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TROPICAL STORM ERIN DISCUSSION NUMBER  13
NATIONAL WEATHER SERVICE MIAMI FL
5 PM EDT TUE SEP 04 2001
 
BY OBSERVING THE CIRRUS CLOUDS EMANATING FROM ERIN...ONE CAN SUSPECT
THAT THE SHEAR IS BEGINNING TO DECREASE A LITTLE BIT.  NEVERTHELESS
...THE CLOUD PATTERN CONTINUES TO BE SHAPELESS...DEEP CONVECTION HAS
BEEN INTERMITTENT AND ARC CLOUDS ARE MOVING AWAY FROM THE STORM.  IT
MEANS THAT ERIN...AT LEAST...IS NOT STRENGTHENING AT THIS TIME.  IN
FACT... GIVEN SUCH POOR SATELLITE PRESENTATION...IT WOULD BE
TEMPTING TO LOWER THE INITIAL INTENSITY BUT SATELLITE ESTIMATES
REMAIN AT 45 KNOTS.  MODELS INSIST ON GETTING RID OF THE UPPER-LOW
AHEAD OF THE TROPICAL CYCLONE PROVIDING A BETTER ENVIRONMENT FOR
STRENGTHENING IN A DAY OR TWO.  ASSUMING THAT ERIN SURVIVES THAT
LONG...STRENGTHENING IS FORECAST BEYOND 36 HOURS.  BOTH SHIPS AND
GFDL MODELS INTENSIFY ERIN.

THE LOW-LEVEL CENTER CONTINUES TO BE VERY DIFFICULT TO LOCATE ON
SATELLITE IMAGERY.  HOWEVER...IT APPEARS THAT THE INITIAL LOCATION
IN THE PREVIOUS ADVISORY WAS TOO FAR TO THE WEST OR ERIN HAS SLOWED
DOWN.  TAKING A LONGER TERM AVERAGE THE BEST ESTIMATE OF THE INITIAL
MOTION IS 275/14.  A SOLID 500 MB RIDGE TO THE NORTH OF ERIN HAS 
BEEN PROVIDING A WEST TO WEST-NORTHWEST TRACK SO FAR BUT THE RIDGE 
IS FORECAST TO WEAKEN AND CONSEQUENTLY THE STEERING CURRENTS WILL 
ALSO WEAKEN.  THE MAIN IMPACT WILL BE FOR ERIN TO DECREASE ITS 
FORWARD SPEED AS IT MOVES TOWARD THE WEST-NORTHWEST WELL NORTH OF 
THE LEEWARD ISLANDS IN A DAY OR TWO. IT THEN SHOULD CONTINUE TOWARD 
THE NORTHWEST OVER OPEN WATERS. IN THE LONG RANGE...THE FORECAST 
TRACK COULD BECOME EVEN MORE COMPLICATED IF ERIN SLOWS DOWN 
CONSIDERABLY AND MISSES THE TROUGH AND A RIDGE REDEVELOPS TO THE 
NORTH. 
 
THE REAL INTENSITY AND LOCATION OF ERIN WILL BE KNOWN TOMORROW WHEN 
AN AIR FORCE PLANE REACHES THE TROPICAL CYCLONE.
 
FORECASTER AVILA
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     04/2100Z 17.2N  54.1W    45 KTS
12HR VT     05/0600Z 17.7N  56.2W    45 KTS
24HR VT     05/1800Z 19.0N  58.5W    45 KTS
36HR VT     06/0600Z 20.5N  60.5W    55 KTS
48HR VT     06/1800Z 21.5N  62.0W    60 KTS
72HR VT     07/1800Z 23.0N  64.0W    65 KTS
 
 
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