Tropical Storm ERIKA

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TROPICAL STORM ERIKA DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL062009
500 AM AST THU SEP 03 2009
 
ALTHOUGH THE HURRICANE HUNTERS FIXED A FLIGHT-LEVEL CENTER...THERE
ARE A FAIR NUMBER OF SURFACE OBSERVATIONS OVER THE NORTHEASTERN
CARIBBEAN SEA REGION...AND THESE SHOW THAT THE SURFACE CIRCULATION
IS AT BEST POORLY-DEFINED.  THE SYSTEM MAY BE OPENING UP INTO A
TROUGH ORIENTED FROM SOUTH-SOUTHWEST TO NORTH-NORTHEAST.  THERE
WERE A FEW 34 TO 37-KT SFMR SURFACE WIND READINGS SOME 40-75 N MI
NORTH AND NORTHEAST OF THE FLIGHT-LEVEL CENTER...SO THE INTENSITY
IS MAINTAINED AT 35 KT.  EVEN THOUGH THERE MAY NO LONGER BE A
DEFINITE SURFACE CENTER OF CIRCULATION...WE WILL MAINTAIN
ADVISORIES ON THE SYSTEM PENDING ADDITIONAL DATA FROM THE
RECONNAISSANCE AIRCRAFT AND VISIBLE IMAGERY.  RADIOSONDE DATA FROM
THE EASTERN CARIBBEAN SEA AREA SUGGEST THAT THE STORM HAS BEEN
EXPERIENCING WESTERLY SHEAR DUE TO FLOW AT ABOUT THE 250 MB
LEVEL...WHICH IS LIKELY JUST BELOW THE TROPICAL CYCLONE OUTFLOW
LAYER.  GLOBAL MODELS SHOW INCREASING WEST-SOUTHWESTERLY SHEAR OVER
ERIKA DURING THE NEXT COUPLE OF DAYS DUE TO STRENGTHENING
UPPER-TROPOSPHERIC WINDS.  IN SPITE OF THE STRONG SHEAR...THE GFDL
AND HWRF MODELS CONTINUE TO FORECAST THAT ERIKA WILL INTENSIFY
SIGNIFICANTLY.  THE SHIPS AND LGEM GUIDANCE ARE MUCH LESS
AGGRESSIVE...PARTICULARLY THE LATTER.  THE OFFICIAL INTENSITY
FORECAST IS THE SAME AS THAT FROM THE PREVIOUS ADVISORY AND
ANTICIPATES THAT THE TROPICAL CYCLONE WILL DISSIPATE BY 72
HOURS.

BECAUSE OF THE UNCERTAINTIES IN THE LOCATION...OR EXISTENCE...OF THE
SURFACE CENTER...THE INITIAL MOTION ESTIMATE OF 295/6 IS MORE OF AN
EDUCATED GUESS.  A WEAK MID-LEVEL RIDGE LIES TO THE NORTH OF ERIKA. 
AS THE MODELS BUILD THIS RIDGE SOMEWHAT DURING THE NEXT COUPLE OF
DAYS...A SLIGHT INCREASE IN FORWARD SPEED IS LIKELY.  LATER IN THE
PERIOD A MORE NORTHWESTWARD TRACK IS FORECAST AS THE CYCLONE NEARS
THE WESTERN PERIPHERY OF THE RIDGE.  THE OFFICIAL FORECAST IS
SHIFTED A LITTLE NORTH OF THE PREVIOUS ONE PRIMARILY DUE TO A
SLIGHT NORTHWARD RELOCATION OF THE INITIAL POSITION.

EVEN IF ERIKA LOSES ITS TROPICAL CYCLONE STATUS...IT COULD MAINTAIN
VIGOROUS DEEP CONVECTION WHICH WOULD PRODUCE VERY HEAVY RAINS OVER
THE NORTHERN LESSER ANTILLES AND PUERTO RICO.
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      03/0900Z 16.5N  62.7W    35 KT
 12HR VT     03/1800Z 17.1N  64.1W    35 KT
 24HR VT     04/0600Z 17.9N  65.6W    30 KT
 36HR VT     04/1800Z 18.7N  67.1W    30 KT
 48HR VT     05/0600Z 19.5N  68.5W    30 KT
 72HR VT     06/0600Z 21.0N  70.8W    25 KT...REMNANT LOW
 96HR VT     07/0600Z 22.5N  73.0W    20 KT...REMNANT LOW
120HR VT     08/0600Z...DISSIPATED
 
$$
FORECASTER PASCH
 
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