Tropical Storm FIONA

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TROPICAL STORM FIONA DISCUSSION NUMBER   7
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082010
500 AM AST WED SEP 01 2010
 
RECONNAISSANCE DATA...RADAR DATA FROM GUADELOUPE...AND SATELLITE
IMAGERY INDICATE FIONA HAS BECOME A LITTLE BETTER ORGANIZED THIS
MORNING. DEEP CONVECTION HAS INCREASED IN THE SOUTHERN SEMICIRCLE
AND THE CENTER HAS BECOME BETTER DEFINED IN RADAR IMAGERY AND IN
FLIGHT-LEVEL WIND DATA. THE MAXIMUM 850 MB FLIGHT-LEVEL WINDS
OBSERVED THUS FAR DURING THE CURRENT MISSION HAS BEEN 52 KT IN THE
NORTHEAST QUADRANT...WHICH EQUATES TO ABOUT A 42-KT SURFACE WIND.
THE LOWEST PRESSURES OBSERVED THUS FAR HAVE BEEN 1002 MB AT 0602Z
AND 1004 MB AT 0803Z. THE INITIAL INTENSITY IS SET AT 40 KT...WHICH
IS LOWER THAN THE 50-KT ADT INTENSITY ESTIMATE FROM UW-CIMSS.

THE INITIAL MOTION IS 290/12 KT USING RECON AND RADAR FIX POSITIONS.
FIONA APPEARS TO HAVE DECREASED ITS RATE OF CLOSURE ON EARL LOCATED
TO ITS NORTHWEST. OVER THE PAST 48 HOURS...THE DISTANCE BETWEEN
FIONA AND EARL HAS BEEN DECREASING BY ABOUT 100 NMI EVERY 12
HOURS...AND THE TWO CYCLONES ARE CURRENTLY ABOUT 750 NMI APART.
RIDDING BETWEEN THE TWO SYSTEMS HAS OBVIOUSLY INCREASED...WHICH HAS
RESULTED IN THE SIGNIFICANT DECREASE IN THE FORWARD SPEED OF FIONA
FROM MORE THAN 20 KT DOWN TO THE CURRENT 12 KT IN THE PAST 12
HOURS. THE POSSIBLE CAUSE OF THE RIDGING TO THE NORTHWEST OF FIONA
IS THE UPPER-LEVEL OUTFLOWS FROM BOTH CYCLONES CONVERGING TO THE
NORTHWEST OF FIONA. THIS RIDGING MAY ALLOW KEEP FIONA FROM
INTERACTING WITH THE LARGER CIRCULATION OF EARL LIKE MOST OF THE
GLOBAL MODELS HAVE BEEN FORECASTING THE PAST COUPLE OF DAYS. ALL OF
THE MODEL GUIDANCE...EXCLUDING NOGAPS...NOW SLOWS DOWN FIONA BY DAY
3...WITH THE CYCLONE POSSIBLY GETTING TRAPPED IN THE SUBTROPICAL
RIDGE. AS RESULT OF THESE NEW SCENARIO...A 96-HOUR POSITION HAS
BEEN ADDED AND DISSIPATION HAS BEEN DELAYED UNTIL 120 HOURS. IF
FIONA BECOMES A STRONGER SYSTEM THAN CURRENTLY FORECAST...THE
CYCLONE COULD SURVIVE AND END UP MAKING A CLOCKWISE LOOP OR EVEN
BECOME STATIONARY BY 120 HOURS. THE OFFICIAL FORECAST TRACK IS
SIMILAR TO THE PREVIOUS ADVISORY..EXCEPT SLOWER AT 48 HOURS AND
BEYOND...AND IS ALONG THE WESTERN EDGE OF THE GUIDANCE SUITE.

SOME DRY MID-LEVEL AIR APPEARS TO BE GETTING DRAWN INTO THE NORTHERN
QUADRANT OF THE FIONA...WHICH MAY BE RESPONSIBLE FOR THE LACK OF
SIGNIFICANT CONVECTION IN THAT REGION. HOWEVER...UPPER-LEVEL
OUTFLOW HAS IMPROVED SOME AND BECOME A LITTLE MORE CIRCULAR IN
APPEARANCE. THE OFFICIAL INTENSITY FORECAST WAS NUDGED UPWARD
SLIGHTLY FROM THE PREVIOUS ADVISORY...BUT IS LOWER THAN THE SHIPS
AND LGEM MODELS WHICH BRING FIONA UP TO A 60-KT SYSTEM BY 48
HOURS...AND MUCH LOWER THAN THE GFDL MODEL WHICH MAKES FIONA AN
85-KT HURRICANE AT THAT SAME TIME.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/0900Z 17.4N  60.2W    40 KT
 12HR VT     01/1800Z 18.9N  62.5W    45 KT
 24HR VT     02/0600Z 21.2N  64.7W    50 KT
 36HR VT     02/1800Z 23.5N  66.5W    50 KT
 48HR VT     03/0600Z 26.3N  67.3W    40 KT
 72HR VT     04/0600Z 29.2N  67.5W    35 KT
 96HR VT     05/0600Z 31.5N  67.0W    30 KT
120HR VT     06/0600Z...DISSIPATED
 
$$
FORECASTER STEWART
 
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