF GUIDANCE DAYS 3-4 WED-THU.
THE BLEND DOWNPLAYS THE FARTHER SWWD 12Z ECMWF EVOLUTION OVER THE
GULF.  PREFS FOR THE ENERGY THAT MAY EJECT FROM THE WRN TROUGH AND
REASONABLE AGREEMENT DOWNSTREAM LEAD TO TRANSITIONING THE BLEND
TOWARD THE 12Z NAEFS/ECMWF MEANS BY NEXT WEEKEND.


...WEATHER HIGHLIGHTS/THREATS...

DEPENDING ON EVOLUTION OF THE POTENTIAL GULF OF MEXICO FEATURE
THERE MAY BE SOME HVY RNFL OVER PORTIONS OF THE ERN HALF OF THE
GULF COAST AND FLORIDA WITH LOW CONFIDENCE IN THE NWD EXTENT OF
THE RNFL SHIELD.  COLD FRONT PUSHING SEWD FROM THE GRTLKS/PLAINS
MAY BE ACCOMPANIED BY SOME SHWRS/TSTMS AND ONCE REACHING THE
SOUTHEAST COULD INTERACT WITH THE GULF FEATURE IN SOME FASHION.
WEST COAST UPR TROUGH WILL SUPPORT PERIODS OF RNFL OVER THE PAC
NW.  LATE IN THE PERIOD THE LEADING COLD FRONT EMERGING INTO THE
NRN PLAINS MAY PROMOTE CONVECTION OVER PARTS OF THE NRN-CNTRL
PLAINS AND UPR MS VLY.  EXPECT BELOW NORMAL DAYTIME TEMPS OVER THE
WEST COAST STATES TO EXPAND EWD BY FRI-SUN.  IN THE WARM SECTOR
THE NRN HIGH PLAINS AND VICINITY MAY SEE TEMPS REACH 10-15F ABOVE
NORMAL WED-FRI.  MEANWHILE COLD FRONT AND TRAILING HIGH PRESSURE
OVER THE EAST SHOULD RESULT IN ERN TEMPS TRENDING TO NEAR/BELOW
NORMAL LEVELS AFTER STARTING OUT WELL ABOVE NORMAL ON WED.

RAUSCH

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