[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 201500
TCDAT3
TROPICAL STORM LISA DISCUSSION NUMBER   5
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT MON SEP 20 2004

LISA REMAINS A SMALLER THAN AVERAGE TROPICAL CYCLONE. BANDING
FEATURES HAVE CONTINUED TO IMPROVE THIS MORNING...INCLUDING AND
OCCASIONAL HINT OF A BANDING EYE FEATURE IN VISIBLE IMAGERY.
MICROWAVE IMAGERY FROM 20/0903Z AMSU AND 20/0955Z SSMI OVERPASSES
REVEALED A SMALL MID-LEVEL EYE WAS PRESENT. SUCH FEATURES ARE
TYPICALLY ASSOCIATED WITH 45-55 KT TROPICAL CYCLONES. THE INITIAL
INTENSITY ESTIMATE WAS INCREASED TO 50 KT BASED ON RECENT UW-CIMSS
AND CIRA AMSU PRESSURE ESTIMATES OF 999.1 AND 998 MB...
RESPECTIVELY. THEREFORE...THE INITIAL INTENSITY HAS BEEN MADE
HIGHER THAN THE DVORAK SATELLITE INTENSITY ESTIMATES OF 35 KT FROM
BOTH TAFB AND SAB...AND 30 KT FROM AFWA.

THE INITIAL MOTION ESTIMATE REMAINS 275/10.  THERE REMAINS NO
SIGNIFICANT CHANGE TO THE PREVIOUS TRACK FORECAST OR REASONING.
LISA IS EXPECTED TO MOVE IN A WEST TO WEST-NORTHWESTWARD MOTION
THROUGHOUT THE FORECAST PERIOD AS THE SMALL CYCLONE MOVES AROUND
THE PERIPHERY OF A NARROW SUBTROPICAL TO THE NORTH. THE RIDGE
CURRENTLY BETWEEN LISA AND KARL TO THE NORTHWEST IS FORECAST BY
MOST OF THE MODELS TO REMAIN INTACT TO SOME DEGREE. THE NOGAPS
MODEL IS THE ONLY MODEL THAT WEAKENS THE RIDGE AND PULLS LISA
NORTHWARD AFTER 72 HOURS. HOWEVER...NOGAPS IS THE ONLY MODEL THAT
EVEN IDENTIFIES LISA BEYOND 24-36 HOURS...AND IT APPEARS TO BE
SUFFERING FROM ITS USUAL OVERDEVELOPMENT OF THE CYCLONE...WHICH
ALLOWS LISA TO ERODE THE NARROW RIDGE AND MOVE THROUGH IT. THIS
SOLUTION HAS BEEN DISCOUNTED GIVEN THE CURRENT VERY SMALL DIAMETER
OF LISA...AND THE STRONG HIGH PRESSURE SYSTEM TO THE NORTH EXPECTED
TO KEEP THE DIAMETER OF THE CYCLONE SMALLER THAN AVERAGE THROUGHOUT
THE FORECAST PERIOD. THE OFFICIAL FORECAST TRACK IS JUST AN UPDATE
OF THE PREVIOUS TRACK AND IS CLOSE TO THE GFDL AND DEEP BAM MODELS.

SMALL COMPACT TROPICAL CYCLONES LIKE LISA ARE SUSCEPTIBLE TO SHARP
CHANGES IN INTENSITY...BOTH STRENGTHENING AND WEAKENING. THE
VERTICAL SHEAR IS FORECAST TO REMAIN AROUND 10 KT OR LESS FOR THE
NEXT 24 HOURS...SO STRENGTHENING INTO A HURRICANE SEEMS
LIKELY...PERHAPS EVEN BY LATER TODAY. BY 36 HOURS...LOW TO MODERATE
NORTHEASTERLY VERTICAL SHEAR IS EXPECTED TO INHIBIT INTENSIFICATION
...SO ONLY SLOW STRENGTHENING IS FORECAST IN THE LONGER TIME
PERIODS. IF LISA REMAINS SOUTH OF 15N LATITUDE WHERE THE SHEAR IS
EXPECTED TO BE THE STRONGEST...THEN THE INTENSITY WILL LILELY BE
LESS THAN WHAT IS CURRENTLY FORECAST AND INDICATED BY THE SHIPS
INTENSITY MODEL.

FORECASTER STEWART

FORECAST POSITIONS AND MAX WINDS

INITIAL      20/1500Z 13.6N  35.9W    50 KT
 12HR VT     21/0000Z 13.8N  37.3W    55 KT
 24HR VT     21/1200Z 14.3N  39.1W    65 KT
 36HR VT     22/0000Z 14.8N  40.6W    70 KT
 48HR VT     22/1200Z 15.5N  42.4W    75 KT
 72HR VT     23/1200Z 16.8N  45.5W    75 KT
 96HR VT     24/1200Z 17.0N  48.5W    75 KT
120HR VT     25/1200Z 17.0N  51.5W    75 KT


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