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| #375003 (Received by flhurricane at: 10:40 PM 09.Sep.2010) TCDAT1 TROPICAL DEPRESSION IGOR DISCUSSION NUMBER 7 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL112010 1100 PM AST THU SEP 09 2010 INFRARED SATELLITE IMAGES INDICATE A BURST OF DEEP CONVECTION OVER THE WESTERN SEMICIRCLE OF IGOR DURING THE LAST SEVERAL HOURS...WITH CLOUD TOP TEMPERATURES AS COLD AS -90 C. THIS BURSTING PATTERN SUGGESTS THAT EASTERLY VERTICAL WIND SHEAR CONTINUES OVER THE CYCLONE...AND THIS IS CONFIRMED BY UW-CIMSS SHEAR ANALYSES SHOWING ABOUT 20 KT OF EASTERLY SHEAR. DVORAK INTENSITY ESTIMATES ARE AT 2.5/35 KT FROM TAFB...AND 3-HOURLY AVERAGED ADT VALUES ARE CLOSE TO THIS VALUE. A STRICT INTERPRETATION OF THE SATELLITE ESTIMATES SUGGEST THAT IGOR MAY BE NEARING TROPICAL STORM STRENGTH AGAIN. HOWEVER...GIVEN THE LACK OF PERSISTENCE OF THE CURRENT BURST OF CONVECTION...THE INITIAL INTENSITY IS HELD AT 30 KT FOR THIS ADVISORY. A REFORMATION OF THE LOW-LEVEL CENTER DURING THE LAST 12 HOURS HAS MADE DISCERNING AN INITIAL MOTION ESTIMATE MORE DIFFICULT THAN NORMAL. GIVEN THE DISCONTINUITY IN CENTER FIXES...A LONGER-TERM MOTION WAS COMPUTED AND YIELDS AN UNCERTAIN 295/11. GLOBAL DYNAMICAL MODELS ARE GENERALLY IN GOOD AGREEMENT THAT IGOR WILL BE STEERED ON A MORE WESTWARD TRACK AT A FASTER FORWARD SPEED BY THE FLOW ON THE SOUTH SIDE OF A SUBTROPICAL RIDGE THROUGH ABOUT 72 HOURS. THEREAFTER...IGOR IS EXPECTED TO TURN MORE WEST- NORTHWESTWARD AND SLOW LATE IN THE FORECAST PERIOD AS IT APPROACHES A WEAKNESS IN THE SUBTROPICAL RIDGE OVER THE CENTRAL ATLANTIC. THE SPREAD IN THE MODEL GUIDANCE HAS INCREASED THIS CYCLE IN RESPONSE TO THE DIFFERENCES IN THE WAY MODELS HANDLE THE EVOLUTION OF THIS WEAKNESS. THE REFORMATION OF THE CENTER AND A NOTABLE SOUTHWARD AND WESTWARD SHIFT TO THE GUIDANCE HAS PROMPTED A FAIRLY SIGNIFICANT THE OFFICIAL NHC TRACK FORECAST IN THAT DIRECTION... THOUGH THE TRACK FORECAST REMAINS ON THE NORTHERN SIDE OF THE GUIDANCE ENVELOPE. A CONTINUATION OF MODERATE EASTERLY SHEAR SHOULD SLOW OR POSSIBLY EVEN HALT FURTHER INTENSIFICATION FOR AT LEAST THE NEXT 24 HOURS. SHIPS MODEL OUTPUT INDICATES A REDUCTION IN THE SHEAR...A FAIRLY MOIST ATMOSPHERE...AND RELATIVELY WARM SEA SURFACE TEMPERATURES AHEAD OF IGOR FROM 24-72 HOURS. THESE INGREDIENTS SHOULD FAVOR A STEADIER RATE OF INTENSIFICATION DURING THAT TIME. THEREAFTER...THE INTENSITY FORECAST BECOMES A BIT TRICKIER. THE STATISTICAL GUIDANCE INDICATES POSSIBLE NEGATIVE EFFECTS OF SOUTHWESTERLY SHEAR IN ASSOCIATION WITH THE AFOREMENTIONED WEAKNESS OVER THE CENTRAL ATLANTIC WHILE SEVERAL OF THE GLOBAL MODELS SHOW LESS SHEAR. THE OFFICIAL NHC INTENSITY FORECAST IS A BIT LOWER EARLY IN THE PERIOD GIVEN CURRENT TRENDS BUT REMAINS CLOSE TO BOTH THE PREVIOUS FORECAST AND THE STATISTICAL-DYNAMICAL INTENSITY GUIDANCE. REGARDLESS OF THE EXACT INTENSITY...GLOBAL MODELS ARE IN GOOD AGREEMENT THAT IGOR WILL GROW IN SIZE...POTENTIALLY BECOMING A LARGE-SIZED HURRICANE. FORECAST POSITIONS AND MAX WINDS INITIAL 10/0300Z 15.4N 28.0W 30 KT 12HR VT 10/1200Z 15.9N 30.0W 30 KT 24HR VT 11/0000Z 16.3N 33.1W 35 KT 36HR VT 11/1200Z 16.5N 36.1W 40 KT 48HR VT 12/0000Z 16.7N 39.0W 50 KT 72HR VT 13/0000Z 17.5N 44.0W 65 KT 96HR VT 14/0000Z 18.5N 48.0W 80 KT 120HR VT 15/0000Z 19.5N 50.5W 85 KT $$ FORECASTER KIMBERLAIN/MUSHER |
