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TROPICAL STORM ERIN DISCUSSION NUMBER  11
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT TUE SEP 04 2001
 
TROPICAL STORM ERIN HAS TRACKED WEST-NORTHWESTWARD AND HAS BECOME 
BETTER ORGANIZED AS THE UPPER-LEVEL SHEAR HAS DECREASED.  THE 
INITIAL INTENSITY WAS INCREASED TO 45 KT BASED ON CONSENSUS 
SATELLITE INTENSITY ESTIMATES OF 45 KT...T3.0...FROM ALL THREE 
SATELLITE AGENCIES...AND A 41 KT BUOY REPORT AT 04/02Z ABOUT 40 NM 
NORTHEAST OF THE CENTER. UPPER-LEVEL OUTFLOW HAS IMPROVED AND IS 
MORE CIRCULAR IN APPEARANCE NOW.
 
THE INITIAL MOTION ESTIMATE REMAINS AT 285/16.  DUE TO THE 
UNCERTAINTY IN THE CENTER LOCATION AT NIGHT...THE POSITION WAS 
BIASED SLIGHTLY TO THE RIGHT OF THE SATELLITE FIXES AND CLOSER TO 
THE PAST TRACK AND PREVIOUS FORECAST TRACK.  HOWEVER...NIGHTTIME 
VISIBLE IMAGERY SUGGESTS THAT THE CENTER COULD BE MORE TO THE LEFT 
OF THE INDICATED POSITION AND THE OFFICIAL FORECAST TRACK. 
HOWEVER...THERE REMAINS STRONG AGREEMENT BY THE AVN AND NOGAPS 
MODELS ON TAKING ERIN ON A GENERAL WEST-NORTHWEST TRACK THROUGH 48 
HOURS...AND THEN TAKE THE SYSTEM A LITTLE MORE NORTHWESTWARD AFTER 
THAT.  THE GFDL REMAINS THE WESTERNMOST MODEL AND MOVES ERIN TO 
WITHIN 100 NM OF THE NORTHERN LESSER ANTILLES AS A CATEGORY 1 
HURRICANE IN 36 TO 48 HOURS.  IF A MORE WESTWARD CENTER POSITION 
BECOMES EVIDENT IN THE FIRST FEW VISIBLE IMAGES...THEN THE FORECAST 
TRACK WILL HAVE TO BE SHIFTED MORE WESTWARD IN THE NEXT ADVISORY.  
OVERALL...THE SUBTROPICAL RIDGE IS FORECAST TO REMAIN INTACT THROUGH 
48 HOURS ANS THEN WEAKEN SLIGHTLY BY 72 HOURS AS AN UPPER-LEVEL 
TROUGH MOVES OFF THE U.S. EAST COAST AND ERODES THE RIDGE 
SLIGHTLY... ALLOWING FOR ERIN TO MOVE A LITTLE MORE POLEWARD AND 
MAKE A SMALL "STAIR-STEP" IN THE TRACK.

ALTHOUGH THE SHEAR HAS WEAKENED SOME THIS MORNING...THE GENERAL 
UPPER-LEVEL FLOW PATTERN SHOULD MAINTAIN SOME SHEAR ON THE SYSTEM 
THROUGH 48 HOURS.  AFTER THAT...THE UPPER-LEVEL WINDS BECOME A 
LITTLE MORE FAVORABLE FOR DEVELOPMENT AS THE UPPER-LEVEL LOW 
CURRENTLY NORTHEAST OF PUERTO RICO WEAKENS AND SHIFTS WESTWARD.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     04/0900Z 17.2N  51.1W    45 KTS
12HR VT     04/1800Z 17.8N  53.6W    45 KTS
24HR VT     05/0600Z 18.9N  56.5W    50 KTS
36HR VT     05/1800Z 20.4N  59.0W    55 KTS
48HR VT     06/0600Z 21.7N  60.5W    60 KTS
72HR VT     07/0600Z 23.5N  62.0W    65 KTS
 
 
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