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Tropical Storm LISA (Text)

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TROPICAL STORM LISA DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT TUE SEP 21 2004
 
NORTHERLY SHEAR THAT HAD PREVIOUSLY BEEN AFFECTING LISA APPEARS TO
HAVE ABATED...AT LEAST FOR THE TIME BEING. THIS HAS ALLOWED OUTFLOW
TO REDEVELOP IN THE NORTHERN SEMICIRCLE. THE OVERNIGHT BURST OF
DEEP CONVECTION...INDICATED BY CLOUD TOPS COLDER THAN -80C...HAS
PERSISTED FOR MORE THAN 9 HOURS NOW. AN UW-CIMSS AMSU PRESSURE
ESTIMATE OF 987.4 MB...ALONG WITH A CIMSS 3-HR AVERAGE AODT OF
T4.1...OR 66 KT...SUGGEST THAT LISA COULD BE A HURRICANE. HOWEVER
...SUBJECTIVE DVORAK ESTIMATES ARE ONLY 45-55 KT FROM THE THREE
SATELLITE AGENCIES. THEREFORE...THE INITIAL INTENSITY IS BASED ON A
BLEND OF THESE VARIOUS INTENSITY ESTIMATES FOR THE TIME BEING.

THE INITIAL MOTION ESTIMATE IS 285/09.  THERE REMAINS NO SIGNIFICANT
CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. MOST OF THE NHC
MODEL GUIDANCE HAS BEEN OF LITTLE HELP...EXCEPT FOR THE GFDL...GFDN
AND CANADIAN MODELS. THE GFS...UKMET...AND NOGAPS HAVE ALREADY
MISSED THE SHARP NORTHWARD TURN OF HURRICANE KARL LOCATED TO THE
NORTHWEST OF LISA. THE 12 LOCATION OF KARL WAS ALREADY 18 HOURS
AHEAD OF AND AT LEAST 2 DEGREES OF LONGITUDE EAST OF THOSE MODELS.
THIS SUGGESTS THAT RIDGE LOCATED BETWEEN LISA AND KARL IS STRONGER
THAN THOSE MODELS HAVE BEEN FORECASTING...SO THE OFFICIAL ELANS
MORE HEAVILY TOWARD THE GFDL-GFDN-CANADIAN SOLUTIONS. THE RIDGE TO
THE NORTH OF TINY LISA IS EXPECTED TO REMAIN INTACT AND STRONG
ENOUGH TO STEER THE CYCLONE WEST-NORTHWEST OR WESTWARD UNTIL THE
END OF THE FORECAST PERIOD WHEN A TROUGH IS FORECAST TO DIG
SOUTHWARD BETWEEN 50-55W LONGITUDE AND WEAKEN THE RIDGE SLIGHTLY.
THIS MAY ALLOW LISA TO BRIEFLY GAIN SOME LATITUDE BEFORE THE TROUGH
LIFTS OUT AND THE RIDGE BUILDS BACK IN TO THE NORTH.
 
MODERATE TO WEAK NORTH TO NORTHEASTERLY SHEAR IS EXPECTED TO AFFECT
LISA FOR THE NEXT 36 HOURS OR SO. AS SUCH...THE INTENSITY IS HELD
CONSTANT DUE TO THE SMALL SIZE OF THE CYCLONE. AFTER THAT...THE
SHEAR IS EXPECTED TO WEAKEN TO 10-15 KT FROM THE EAST AND SOUTHEAST
...WHICH MAY ALLOW FOR SOME MODEST STRENGTHENING TO OCCUR...ASSUMING
LISA SURVIVES THE AFOREMENTIONED SHEAR PATTERN. NOTE-- SINCE LISA
IS A VERY SMALL CYCLONE...IT WILL BE SUBJECT TO RAPID FLUCTUATIONS
IN STRENGTH.
 
FORECASTER STEWART
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      21/1500Z 14.2N  39.7W    60 KT
 12HR VT     22/0000Z 14.6N  41.1W    60 KT
 24HR VT     22/1200Z 15.2N  42.7W    60 KT
 36HR VT     23/0000Z 15.7N  44.3W    60 KT
 48HR VT     23/1200Z 15.9N  45.6W    65 KT
 72HR VT     24/1200Z 16.0N  48.0W    65 KT
 96HR VT     25/1200Z 16.5N  50.0W    70 KT
120HR VT     26/1200Z 17.5N  52.0W    70 KT
 
 
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