Tropical Storm FIONA

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TROPICAL STORM FIONA DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082010
500 PM AST WED SEP 01 2010
 
THOUGH THE SATELLITE APPEARANCE OF FIONA LOOKS QUITE DISHEVELED WITH
THE SURFACE CIRCULATION POPPING OUT FROM UNDER THE DEEP
CONVECTION...THE AIR FORCE HURRICANE HUNTER AIRCRAFT OBSERVATIONS
SHOW THAT THE SYSTEM STILL IS MAINTAINING MODERATE TROPICAL STORM
INTENSITY. PEAK 850 MB FLIGHT LEVEL WINDS OF 57 KT AND SFMR SURFACE
WIND ESTIMATES OF ABOUT 50 KT INDICATE THAT FIONA REMAINS AT 50 KT.
AIRCRAFT OBSERVATIONS ALSO INDICATE THAT FIONA IS RATHER SMALL IN
HORIZONTAL EXTENT WITH TROPICAL STORM FORCE WINDS EXTENDING OUT
LESS THAN 100 NM ON AVERAGE AROUND THE SYSTEM.

CURRENT MOTION OF THE CYCLONE IS A SPEEDY 17 KT TOWARD THE
NORTHWEST.  THE SYSTEM IS BEING ADVECTED ALONG MAINLY BY THE RIDGE
TO ITS NORTHEAST WITH SOME CONTRIBUTION OF STEERING AS WELL BY THE
MUCH LARGER HURRICANE EARL.  THE MODEL GUIDANCE IS IN GOOD
AGREEMENT THROUGH TWO DAYS THAT THE CYCLONE WILL CONTINUE TOWARD
THE NORTHWEST AT A SLOWER FORWARD SPEED.  AFTER THAT...THE
SOLUTIONS DIVERGE WITH SOME GUIDANCE QUICKLY HAVING FIONA ABSORBED
WITHIN THE CIRCULATION OF EARL AND SOME KEEPING FIONA AS A DISTINCT
VORTEX AND EVENTUALLY BECOMING A FRONTAL LOW IN ABOUT FOUR TO FIVE
DAYS.  THE FORECAST TRACK IS CLOSE TO THE MODEL CONSENSUS...MINUS
THE GFDN HURRICANE MODEL WHICH TAKES FIONA UNREALISTICALLY DUE
WEST...AND IS SIMILAR TO THE PREVIOUS ADVISORY.

THE INTENSITY FORECAST IS...AS USUAL...A DIFFICULT CALL. DESPITE THE
CYCLONE REMAINING OVER QUITE WARM WATERS FOR THE NEXT SEVERAL
DAYS...THE VERTICAL SHEAR APPEARS TO BE THE LIMITING FACTOR FOR THE
INTENSITY OF FIONA. A RESEARCH MISSION BY THE NSF GULFSTREAM-V JET
SHOWED UNIDIRECTIONAL 35 KT EASTERLIES ADVECTING OVER THE TOP OF
THE CYCLONE. SUCH UPPER LEVEL FLOW...ALONG WITH THE FAST FORWARD
SPEED...SHOULD LIMIT ANY INTENSIFICATION IN THE NEAR TERM AND THE
FORECAST INTENSITY SHOWS LITTLE CHANGE FOR THE NEXT TWO DAYS. AFTER
THAT...IT IS ANTICIPATED THAT FIONA WILL GRADUALLY DECAY AS IT GETS
ENTRAINED INTO THE CIRCULATION OF EARL AND VERTICAL WIND SHEAR
INCREASES. THE INTENSITY FORECAST IS SIGNIFICANTLY BELOW THE MODEL
CONSENSUS BUT IS QUITE SIMILAR TO THE PREVIOUS ADVISORY. IF CURRENT
TRENDS CONTINUE...IT IS EVEN POSSIBLE THAT FIONA WILL NO LONGER
HAVE A CLOSED SURFACE CIRCULATION. UNDER THIS SCENARIO...
ADVISORIES...AND FIONA...WOULD CEASE TO EXIST.
  
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      01/2100Z 20.2N  62.9W    50 KT
 12HR VT     02/0600Z 21.6N  64.6W    50 KT
 24HR VT     02/1800Z 23.8N  66.3W    50 KT
 36HR VT     03/0600Z 25.9N  67.4W    50 KT
 48HR VT     03/1800Z 27.4N  67.6W    45 KT
 72HR VT     04/1800Z 30.0N  66.5W    35 KT
 96HR VT     05/1800Z 33.5N  64.5W    25 KT
120HR VT     06/1800Z...DISSIPATED
 
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FORECASTER LANDSEA/BLAKE
 
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