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Tropical Storm IGOR (Text)

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TROPICAL STORM IGOR DISCUSSION NUMBER   9
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112010
1100 AM AST FRI SEP 10 2010
 
IGOR REMAINS A SHEARED TROPICAL CYCLONE. DURING THE PAST FEW
HOURS...HOWEVER...THE SHEAR HAS BEGUN TO SHOW SIGNS OF ABATING AND
THE LOW-LEVEL CENTER IS NOW FARTHER INTO THE EASTERN PORTION OF THE
CENTRAL DEEP CONVECTION. THE UPGRADE TO STORM STATUS IS BASED ON A
SATELLITE INTENSITY ESTIMATE OF 35 KT FROM TAFB AND A 47-KT
UW-CIMSS 3-HR AVERAGE ADT.
 
THE INITIAL MOTION IS 280/14. THE INITIAL POSITION AND MOTION ARE
STILL SOMEWHAT UNCERTAIN DUE TO MULTIPLE CIRCULATIONS ROTATING
AROUND THE CENTER OF A LARGER CYCLONIC GYRE. HOWEVER...THE MOTION
IS EXPECTED TO BE BASICALLY WESTWARD FOR THE NEXT 96 HOURS OR SO
ALONG THE SOUTHERN PERIPHERY OF A STRONG SUBTROPICAL RIDGE LOCATED
TO THE NORTH OF THE CYCLONE. BY DAY 5...A SERIES OF WEAK SHORTWAVE
TROUGHS MOVING THROUGH THE BASE OF A BROAD LONGWAVE TROUGH FORECAST
TO BE OFF THE U.S. EAST COAST IS FORECAST BY ALL OF THE GLOBAL
MODELS TO CREATE A WEAKNESS IN THE RIDGE BETWEEN 50W-60W LONGITUDE.
THIS IS EXPECTED TO WEAKEN THE STEERING FLOW AND CAUSE IGOR TO SLOW
DOWN AND ALSO TURN WEST-NORTHWESTWARD. THE OFFICIAL FORECAST TRACK
IS SIMILAR TO THE PREVIOUS ADVISORY AND THE TVCN MODEL CONSENSUS.  
 
CURRENT SHEAR ANALYSES FROM SHIPS AND UW-CIMSS...ALONG WITH WATER
VAPOR WINDS...INDICATE THE VERTICAL SHEAR ACROSS IGOR HAS DECREASED
FROM 20 KT DOWN TO 10 KT DURING THE PAST 24 HOURS. THIS DECEASING
SHEAR TENDENCY IS EXPECTED TO CONTINUE THROUGHOUT THE FORECAST
PERIOD AS IGOR MOVES OVER GRADUALLY INCREASING SSTS...ESPECIALLY
AFTER 48 HOURS. THE RESULT SHOULD BE STEADY INTENSIFICATION WITH
IGOR POSSIBLY REACHING HURRICANE STATUS ON SUNDAY AND MAJOR
HURRICANE STATUS BY DAY 5...AS INDICATED BY THE SHIPS...GFDL...AND
HWRF MODELS. THE OFFICIAL INTENSITY FORECAST IS A LITTLE BELOW THE
GUIDANCE THROUGH 48 HOURS DUE TO UNCERTAINTY ABOUT WHEN IGOR WILL
DEVELOP A CONSOLIDATED CENTER OF CIRCULATION...BUT IS CLOSE TO THE
GUIDANCE THEREAFTER.  

 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      10/1500Z 16.4N  31.0W    35 KT
 12HR VT     11/0000Z 16.6N  33.2W    40 KT
 24HR VT     11/1200Z 16.8N  36.2W    45 KT
 36HR VT     12/0000Z 17.0N  39.2W    50 KT
 48HR VT     12/1200Z 17.1N  41.7W    60 KT
 72HR VT     13/1200Z 17.3N  46.2W    70 KT
 96HR VT     14/1200Z 18.2N  50.3W    85 KT
120HR VT     15/1200Z 19.5N  53.5W    95 KT
 
$$
FORECASTER STEWART
 
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