000
WTNT43 KNHC 020853
TCDAT3
TROPICAL STORM FIONA DISCUSSION NUMBER  11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL082010
500 AM AST THU SEP 02 2010
 
AFTER LITTLE TO NO CONVECTION NEAR THE CENTER OF FIONA...A STRONG
BURST OF DEEP CONVECTION DEVELOPED OVER THE CENTER FOR A FEW HOURS
EARLIER THIS MORNING...WHICH HAS NOW BEEN SHEARED SLIGHTLY TO THE
SOUTHWEST OF THE CENTER. HOWEVER...NIGHTTIME VISIBLE IMAGERY AND A
02/0156Z ASCAT OVERPASS INDICATE FIONA HAS A SMALL BUT VERY
WELL-DEFINED CIRCULATION. THE INITIAL INTENSITY OF 45 KT IS BASED
ON A BELTON OF SATELLITE INTENSITY ESTIMATES OF 35 KT FROM FROM
TAFB AND SAB...AT LEAST 40 KT FROM THE ASCAT PASS...AND UW-CIMSS
ADT ESTIMATE OF 49 KT.
 
THE INITIAL MOTION ESTIMATE IS 320/15 BASED ON MOSTLY ON MICROWAVE
FIXES AND CONTINUITY WITH THE PREVIOUS ADVISORY MOTION. THE FUTURE
TRACK OF FIONA IS HEAVILY DEPENDENT ON THE INTENSITY AND STRUCTURE
OF THE CYCLONE. IT IS UNLIKELY THAT FIONA WILL GROW MUCH MORE IN
HORIZONTAL SIZE DUE TO THE LARGE CIRCULATION OF POWERFUL HURRICANE
EARL SITUATED ABOUT 750 NMI TO THE NORTHWEST. HOWEVER...ALL OF THE
GLOBAL MODELS ARE NOW FORECASTING FIONA TO ROUND THE WESTERN
PERIPHERY OF THE SUBTROPICAL RIDGE LOCATED EAST-WEST BETWEEN 27N
AND 30N LATITUDE...AND IN 36-48 HOURS MOVE UNDERNEATH A REGION OF
LOW VERTICAL SHEAR BETWEEN THE STRONG NORTHERLY OUTFLOW FROM EARL
AND THE SOUTHWESTERLY FLOW AHEAD OF A LARGE MID-LATITUDE TROUGH
FORECAST TO MOVE OFF THE U.S. EAST COAST IN 24 HOURS AND CAPTURE
HURRICANE EARL. IF THIS PATTERN EVOLVES...FIONA WILL REMAIN A
DISTINCT ENTITY AND NOT BECOME ABSORBED BY THE LARGE CIRCULATION OF
EARL AS QUICKLY AS MOST OF THE GLOBAL MODELS ARE FORECASTING. THE
HWRF AND GFDL MODELS KEEP FIONA A SEPARATE SYSTEM THROUGH 120 HOURS
AND EVEN MAKE THE CYCLONE A HURRICANE IN 24-36 HOURS. THE OFFICIAL
FORECAST TRACK IS SIMILAR TO THE PREVIOUS TRACK THROUGH 24 HOURS...
AND THEN A LITTLE TO EAST OR RIGHT OF TRACK AFTER THAT. ON THE
FORECAST TRACK...FIONA IS EXPECTED TO PASS VERY NEAR BERMUDA IN
ABOUT 48 HOURS...WHICH HAS PROMPTED THE ISSUANCE OF A TROPICAL
STORM WATCH FOR THAT ISLAND BY THE BERMUDA WEATHER SERVICE.

AS MENTIONED ABOVE...FIONA MAY REMAIN JUST WEST OF THE STRONGEST
NORTHEASTERLY VERTICAL WIND SHEAR ASSOCIATED WITH THE OUTFLOW OF
EARL FOR THE NEXT 24 HOURS. ALTHOUGH THERE MAY BE FLUCTUATIONS IN
THE INTENSITY OF FIONA...THE SHEAR IS NOT EXPECTED TO BE STRONG
ENOUGH TO STRIP AWAY ALL OF THE CONVECTION. BY 36-48 HOURS...FIONA
MAY MOVE INTO THE AFOREMENTIONED REGION OF LOW SHEAR BENEATH A
SMALL UPPER-LEVEL ANTICYCLONE FORECAST BY ALL THE MODELS TO DEVELOP
NEAR BERMUDA. THIS WOULD PROVIDE A BRIEF WINDOW OF OPPORTUNITY FOR
MORE SIGNIFICANT DEVELOP THAN CURRENTLY INDICATED. THE OFFICIAL
FORECAST CALLS FOR FIONA TO MAINTAIN ITS CURRENT INTENSITY FOR THE
NEXT 48 HOURS...ALTHOUGH THERE MAY SOME FLUCTUATIONS DURING THAT
TIME AS THE SHEAR WAXES AND WANES. THIS SCENARIO IS CONSISTENT WITH
THE GFDL AND HWRF MODELS.

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      02/0900Z 22.6N  65.4W    45 KT
 12HR VT     02/1800Z 24.6N  66.6W    45 KT
 24HR VT     03/0600Z 27.1N  67.3W    45 KT
 36HR VT     03/1800Z 29.7N  67.0W    45 KT
 48HR VT     04/0600Z 32.4N  65.8W    45 KT
 72HR VT     05/0600Z 36.5N  62.0W    35 KT
 96HR VT     06/0600Z...DISSIPATED
 
$$
FORECASTER STEWART