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| #375206 (Received by flhurricane at: 10:37 PM 10.Sep.2010) TCDAT1 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 |
