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TROPICAL STORM IRIS DISCUSSION NUMBER   7
NATIONAL WEATHER SERVICE MIAMI FL
5 AM EDT SAT OCT 06 2001
 
THE LATEST DATA FROM AN AIR FORCE RESERVE HURRICANE HUNTER INDICATES
THAT IRIS HAS CHANGED LITTLE IN STRENGTH DURING THE NIGHT.  THE
AIRCRAFT REPORTED A DROPSONDE MINIMUM PRESSURE OF 1002 MB ON THE
FIRST FIX AND AN EXTRAPOLATED MINIMUM PRESSURE OF 998 MB ON THE
SECOND.  MAXIMUM FLIGHT-LEVEL WINDS AT 850 MB WERE 59 KT.  THE
AIRCRAFT ALSO REPORTED THAT THE INNER EYEWALL SEEN EARLIER IS
COMING AND GOING...WHICH MAY PARTIALLY EXPLAIN WHY THE STORM IS
ONLY SLOWLY STRENGTHENING.  THE INITIAL INTENSITY REMAINS 55 KT.
 
IRIS IS MOVING BETWEEN 280/285 DEGREES AT ABOUT 14 KT ON THE SOUTH
SIDE OF A LOW/MID LEVEL RIDGE.  LARGE-SCALE MODELS INDICATE THIS
RIDGE SHOULD PERSIST THROUGH 48 HR...THEN STRENGTHEN SOME AS
DEEP-LAYER TROUGHING OVER THE EASTERN U.S. IS REPLACE BY DEEP
LAYER RIDGING.  NHC HURRICANE GUIDANCE RESPONDS TO THIS PATTERN
BY BEING TIGHTLY CLUSTERED AROUND A WEST-NORTHWEST MOTION THROUGH
48 HR AND A TURN TO THE WEST THEREAFTER.  THE OFFICIAL FORECAST
IS AN UPDATE OF THE PREVIOUS PACKAGE AND IS ALONG THE RIGHT SIDE
OF THE GUIDANCE.
 
SATELLITE IMAGERY INDICATES THAT CONVECTION HAS INCREASED OVER
THE PAST SEVERAL HOURS...ALTHOUGH IT IS HAVING TROUBLE WRAPPING
AROUND THE CENTER.  IRIS REMAINS IN DEEP EASTERLY FLOW TO THE NORTH
OF AN UPPER LEVEL LOW.  THIS PATTERN LOOKS GENERALLY FAVORABLE FOR
STRENGTHENING...ALTHOUGH THERE IS STILL THE CHANCE OF EASTERLY
SHEAR INTERFERING WITH THE DEVELOPMENT OF THE TIGHT INNER CORE.
LARGE-SCALE MODELS INDICATE CONDITIONS SHOULD BECOME EVEN MORE
FAVORABLE...AND THE INTENSITY FORECAST THUS CALLS FOR STRENGTHENING.
SOME OF THE DEVELOPMENT EARLY IN THE FORECAST PERIOD MAY COME IN
BURSTS UNTIL THE CENTRAL CORE BECOMES WELL DEFINED.
 
THE WIND RADII HAVE BEEN REVISED SOMEWHAT BASED ON THE AIRCRAFT
DATA.
 
FORECASTER BEVEN
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL     06/0900Z 16.3N  69.7W    55 KTS
12HR VT     06/1800Z 16.8N  72.0W    65 KTS
24HR VT     07/0600Z 17.8N  75.0W    70 KTS
36HR VT     07/1800Z 18.6N  77.5W    75 KTS
48HR VT     08/0600Z 19.0N  80.0W    75 KTS
72HR VT     09/0600Z 19.5N  85.5W    85 KTS
 
 
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