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TROPICAL STORM IRIS DISCUSSION NUMBER   6
NATIONAL WEATHER SERVICE MIAMI FL
11 PM EDT FRI OCT 05 2001
 
THE LAST RECON FIX JUST BEFORE 00Z INDICATED THAT THE EARLIER 
INTENSIFICATION HAD LEVELED OFF...WITH A PRESSURE OF 1003 MB AND 
PEAK FLIGHT-LEVEL WINDS...AT 1500 FT...OF ABOUT 63 KT.  SINCE THAT 
TIME THERE HAS BEEN ANOTHER CONVECTIVE BURST JUST NORTHEAST OF THE 
CENTER AND THE RADAR PRESENTATION IMPROVED A BIT AS WELL.  ON THIS 
BASIS...THE WINDS ARE INCREASED TO 55 KT...AND I SUSPECT THAT THE 
NEXT RECON AT 06Z WILL FIND THAT IRIS IS VERY CLOSE TO HURRICANE 
STRENGTH.  

IRIS IS EMBEDDED IN A WELL-DEFINED EASTERLY STEERING FLOW SOUTH OF A 
MID-LEVEL RIDGE AND NORTH OF AN UPPER-LEVEL LOW.  THE INITIAL MOTION 
IS 275/16.  OVER THE NEXT 36 HOURS A MID-LEVEL TROUGH CURRENTLY OVER 
THE MIDWEST STATES WILL MOVE EASTWARD...BUT GLOBAL MODELS ARE IN 
AGREEMENT THAT THIS TROUGH WILL NOT DIG SOUTH AND HAVE AN 
APPRECIABLE EFFECT ON THE TRACK OF IRIS.  THIS TROUGH SHOULD WEAKEN 
THE RIDGE A LITTLE THOUGH...ENOUGH TO BRING IRIS BACK TO A MORE 
WEST-NORTHWESTERLY MOTION LATER TOMORROW AND INTO SUNDAY.  AFTER THE 
TROUGH MOVES BY...RIDGING OVER THE SOUTHEASTERN STATES IS EXPECTED 
TO KEEP IRIS ON A WEST TO WESTNORTHWEST TRACK INTO THE NORTHWESTERN 
CARIBBEAN.  MODEL GUIDANCE IS TIGHTLY CLUSTERED AND THE OFFICIAL 
FORECAST IS NOT FAR FROM THE PREVIOUS FORECAST.

IRIS IS CURRENTLY IN A LIGHT EASTERLY SHEAR ENVIRONMENT.  THERE ARE 
STRONGER UPPER-LEVEL WINDS TO ITS SOUTH...AND IF IRIS CAN AVOID 
THESE THEN IT SHOULD CONTINUE TO STRENGTHEN.  THE SHIPS MODEL BRINGS 
IRIS TO 95 KT IN 72 HOURS.  THE GFDL SHOWS SIGNIFICANTLY LESS 
STRENGTHENING...BUT KEEPS IRIS ON A MORE SOUTHERLY TRACK WHERE THE 
SHEAR IS HIGHER.  THE OFFICIAL FORECAST LEANS TOWARDS THE SHIPS 
GUIDANCE.  ONE CAUTION HOWEVER...THE RECON DATA INDICATED THAT THE 
CLOSED CIRCULATION IS STILL VERY SMALL AND FRAGILE...SO THAT IF IRIS 
ENCOUNTERS LESS FAVORABLE UPPER FLOW THE CYCLONE COULD DISSIPATE 
VERY QUICKLY.
 
FORECASTER FRANKLIN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     06/0300Z 15.8N  68.5W    55 KTS
12HR VT     06/1200Z 16.2N  71.0W    65 KTS
24HR VT     07/0000Z 17.2N  74.0W    70 KTS
36HR VT     07/1200Z 18.1N  76.4W    75 KTS
48HR VT     08/0000Z 19.0N  79.0W    75 KTS
72HR VT     09/0000Z 19.5N  84.5W    85 KTS
 
 
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