[Tropical] Tropical Cyclone Discussion

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WTNT41 KNHC 172045
TCDAT1
TROPICAL DEPRESSION JEANNE DISCUSSION NUMBER  17
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
5 PM EDT FRI SEP 17 2004

A USAF RESERVE UNIT AIRCRAFT LOCATED THE CENTER JUST OFFSHORE FROM
HISPANIOLA WITH 1002 MB CENTRAL PRESSURE AND MAXIMUM 700-MB FLIGHT
LEVEL WINDS OF ONLY 34 KT NORTHEAST AND NORTHWEST OF THE CENTER.
JEANNES INTERACTION WITH LAND HAS TAKEN ITS TOLL AND THE STORM IS
DOWNGRADED TO A TROPICAL DEPRESSION BASED ON THIS DATA.

THE INITIAL MOTION ESTIMATE IS 295/08.  THE TRACK FORECAST SCENARIO
REMAINS THE SAME FOR THE FIRST 3 DAYS WITH A NORTHWEST TO NORTHWARD
MOTION INTO A WEAKNESS DEVELOPING IN THE MID-LEVEL RIDGE TO THE
NORTH OF JEANNE.  THE GFS AND GFDL EVEN SHOW SOME EASTWARD
COMPONENT OF MOTION AFTER 2 DAYS.  THEREAFTER THERE IS AN EASTWARD
SHIFT TO ALL OF THE GUIDANCE.  THE UKMET AND NOGAPS SHOW A SLOW
WESTWARD DRIFT FOR 4 AND 5 DAYS WHILE THE GFDL AND GFS SHOW A
NORTHEAST OR EASTWARD MOTION.  EXACTLY WHAT HAPPENS IS DEPENDENT ON
THE EVOLUTION OF THE REMAINS OF IVAN AND THERE IS DISAGREEMENT ON
THIS.  THE OFFICIAL FORECAST MOVES JEANNE NORTHWEST TO NORTHWARD
FOR 3 DAYS AND THEN HOLDS THE MOTION STATIONARY SEVERAL HUNDRED
MILES NORTHEAST OF THE NORTHERN BAHAMAS.  THIS IS A SIGNIFICANT
EASTWARD SHIFT FROM THE PREVIOUS ADVISORY AND IMPLIES A REDUCED
RISK TO THE SOUTHEASTERN U.S. DURING THE NEXT 5 DAYS...ALTHOUGH THE
MODEL SPREAD INDICATES INCREASED UNCERTAINTY ASSOCIATED WITH THIS
PARTICULAR FORECAST.

SOME OF THE CURRENT WEAKENING MAY ALSO BE DUE TO 18 KT OF VERTICAL
SHEAR AS DIAGNOSED BY THE SHIPS MODEL.  DECREASING SHEAR IS
FORECAST THROUGH 36 HOURS AND MOST GUIDANCE CALLS FOR MODEST
STRENGTHENING.  THE OFFICIAL FORECAST IS TO 55 KT IN 48 HOURS
COMPARED TO THE PREVIOUS ADVISORY WHICH CALLED FOR A HURRICANE IN
36 HOURS BUT WHICH STARTED WITH A WIND SPEED THAT WAS 25 KT TOO
HIGH.

FORECASTER LAWRENCE

FORECAST POSITIONS AND MAX WINDS

INITIAL      17/2100Z 20.2N  71.8W    30 KT
 12HR VT     18/0600Z 21.0N  73.0W    35 KT
 24HR VT     18/1800Z 22.0N  74.0W    45 KT
 36HR VT     19/0600Z 23.7N  74.7W    50 KT
 48HR VT     19/1800Z 25.8N  74.7W    55 KT
 72HR VT     20/1800Z 29.0N  75.0W    55 KT
 96HR VT     21/1800Z 29.0N  75.0W    55 KT
120HR VT     22/1800Z 29.0N  75.0W    50 KT


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