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TROPICAL STORM ERIN DISCUSSION NUMBER   3
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT SUN SEP 02 2001

TROPICAL DEPRESSION SIX HAS BECOME CONSIDERABLY BETTER ORGANIZED 
DURING THE PAST 6 HOURS AND HAS STRENGTHENED INTO TROPICAL STORM
ERIN.  THE INITIAL INTENSITY OF 40 KT IS BASED ON DVORAK SATELLITE 
INTENSITY ESTIMATES OF 35 KT...T2.5...FROM TAFB AND 45 KT...T3.0... 
FROM SAB.  THE UPPER-LEVEL OUTFLOW IS QUITE IMPRESSIVE FOR A 
TROPICAL CYCLONE SO EARLY IN THE DEVELOPMENT STAGE.
 
THE INITIAL MOTION ESTIMATE IS 280/17.  DURING THE PAST 9 HOURS...
THE MOTION HAS ACTUALLY BEEN ALMOST DUE WEST AT 270 DEGREES.
HOWEVER...DUE TO THE UNCERTAINTY ON THE WHERE THE ACTUAL CENTER OF
ERIN IS LOCATED WITHIN THE CONVECTIVE CLOUD MASS USING INFRARED
IMAGERY...I CHOSE TO KEEP THE CENTER A LITTLE CLOSER TO THE PREVIOUS
TRACK MOTION.  THE OFFICIAL FORECAST TRACK IS SOUTH OF THE PREVIOUS
FORECAST TRACK AND A LITTLE SOUTH OF ALL OF THE NHC MODEL GUIDANCE.
WHILE IT IS NORMAL PROCEDURE TO REMAIN SOMEWHERE INSIDE THE GUIDANCE
ENVELOPE...THE GLOBAL MODELS...ESPECIALLY THE UKMET...AVN...AS WELL
AS THE BAM MODELS APPEAR TO HAVE A SHARP POLEWARD BIAS BASED ON
MODEL TRENDS OVER THE PAST 24 HOURS.  THE INITIAL POSITION IS
ALREADY WELL SOUTH OF THE UKMET POSITIONS FROM THE 01/12Z AND 02/00Z
MODELS RUNS.  THE 00Z GFDL RUN LOSES ERIN IN 24 HOURS AS DOES THE
AVN AND...TO A CERTAIN DEGREE...SO DOES THE UKMET.  ONLY THE NOFAPS
MODEL SEEMS TO HVE A GOOD HANDLE ON ERIN OVERALL.  GIVEN HOW WELL
DEVELOPED THE CYCLONE APPEARS IN SATELLITE IMAGERY AND THE MORE
RECENT WESTWARD MOTION...A SLIGHT SOUTHWARD SHIFT IN THE TRACK
APPEARS TO BE IN ORDER.  THE OFFICIAL TRACK IS CLOSE TO THE FORECAST 
TRACK IN ADVISORY NUMBER 1 AND A LITTLE NORTH OF THE 01/18Z GFDL 
TRACK. HOWEVER...IF THE WESTWARD TRACK CONTINUES...THEN THE FORECAST 
TRACKWILL HAVE TO BE BROUGHT EVEN FARTHER SOUTH ON THE NEXT 
ADVISORY.

GIVEN THE PRESENT CIRCULAR OUTFLOW PATTERN AND THE LACK OF ANY 
SIGNIFICANT UPPER-LEVEL SHEAR AHEAD OF ERIN...A TYPICAL RATE OF ONE 
T-NUMBER PER DAY INTENSIFICATION SEEMS QUITE REASONABLE.  AFTER 48 
HOURS...THE AVN AND UKMET DEVELOP SOUTHERLY 20 TO 30 KT UPPER-LEVEL 
FLOW AHEAD OF OR OVER ERIN. IT IS UNCERTAIN WHETHER OR NOT THIS FLOW 
WILL BE PART OF THE SURROUNDING ENVIRONMENT OR JUST PART OF THE 
OUTFLOW ON THE WEST SIDE OF ERIN.  THE INTENSITY WAS FLATTENED OUT 
AFTER 48 HOURS IN ANTICIPATION OF THE SOUTHERLY WINDS PRODUCING 
SHEAR ON ERIN LIKE THE SHIPS INTENSITY MODEL IS FORECASTING. 
HOWEVER...BY THE TIME ERIN REACHES THE 72 HOUR FORECAST POSITION... 
THE CYCLONE COULD BE EVEN STRONGER THAN INDICATED WHICH WOULD ALLOW 
IT TO FIGHT OFF THE SOUTHERLY SHEAR AND CONTINUE TO STRENGTHEN.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     02/0900Z 13.4N  38.5W    40 KTS
12HR VT     02/1800Z 13.9N  40.9W    50 KTS
24HR VT     03/0600Z 14.5N  43.8W    60 KTS
36HR VT     03/1800Z 15.1N  46.5W    70 KTS
48HR VT     04/0600Z 15.8N  49.1W    80 KTS
72HR VT     05/0600Z 17.5N  54.0W    80 KTS
 
 
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