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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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