000
WTNT41 KNHC 012047
TCDAT1
TROPICAL STORM ERIKA DISCUSSION NUMBER   1
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL062009
500 PM EDT TUE SEP 01 2009
 
AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT HAS BEEN INVESTIGATING
THE LOW PRESSURE AREA EAST OF THE LEEWARD ISLANDS AND HAS FOUND A
MINIMUM PRESSURE OF ABOUT 1007 MB AND A CLOSED...ALBEIT...BROAD
CIRCULATION.  PEAK FLIGHT-LEVEL WINDS WERE 52 KT WITH SFMR DATA OF
ABOUT 45 KT.  THUS...TROPICAL STORM ERIKA HAS FORMED WITH AN
INITIAL INTENSITY OF 45 KT.
 
SATELLITE IMAGES SHOW THAT ERIKA IS EXPERIENCING MODERATE
SOUTHWESTERLY SHEAR WITH THE CENTER NEAR THE WESTERN EDGE OF THE
DEEP CONVECTION.  SOME STRENGTHENING OF THE STORM SEEMS POSSIBLE
DURING THE NEXT DAY OR TWO WHILE THE SSTS ARE WARM AND SHEAR
REMAINS MODERATE.  HOWEVER...IN ABOUT TWO DAYS...GLOBAL MODELS
SIGNIFICANTLY INCREASE THE SHEAR NEAR THE TROPICAL CYCLONE AS
UPPER-LEVEL WESTERLIES FROM A MID-OCEANIC TROUGH OVER THE WESTERN
ATLANTIC IMPINGE ON THE SYSTEM.  MOST OF THE GLOBAL MODELS SHOW
ERIKA WEAKENING AFTER TWO DAYS DUE TO THE SHEAR.  THE TWO BIG
OUTLIERS IN THE MODEL GUIDANCE ARE THE GFDL/HWRF...WHICH FORM AN
ANTICYCLONE NEAR ERIKA AND MAKE IT A HURRICANE. THE NHC FORECAST
WILL SHOW SLOW INTENSIFICATION OVER THE FIRST COUPLE OF DAYS...THEN
WEAKENING THEREAFTER AS THE SHEAR TAKES OVER. THE NHC FORECAST WILL
BE CONSERVATIVE AT THIS TIME...BUT IT IS OF NOTE THAT ALMOST ALL OF
THE RELIABLE INTENSITY GUIDANCE SHOW ERIKA EVENTUALLY BECOMING A
HURRICANE...DESPITE THE SHEAR.
 
THE INITIAL MOTION ESTIMATE IS 300/8...THOUGH THE CENTER APPEARS TO
HAVE DONE SOME REFORMATION SINCE YESTERDAY.  MID-LEVEL RIDGING
OVER THE WEST-CENTRAL ATLANTIC SHOULD CAUSE ERIKA TO MOVE ALONG
THAT GENERAL COURSE FOR THE NEXT COUPLE OF DAYS.  THEREAFTER...
THE STORM COULD TURN MORE TO THE NORTHWEST AS IT NEARS THE
SOUTHWESTERN PERIPHERY OF THE RIDGE.  THIS FORECAST IS RATHER
UNCERTAIN AS THE TRACK PROBABLY HAS A STRONG DEPENDENCE ON THE
FUTURE INTENSITY OF ERIKA.  A WEAKER SYSTEM WOULD LIKELY MOVE MORE
TO THE WEST...SIMILAR TO THE UKMET OR GFS MODELS.  HOWEVER...A
STRONGER SYSTEM WOULD LIKELY FEEL THE STRONG UPPER-LEVEL WINDS AND
MOVE MORE TO THE RIGHT...LIKE THE HWRF/GFDL.  THE OFFICIAL FORECAST
IS LEFT OF THE DYNAMICAL CONSENSUS DUE TO THE STRONG SHEAR FORECAST
IN THE LONGER-TERM.
 
BECAUSE OF THE PROXIMITY TO THE NORTHERN LEEWARD ISLANDS...TROPICAL
STORM WATCHES HAVE BEEN ISSUED IN THAT AREA. 

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/2100Z 17.2N  57.3W    45 KT
 12HR VT     02/0600Z 17.7N  58.2W    45 KT
 24HR VT     02/1800Z 18.4N  60.3W    50 KT
 36HR VT     03/0600Z 19.1N  62.0W    55 KT
 48HR VT     03/1800Z 19.8N  63.8W    60 KT
 72HR VT     04/1800Z 21.0N  66.5W    55 KT
 96HR VT     05/1800Z 23.0N  69.5W    50 KT
120HR VT     06/1800Z 25.0N  72.0W    45 KT
 
$$
FORECASTER BLAKE/FRANKLIN