Tropical Storm IGOR

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TROPICAL STORM IGOR DISCUSSION NUMBER  11
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL   AL112010
1100 PM AST FRI SEP 10 2010
 
CONVENTIONAL SATELLITE IMAGES INDICATE THAT IGOR IS MAINTAINING A
BURSTING PATTERN...CHARACTERISTIC OF A SHEARED TROPICAL CYCLONE. 
THE PARTIALLY EXPOSED LOW-LEVEL CENTER LIES NEAR THE NORTHEAST SIDE
OF A LARGE AND CIRCULAR CONVECTIVE MASS IN THIS IMAGERY...BUT
SEVERAL EARLIER MICROWAVE OVERPASSES INDICATED THE POSSIBILITY OF
THE LOW-LEVEL CENTER LYING UNDERNEATH THE CONVECTION.  AN EXPANDING
UPPER-LEVEL OUTFLOW IS WELL ESTABLISHED IN ALL QUADRANTS EXCEPT TO
THE EAST.  DVORAK INTENSITY ESTIMATES ARE AT 3.0/45 KT AND 3.5/55
KT FROM SAB AND TAFB...AND THE LATEST 3-HOURLY AVERAGED ADT VALUE
IS 3.3.  GIVEN THE UNCERTAINTY WITH REGARD TO THE LOCATION OF THE
CENTER...THE ADVISORY INTENSITY IS INCREASED CONSERVATIVELY TO 45
KT.
 
THE INITIAL MOTION ESTIMATE OF 285/18 IS SOMEWHAT UNCERTAIN AND
REPRESENTS A LONGER-TERM AVERAGE...GIVEN DIFFERENCES BETWEEN
CONVENTIONAL AND MICROWAVE SATELLITE FIXES. THE FORECAST REASONING
REMAINS UNCHANGED. DYNAMICAL MODELS SHOW IGOR MOVING ON A WESTWARD
COURSE ON THE SOUTH SIDE OF A SUBTROPICAL RIDGE OVER THE EAST
CENTRAL ATLANTIC DURING THE NEXT 72 HOURS.  DURING THIS
PERIOD...THE MODEL GUIDANCE IS IN GOOD AGREEMENT THAT IGOR SHOULD
SLOW ITS FORWARD SPEED AND THEN TURN WEST-NORTHWESTWARD BEYOND 72
HOURS IN RESPONSE TO A LARGE BREAK IN THE SUBTROPICAL RIDGE OVER
THE WESTERN AND CENTRAL ATLANTIC.  THE OFFICIAL NHC TRACK LIES NEAR
THE MIDDLE OF THE GUIDANCE ENVELOPE...BUT IS A BIT NORTH OF THE
MULTI-MODEL CONSENSUS WHICH HAS TRENDED SLIGHTLY SOUTHWARD THIS
CYCLE.
 
UW-CIMSS ANALYSES STILL INDICATE AROUND 15 KT OF NORTHEASTERLY
VERTICAL WIND SHEAR OVER IGOR...A LESS THAN IDEAL ENVIRONMENT FOR
INTENSIFICATION.  THE STATISTICAL-DYNAMICAL GUIDANCE INDICATES A
QUICK REDUCTION TO THE SHEAR DURING THE NEXT 24 HOURS...AND A LOW
SHEAR ENVIRONMENT OVER GRADUALLY WARMER WATERS ARE LIKELY TO LEAD
TO A STEADIER STRENGTHENING OVER THE NEXT COUPLE OF DAYS.  THE
INTENSITY GUIDANCE PRESENTS A DILEMMA LATER IN THE FORECAST
PERIOD...HOWEVER.  THE STATISTICAL MODEL OUTPUT INDICATES POSSIBLE
SOUTHWESTERLY SHEAR RELATED TO THE LONGWAVE TROUGH FORECAST OVER
THE WEST CENTRAL ATLANTIC...AS DO A FEW OF THE GLOBAL DYNAMICAL
MODELS.  ON THE OTHER HAND...THE HWRF AND ECMWF SOLUTIONS A
CONTINUATION OF THE LOW SHEAR ENVIRONMENT AND A STRONGER IGOR.  THE
OFFICIAL NHC INTENSITY FORECAST IS ABOUT THE SAME AS THE PREVIOUS
ONE...EXCEPT SLIGHTLY REDUCED LATE IN THE PERIOD IN DEFERENCE TO
THE STATISTICAL MODELS.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INITIAL      11/0300Z 17.1N  34.9W    45 KT
 12HR VT     11/1200Z 17.2N  37.2W    50 KT
 24HR VT     12/0000Z 17.4N  40.1W    55 KT
 36HR VT     12/1200Z 17.5N  42.8W    65 KT
 48HR VT     13/0000Z 17.6N  45.3W    75 KT
 72HR VT     14/0000Z 18.0N  49.5W    85 KT
 96HR VT     15/0000Z 19.0N  53.0W    90 KT
120HR VT     16/0000Z 20.5N  56.0W   100 KT
 
$$
FORECASTER KIMBERLAIN
 
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