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| #375152 (Received by flhurricane at: 4:46 PM 10.Sep.2010) TCDAT1 TROPICAL STORM IGOR DISCUSSION NUMBER 10 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL112010 500 PM AST FRI SEP 10 2010 THE UPPER-LEVEL SHEAR THAT HAS BEEN PLAGUING IGOR FOR THE PAST SEVERAL DAYS HAS FINALLY STARTED TO ABATE AND CIRRUS OUTFLOW HAS NOW BECOME PRONOUNCED IN ALL QUADRANTS. HOWEVER...SOME MID-LEVEL SHEAR AND DRY AIR ENTRAINMENT IS STILL AFFECTING THE CYCLONE... WHICH HAS RESULTED IN A VERY ASYMMETRIC CONVECTIVE CLOUD PATTERN WITH THE LOW-LEVEL CENTER DISPLACED AT LEAST 30 NMI TO THE EAST OF THE COLDEST CLOUD TOPS. INTENSITY ESTIMATES ARE 30 KT FROM SAB...55 KT FROM TAFB...AND UW-CIMSS 3-HR AVERAGE ADT VALUES OF 49 KT AND AMSU ESTIMATES OF 53 KT. THE INITIAL INTENSITY HAS BEEN CONSERVATIVELY INCREASED TO 40 KT...WHICH WILL BE TOO LOW IF THE LOW-LEVEL CENTER IS ACTUALLY CLOSER TO THE DEEP CONVECTION. THE INITIAL MOTION IS 280/18. THERE IS NO CHANGE TO THE PREVIOUS TRACK FORECAST OR REASONING. IGOR IS EXPECTED TO MOVE WESTWARD... ACCOMPANIED BY A GRADUAL DECREASE IN FORWARD SPEED...FOR THE NEXT 96 HOURS AS THE CYCLONE MOVES ALONG THE SOUTHERN PERIPHERY OF A LARGE AND STRONG SUBTROPICAL RIDGE SITUATED TO THE NORTH. BY DAY 5...A BROAD LONGWAVE TROUGH IS FORECAST TO MOVE OFF THE U.S. EAST COAST AND CREATE A WEAKNESS IN THE RIDGE TO THE NORTHEAST OF THE LESSER ANTILLES. THIS SHOULD CAUSE THE STEERING FLOW TO WEAKEN AND ALLOW WHAT IS EXPECTED TO BE A LARGE HURRICANE IGOR AT THAT TIME TO GRADUALLY TURN WEST-NORTHWESTWARD. THE LATEST MODEL GUIDANCE IS MORE TIGHTLY PACKED AROUND THE PREVIOUS NHC TRACK...ESPECIALLY AT DAYS 4 AND 5. IN PARTICULAR...THE GFDL AND ECMWF SHOW LESS OF A NORTHWARD TURN THAT MIGHT BE RELATED TO A FLATTER LONGWAVE TROUGH IN THE LATEST GLOBAL RUNS. THE OFFICIAL FORECAST TRACK REQUIRES LITTLE CHANGE AND IS AN EXTENSION OF THE PREVIOUS ADVISORY. THE VERTICAL SHEAR IS EXPECTED TO CONTINUE TO DECREASE TO 5 TO 10 KT FOR THE NEXT 5 DAYS. THIS FAVORABLE UPPER-LEVEL ENVIRONMENT...IN CONJUNCTION WITH SSTS INCREASING TO 29C BY THE END OF THE FORECAST PERIOD...SHOULD RESULT IN STEADY INTENSIFICATION. IGOR IS EXPECTED TO REACH HURRICANE STATUS BY SUNDAY AND MAJOR HURRICANE STATUS BY DAYS 4 OR 5. THIS SCENARIO IS SUPPORTED BY THE SHIPS...LGEM... HWRF...AND GFDL MODELS. FORECAST POSITIONS AND MAX WINDS INITIAL 10/2100Z 16.7N 33.3W 40 KT 12HR VT 11/0600Z 16.8N 35.4W 45 KT 24HR VT 11/1800Z 17.1N 38.4W 50 KT 36HR VT 12/0600Z 17.2N 41.0W 60 KT 48HR VT 12/1800Z 17.3N 43.4W 70 KT 72HR VT 13/1800Z 17.7N 47.8W 80 KT 96HR VT 14/1800Z 18.7N 51.6W 90 KT 120HR VT 15/1800Z 20.0N 55.0W 105 KT $$ FORECASTER STEWART |
