[Tropical] Tropical Cyclone Discussion

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WTNT42 KNHC 300240
TCDAT2

TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL122011
1100 PM AST MON AUG 29 2011

AT THE TIME OF THE SATELLITE FIXES AT 00Z...A LARGE CURVED
CONVECTIVE BAND WRAPPED ABOUT HALF WAY AROUND THE CIRCULATION...
WHICH RESULTED IN SATELLITE INTENSITY ESTIMATES OF T2.5/35 KT FROM
SAB AND T2.0/30 KT WITH A DATA-T NUMBER OF 2.5/35 KT FROM TAFB.
HOWEVER...SINCE THAT TIME THE CURVED BAND HAS DISSIPATED...AND THE
CONVECTIVE PATTERN HAS DEVOLVED INTO MORE OF A SHEAR PATTERN. A
SMALL BURST OF DEEP CONVECTION HAS DEVELOPED OVER AND JUST WEST OF
THE CENTER...BUT ONE THUNDERSTORM DOES NOT MAKE A TROPICAL STORM.
THEREFORE...THE INTENSITY IS BEING HELD AT 30 KT FOR THIS ADVISORY.

THE INITIAL MOTION IS NOW 290/13 KT BASED ON A BLEND OF VISIBLE AND
MICROWAVE SATELLITE FIXES. THE GLOBAL AND REGIONAL MODELS ARE IN
EXCELLENT AGREEMENT ON A GENERAL WEST-NORTHWESTWARD MOTION OVER THE
NEXT 5 DAYS...BUT THERE ARE SPEED DIFFERENCES IN THE MODELS. THE
GFDL...UKMET...CANADIAN AND ECMWF MOVE THE DEPRESSION THE
FASTEST...WHEREAS THE GFS...HWRF...AND NOGAPS ARE SLOWER.
HOWEVER...THE TREND IN THE MOST RECENT MODEL RUNS HAS BEEN TO
WEAKEN THE MID/UPPER-LEVEL LOW NEAR 30N55W AND LIFT IT OUT TO THE
NORTHEAST SOONER THAN IN PREVIOUS RUNS. THIS RESULTS IN LESS OF A
WEAKNESS IN THE SUBTROPICAL RIDGE...WHICH IS ALSO FORECAST TO
CHANGE ITS CURRENT NORTH-SOUTH CONFIGURATION TO AN EAST-WEST
ORIENTATION BY 72 HOURS. THIS CHANGE IN THE STEERING FLOW SHOULD
KEEP THE CYCLONE MOVING ON A WEST-NORTHWESTWARD TRACK FOR THE NEXT
4 DAYS...AND THEN SLOW DOWN AND POSSIBLY TURN MORE WESTWARD BY DAY
5 AS A NARROW RIDGE DEVELOPS TO THE NORTH AND WEST OF THE SYSTEM.
SINCE THE DEPRESSION IS BASICALLY ON TRACK...THE NEW NHC FORECAST
IS JUST AN UPDATE AND EXTENSION OF THE PREVIOUS ADVISORY TRACK.

THE EROSION OF CONVECTIVE BANDING FEATURES HAS LIKELY BEEN DUE A
NARROW INTRUSION OF DRY AIR COMING IN THE FROM THE NORTHEAST.
HOWEVER...ALL AVAILABLE DIAGNOSTIC AND MODEL FORECAST DATA INDICATE
THAT THE CYCLONE IS SURROUNDED BY VERY MOIST MID-LEVEL AIR WITH
HUMIDITY VALUES OF 70 PERCENT OR HIGHER...WHICH SHOULD ALLOW FOR
DEEP CONVECTION TO RECOVER LATER TONIGHT AND FOR THE SYSTEM TO
REACH TROPICAL STORM STATUS WITHIN THE NEXT 6-12 HOURS. MODERATE
NORTHEASTERLY SHEAR IS ALSO FORECAST TO AFFECT THE CYCLONE FOR THE
NEXT 24 HOURS OR SO...WHICH SHOULD INHIBIT ANY RAPID DEVELOPMENT.
AFTER THAT...HOWEVER...THE SHEAR IS EXPECTED TO SHARPLY DECREASE...
AND AN UPPER-LEVEL ANTICYCLONE IS FORECAST TO DEVELOP OVER THE
SURFACE CIRCULATION. THIS IMPROVEMENT IN THE VENTILATION PROCESS
SHOULD ALLOW FOR THE CYCLONE TO AT LEAST STEADILY INTENSIFY AND
BECOME A HURRICANE IN 48-72 HOURS AS SUGGESTED BY THE SHIPS...
LGEM...HWRF...AND GFDL MODELS.

FORECAST POSITIONS AND MAX WINDS

INIT  30/0300Z 11.0N  30.0W   30 KT  35 MPH
 12H  30/1200Z 11.8N  32.0W   35 KT  40 MPH
 24H  31/0000Z 12.8N  34.7W   45 KT  50 MPH
 36H  31/1200Z 13.9N  37.8W   50 KT  60 MPH
 48H  01/0000Z 14.8N  41.5W   60 KT  70 MPH
 72H  02/0000Z 16.4N  47.5W   70 KT  80 MPH
 96H  03/0000Z 18.3N  52.9W   80 KT  90 MPH
120H  04/0000Z 20.0N  57.0W   90 KT 105 MPH

$$
FORECASTER STEWART


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