Tropical Storm ERIKA

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TROPICAL STORM ERIKA DISCUSSION NUMBER   3
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL062009
500 AM AST WED SEP 02 2009
 
AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT HAS BEEN SEARCHING
FOR THE CENTER OF THE TROPICAL STORM OVER THE PAST FEW HOURS.  THE
CIRCULATION HAS BECOME QUITE DISORGANIZED...BUT IT APPEARS THAT THE
CENTER HAS REFORMED TO THE SOUTHWEST OF THE PREVIOUSLY ESTIMATED
TRACK.  HOWEVER THE FLIGHT-LEVEL WINDS SUGGEST THAT THERE MAY BE
MULTIPLE CENTERS...SO THE ADVISORY POSITION IS HIGHLY UNCERTAIN AND
PROBABLY REPRESENTS A MEAN CENTER...OR THE LOCATION OF THE
LOW-LEVEL VORTICITY MAXIMUM.  THE INITIAL MOTION...270/4...IS OF
COURSE ALSO HIGHLY UNCERTAIN.  TO MAINTAIN SOME CONTINUITY FROM THE
PREVIOUS FORECAST AND FOLLOWING THE TRACK GUIDANCE...A WEST-
NORTHWESTWARD TRACK IS PREDICTED TO RESUME.  ERIKA SHOULD BE
STEERED BY THE FLOW TO THE SOUTH OF A WEAK SUBTROPICAL RIDGE FOR A
COUPLE OF DAYS.  LATER IN THE FORECAST PERIOD...DYNAMICAL GUIDANCE
SHOWS A MORE NORTHWESTWARD MOTION FOR A STRONGER CYCLONE...E.G. THE
GFDL AND HWRF...AND A CONTINUED WEST-NORTHWESTWARD MOTION FOR A
WEAKER SYSTEM...E.G. THE GFS.  THIS IS A LOW-CONFIDENCE TRACK
FORECAST.
 
DATA FROM THE AIRCRAFT INDICATE THAT ERIKA HAS WEAKENED A LITTLE...
WHICH IS NOT SURPRISING CONSIDERING THE DISORGANIZED APPEARANCE OF
THE SYSTEM ON SATELLITE IMAGES.  MODERATE WESTERLY SHEAR HAS BEEN
DISRUPTING THE CYCLONE BUT THE SHEAR IS NOT EXPECTED TO BE STRONG
ENOUGH TO PREVENT SOME STRENGTHENING DURING THE NEXT DAY OR SO.
THEREAFTER...AN UPPER-TROPOSPHERIC TROUGH FARTHER TO THE WEST IS
LIKELY TO IMPART INCREASING SOUTHWESTERLY SHEAR ON ERIKA...WHICH
SHOULD INDUCE WEAKENING.  THE NHC INTENSITY FORECAST IS VERY
SIMILAR TO THE PREVIOUS OFFICIAL FORECASTS...BUT BELOW THE
CONSENSUS OF THE NUMERICAL INTENSITY GUIDANCE.  IT SHOULD BE NOTED
THAT SOME OF THESE NUMERICAL MODELS...SUCH AS THE HWRF AND GFDL...
HAVE SHOWN A HIGH BIAS FOR A COUPLE OF THIS YEAR'S ATLANTIC
TROPICAL CYCLONES.
 
GIVEN THE SOUTHWARD SHIFT IN THE TRACK...A TROPICAL STORM WARNING
HAS BEEN ISSUED FOR THE NORTHERN LEEWARD ISLANDS.

FORECAST POSITIONS AND MAX WINDS
 
INITIAL      02/0900Z 17.0N  59.0W    45 KT
 12HR VT     02/1800Z 17.3N  60.1W    50 KT
 24HR VT     03/0600Z 17.8N  61.5W    55 KT
 36HR VT     03/1800Z 18.4N  62.9W    55 KT
 48HR VT     04/0600Z 19.0N  64.3W    55 KT
 72HR VT     05/0600Z 20.3N  66.9W    50 KT
 96HR VT     06/0600Z 22.0N  69.5W    45 KT
120HR VT     07/0600Z 24.0N  72.0W    45 KT
 
$$
FORECASTER PASCH
 
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