Tropical Depression SEVEN

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TROPICAL DEPRESSION SEVEN DISCUSSION NUMBER   3
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL072011
1100 AM AST SUN AUG 14 2011
 
DURING THE PAST 6 HOURS...CONVECTION HAS BEGUN TO REDEVELOP OVER AND
SOUTHEAST OF THE LOW-LEVEL CENTER OF THE DEPRESSION IN RESPONSE TO
DECREASING VERTICAL WIND SHEAR AND INCREASING MID-LEVEL MOISTURE.
ALTHOUGH DATA T-NUMBERS HAVE DECREASED TO T1.5 AND T1.0 FROM TAFB
AND SAB...RESPECTIVELY...THE INITIAL INTENSITY REMAINS AT 30 KT
BASED ON SATELLITE CURRENT INTENSITY ESTIMATES OF T2.0/30 KT AND
THE IMPROVED CONVECTIVE ORGANIZATION SINCE THE 12Z SATELLITE FIXES.

WATER VAPOR IMAGERY INDICATES THE DEPRESSION IS MOVING INTO A COL
REGION ON THE NORTH SIDE OF AN UPPER-LEVEL LOW THAT IS LOCATED
ABOUT 150 NMI SOUTHWEST OF THE CYCLONE. AS A RESULT...THE PREVIOUS
WESTERLY VERTICAL SHEAR REGIME IS BEING REPLACED BY A MORE
FAVORABLE SOUTHEASTERLY LOW-SHEAR ENVIRONMENT...WHICH SHOULD FAVOR
THE CONTINUED DEVELOPMENT OF DEEP CONVECTION FOR THE NEXT 12-24
HOURS. THE CYCLONE IS EXPECTED TO REMAIN WITHIN A FAVORABLE
UPPER-LEVEL FLOW ENVIRONMENT FOR THE NEXT 48-60 HOURS...WHICH WOULD
NORMALLY FAVOR SIGNIFICANT INTENSIFICATION. HOWEVER...THERMODYNAMIC
CONDITIONS ARE FORECAST TO BE LESS FAVORABLE AS THE CYCLONE MOVES
OVER SUB-26C SSTS BY 48 HOURS. THESE OUT-OF-PHASE CONDITIONS ARE
EXPECTED TO RESULT IN ONLY MODEST STRENGTHENING. BY DAY 4...THE
POST-TROPICAL CYCLONE IS FORECAST TO BE ABSORBED BY A FRONT. THE
OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY AND
CLOSELY FOLLOWS A BLEND OF THE SHIPS AND LGEM INTENSITY MODELS.
 
THE DEPRESSION HAS SLOWED AND MADE AN UNEXPECTED JOG TO THE WEST...
AND THE INITIAL MOTION ESTIMATE IS NOW 285/09. THE TRACK FORECAST
IS LESS STRAIGHTFORWARD THAN THE INTENSITY FORECAST. THE RECENT
WESTWARD JOG IS PROBABLY DUE IN PART TO THE SYSTEM BECOMING MORE
VERTICALLY SHALLOW AFTER THE DEEP CONVECTION WEAKENED EARLIER THIS
MORNING. NOW THAT CONVECTION HAS REDEVELOPED...THE CYCLONE SHOULD
TURN NORTHWESTWARD WITHIN THE NEXT 12 HOURS AS IT COMES UNDER THE
INFLUENCE OF DEEP-LAYER SOUTHEASTERLY STEERING FLOW. BEYOND 12
HOURS...THE TRACK FORECAST BECOMES MORE COMPLICATED. THE 12Z
UPPER-AIR DATA FROM BERMUDA THIS MORNING INDICATES THE MID-LEVEL
FLOW FROM 700-400 MB HAS BACKED AROUND TO THE SOUTH-SOUTHWEST AND
HEIGHTS HAVE FALLEN 40-60 METERS WITHIN THIS LAYER. THIS SUGGESTS
THAT THE SUBTROPICAL RIDGE TO THE NORTH OF THE DEPRESSION HAS
WEAKENED...WHICH SHOULD ALLOW THE SYSTEM TO TURN NORTHWESTWARD OR
EVEN NORTHWARD. HOWEVER...WATER VAPOR IMAGERY INDICATES THAT A
SHORTWAVE TROUGH APPROACHING BERMUDA RAPIDLY FROM THE WEST IS THE
LIKELY CAUSE OF THE EROSION OF THE RIDGE. WITH THE MAIN UPPER-LEVEL
TROUGH STILL OVER THE U.S. OHIO AND TENNESSEE VALLEYS...THE RIDGE
COULD TEMPORARILY BUILD BACK WESTWARD BETWEEN BERMUDA AND THE
DEPRESSION...RESULTING IN A STAIR-STEP TYPE OF MOTION TO THE
NORTHWEST DURING THE NEXT 24 HOURS OR SO. THE MAIN UPPER-LEVEL
TROUGH IS EXPECTED TO LIFT THE CYCLONE QUICKLY TO THE NORTHEAST BY
36-48 HOURS AND TAKE IT ACROSS MUCH COLDER WATER IN 60-72
HOURS...WHICH WILL INDUCE EXTRATROPICAL TRANSITION. THE OFFICIAL
FORECAST TRACK HAS BEEN NUDGED A LITTLE TO THE WEST OF THE PREVIOUS
FORECAST TRACK THROUGH 24 HOURS BASED ON THE MORE WESTWARD INITIAL
POSITION...AND LIES JUST TO THE LEFT OF THE GLOBAL MODEL
CONSENSUS...BUT WELL TO THE EAST OF THE TIGHTLY PACKED BAM MODELS.
 
AS A RESULT OF THE WESTWARD SHIFT IN THE FORECAST TRACK...WHICH
BRINGS THE CYCLONE CLOSER TO BERMUDA...THE BERMUDA WEATHER SERVICE
HAS ISSUED A TROPICAL STORM WARNING FOR BERMUDA.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/1500Z 28.2N  63.2W   30 KT  35 MPH
 12H  15/0000Z 29.6N  63.7W   35 KT  40 MPH
 24H  15/1200Z 31.8N  63.8W   40 KT  45 MPH
 36H  16/0000Z 34.9N  62.4W   45 KT  50 MPH
 48H  16/1200Z 38.6N  58.9W   50 KT  60 MPH
 72H  17/1200Z 47.0N  49.0W   45 KT  50 MPH...POST-TROP/EXTRATROP
 96H  18/1200Z...DISSIPATED
 
$$
FORECASTER STEWART
 
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