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TROPICAL STORM ERIN DISCUSSION NUMBER   7
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT MON SEP 03 2001
 
THE CENTRAL DEEP CONVECTION HAS CONTINUED TO FLUCTUATE EARLY THIS 
MORNING. THERE HAS BEEN LITTLE OVERALL CHANGE IN THE SYSTEM 
ORGANIZATION...ALTHOUGH THE UPPER-LEVEL OUTFLOW PATTERN HAS BECOME 
DISRUPTED SLIGHTLY.  THE INITIAL INTENSITY OF 50 KT IS BASED ON A 
BLEND OF DVORAK SATELLITE CURRENT INTENSITY ESTIMATES OF 45 KT... 
CI3.0...FROM TAFB AND 55 KT...CI3.5...FROM BOTH SAB AND AFWA.

THE INITIAL MOTION ESTIMATE REMAINS 290/14.  THE CENTER IS NOT EASY
TO DETERMINE FROM INFRARED IMAGERY AND WAS PLACED IN THE SOUTHWEST
QUADRANT OF THE LARGE BALL OF DEEP CONVECTION.  THIS IS CONSISTENT
WITH POSITION ESTIMATES FROM NIGHTTIME VISIBLE IMAGERY AND IS ALSO
ALONG THE PREVIOUS FORECAST TRACK.  EARLIER MICROWAVE SATELLITE DATA
INDICATED THAT THE LOW-LEVEL AND UPPER-LEVEL CIRCULATIONS WERE NOT
VERTICALLY ALIGNED AND THAT OCCASIONALLY THE MID-LEVEL CIRCULATION
HAS PUSHED OUT AHEAD OF THE LOW-LEVEL CENTER.  THE RESULT IS A
TROPICAL CYCLONE THAT LOOKS GOOD AT FIRST GLANCE...BUT WHICH STILL
HAS A DISORGANIZED INNER CORE REGION.  IN SPITE OF THE LATTER...THE
LATEST NHC MODEL GUIDANCE HAS MUCH BETTER AGREEMENT ON TAKING ERIN
WEST-NORTHWESTWARD THROUGHOUT THE PERIOD.  THE UKMET MODEL STILL HAS
A SIGNIFICANT RIGHT OF TRACK BIAS...WHEREAS THE NOGAPS MODEL HAS
BEEN THE MOST CONSISTENT WITH VERY LITTLE DEVIATION BETWEEN
SUCCESSIVE MODEL RUNS.  THE SUBTROPICAL RIDGE IS FORECAST BY ALL OF
THE GLOBAL MODELS TO REMAIN INTACT THROUGH THE PERIOD.  HOWEVER...
GRADUAL WEAKENING OFF THE RIDGE ALONG 60W LONGITUDE IN 72 HOURS IS
EXPECTED AS A MID-LEVEL TROUGH MOVES OFF THE U.S. EAST COAST.  THIS
MAY ALLOW ERIN TO SLOW DOWN AND MOVE A LITTLE MORE POLEWARD.  THE
OFFICIAL FORECAST IS BASICALLY AN EXTENSION OF THE PREVIOUS FORECAST
AND IS BETWEEN THE NOGAPS MODEL AND AN AVN-GFDL MODEL CONSENSUS.

THE SURROUNDING UPPER-LEVEL FLOW PATTERN IS EXPECTED TO BE NEUTRAL
AS FAR AS INTENSIFICATION IS CONCERNED.  THE SHIPS INTENSITY MODEL
AND THE AVN MODEL ARE INDICATING SOUTHWESTERLY SHEAR TO GRADUALLY
INCREASE ACROSS THE CYCLONE BY 72 HOURS AS ERIN MOVES CLOSER TO AN
UPPER-LEVEL LOW FORECAST TO BE JUST NORTH OF PUERTO RICO.
THEREFORE... ONLY SLOW BUT STEADY INTENSIFICATION IS EXPECTED
THROUGH THE FORECAST PERIOD.

FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     03/0900Z 15.3N  43.9W    50 KTS
12HR VT     03/1800Z 16.0N  45.9W    55 KTS
24HR VT     04/0600Z 16.9N  48.6W    60 KTS
36HR VT     04/1800Z 17.8N  51.2W    65 KTS
48HR VT     05/0600Z 18.7N  53.6W    70 KTS
72HR VT     06/0600Z 20.5N  57.5W    75 KTS
 
 
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