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Tropical Depression THIRTEEN (Text)

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TROPICAL DEPRESSION THIRTEEN DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       AL132011
1000 AM CDT FRI SEP 02 2011
 
SATELLITE IMAGERY...SURFACE OBSERVATIONS...AND DOPPLER RADAR DATA 
INDICATE THE CONVECTIVE PATTERN ASSOCIATED WITH TROPICAL DEPRESSION
THIRTEEN HAS IMPROVED DURING THE PAST 6 HOURS...WITH A LARGE AREA
OF VERY COLD CLOUD TOPS OF -80C AND COLDER HAVING DEVELOPED IN THE
EASTERN SEMICIRCLE. HOWEVER...THE RECENT INCREASE IN DEEP
CONVECTION HAS YET TO TRANSLATE INTO AN INCREASE IN THE SURFACE
WINDS. AN AIR FORCE RESERVE RECON AIRCRAFT INVESTIGATING HAS ONLY
MANAGED TO FIND 850 MB FLIGHT-LEVEL OF 37-38 KT. OIL RIG
OBSERVATIONS WELL ABOVE THE SURFACE ALSO SUGGEST THE SYSTEM IS
STILL A DEPRESSION.   
 
THE DEPRESSION HAS BEEN ESSENTIALLY STATIONARY...TRAPPED IN A FLAT
RIDGE PATTERN THAT EXTENDS SOUTHWARD FROM A 700-400 MB HEIGHT
CENTER LOCATED OVER THE OHIO AND TENNESSEE VALLEYS. NARROW RIDGE
AXES EXTEND SOUTHEASTWARD TO FLORIDA AND ALSO SOUTHWESTWARD TO
SOUTH TEXAS...WHICH IS EFFECTIVELY BLOCKING ANY SIGNIFICANT MOTION.
A DEEP TROUGH THAT IS AMPLIFYING OFF THE U.S. EAST COAST IS
EXPECTED TO MAINTAIN THIS BLOCKING PATTERN FOR THE NEXT 36 HOURS OR
SO. BY 48 HOURS...THE BLOCKING PATTERN IS EXPECTED TO SLOWLY BREAK
DOWN AS A WEAK SHORTWAVE TROUGH MOVES EASTWARD ACROSS THE SOUTHERN
AND CENTRAL PLAINS STATES. THIS WEAK PROGRESSIVE PATTERN SHOULD
ALLOW FOR THE CYCLONE TO MOVE SLOWLY TO THE NORTH AND THEN TO THE
NORTHEAST. HOWEVER...THE FORWARD SPEED IS ONLY EXPECTED TO BE
AROUND 5 KT...WHICH IS GOING TO EXACERBATE THE HEAVY RAINFALL
THREAT OVER LOUISIANA AND POINTS NORTHEASTWARD. THE FORECAST TRACK
IS SLIGHTLY EAST OF THE PREVIOUS ADVISORY TRACK...MAINLY DUE TO THE
MORE EASTWARD INITIAL POSITION...AND LIES JUST EAST OF THE NHC
MODEL CONSENSUS.
 
WATER VAPOR IMAGERY AND UPPER-AIR OBSERVATIONS INDICATE A SMALL
UPPER-LEVEL LOW OVER SOUTHWESTERN LOUISIANA THAT HAS BEEN PRODUCING
MODERATE TO STRONG SOUTHWESTERLY WIND SHEAR ACROSS THE CYCLONE HAS
BEEN WEAKENING AND LIFTING OUT TO THE NORTHWEST. THIS HAS ALLOWED
THE WIND SHEAR TO DECREASE AND ALSO TURN TO THE SOUTH AND
SOUTHEAST. DISSIPATION OF THIS UPPER-LEVEL LOW AND ITS ATTENDANT
DRY AIR INFLUENCE IS FORECAST TO OCCUR IN THE NEXT 24
HOURS...ALLOWING THE SYSTEM TO STRENGTHEN.  GIVEN
THE SLOW MOTION...LESSENING SHEAR...AND VERY WARM SSTS NEAR 30C THE
CYCLONE COULD BE NEAR HURRICANE STRENGTH BEFORE LANDFALL OCCURS IN
ABOUT 48 HOURS. THE OFFICIAL INTENSITY FORECAST IS SIMILAR TO THE
NHC INTENSITY CONSENSUS MODEL...ICON...WITH THE STATISTICAL MODELS
ABOVE AND THE DYNAMICAL MODELS BELOW THE OFFICIAL FORECAST.


FORECAST POSITIONS AND MAX WINDS
 
INIT  02/1500Z 27.3N  91.5W   30 KT  35 MPH
 12H  03/0000Z 27.6N  91.5W   35 KT  40 MPH
 24H  03/1200Z 28.2N  91.8W   45 KT  50 MPH
 36H  04/0000Z 28.8N  91.6W   55 KT  65 MPH
 48H  04/1200Z 29.7N  91.3W   55 KT  65 MPH...INLAND
 72H  05/1200Z 29.8N  90.4W   50 KT  60 MPH...INLAND
 96H  06/1200Z 31.0N  88.5W   40 KT  45 MPH...INLAND
120H  07/1200Z 33.0N  87.0W   25 KT  30 MPH...POST-TROP/EXTRATROP
 
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FORECASTER STEWART
 
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