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TROPICAL STORM ERIN DISCUSSION NUMBER  14
NATIONAL WEATHER SERVICE MIAMI FL
11 PM EDT TUE SEP 04 2001

THE CLOUD PATTERN OF ERIN HAS NOT BECOME SIGNIFICANTLY BETTER
ORGANIZED.  THERE STILL APPEARS TO BE SOUTHWESTERLY SHEAR OVER THE
SYSTEM...AND THE CENTER IS ESTIMATED TO BE NEAR THE THE SOUTHWEST
EDGE OF THE MAIN AREA OF DEEP CONVECTION.  DVORAK INTENSITY
ESTIMATES REMAIN AT 45 KNOTS.  WATER VAPOR IMAGERY SHOWS THAT THE
UPPER-LEVEL LOW AHEAD OF ERIN HAS MOVED VERY SLOWLY WESTWARD...TO
NEAR 22N60W.  THE IMAGERY...AND SOUNDING DATA FROM THE NORTHERN
LEEWARD ISLANDS...SUGGESTS THAT THE UPPER LOW HAS WEAKENED A BIT.
UNLESS THE LOW DISSIPATES...AS THE AVN MODEL HAS BEEN
PREDICTING...ERIN SHOULD BE MOVING CLOSER TO IT DURING THE NEXT DAY
OR SO.  IT IS NOT CLEAR HOW THIS WOULD AFFECT THE TROPICAL CYCLONE.
GIVEN THE UNCERTAINTIES...NO CHANGE IS MADE TO THE PREVIOUS
INTENSITY FORECAST.  LATER IN THE PERIOD IT IS PRESUMED THAT THE
UPPER-LEVEL ENVIRONMENT WILL BE MORE FAVORABLE FOR STRENGTHENING.

INITIAL MOTION IS ESTIMATED TO BE ABOUT THE SAME AS BEFORE...ONLY
VERY SLIGHTLY TO THE RIGHT...285/14.  A MID-LATITUDE TROUGH WILL BE
MOVING OFF THE NORTHEAST U.S.  COAST BY EARLY TOMORROW.  AT THIS
TIME...IT APPEARS THAT THE TROUGH WILL ONLY WEAKEN THE RIDGE TO THE
NORTH OF ERIN...BUT OTHERWISE BYPASS THE STORM.  HOWEVER...THE AVN
MODEL SHOWS THAT SOME OF THE LOW/MID-LEVEL VORTICITY AT THE BASE OF
THE TROUGH WILL BE LEFT BEHIND OVER THE SOUTHWEST NORTH ATLANTIC
OVER THE NEXT SEVERAL DAYS...AND GET TRAPPED UNDERNEATH THE RIDGE
BUILDING OVER THE U.S. EAST COAST BEHIND THE TROUGH.  THIS WEAKNESS
UNDER THE RIDGE COULD COMPLICATE THE FUTURE STEERING PATTERN FOR
ERIN...AND MOVE THE TROPICAL CYCLONE MORE TO THE RIGHT THAN
PREVIOUSLY THOUGHT.  LATEST GUIDANCE FROM THE AVN AND THE GFDL
HURRICANE MODELS...INITIALIZED AT 18Z...HAS SHIFTED SOMEWHAT TO THE
RIGHT OF THOSE MODEL RUNS INITIALIZED AT 12Z.  THIS IS CLOSER TO THE
U.K.  MET OFFICE TRACK...WHICH EARLIER APPEARED TO BE A NORTHWARD
OUTLIER.  THE OFFICIAL FORECAST IS SHIFTED ONLY SLIGHTLY TO THE
NORTHEAST OF THE PREVIOUS ONE...BUT IS TO THE LEFT OF THE
AVN/GFDL/U.K. MET CONSENSUS.

FORECASTER PASCH

FORECAST POSITIONS AND MAX WINDS
 
INITIAL     05/0300Z 17.6N  55.4W    45 KTS
12HR VT     05/1200Z 18.5N  57.5W    45 KTS
24HR VT     06/0000Z 19.8N  59.5W    45 KTS
36HR VT     06/1200Z 21.0N  61.0W    55 KTS
48HR VT     07/0000Z 22.0N  62.0W    60 KTS
72HR VT     08/0000Z 24.0N  63.5W    65 KTS
 
 
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