Tropical Depression THIRTEEN-E

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TROPICAL DEPRESSION THIRTEEN-E DISCUSSION NUMBER   4
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP132011
700 AM PST SUN NOV 20 2011
 
SATELLITE IMAGERY SHOWS THAT THE DEPRESSION HAS CHANGED LITTLE IN
ORGANIZATION DURING THE PAST SEVERAL HOURS. VISIBLE IMAGERY REVEALS
ONE BAND OF CONVECTION OVER THE NORTHEAST PORTION OF THE
CIRCULATION...WITH SEVERAL WEAKER BANDS WELL REMOVED FROM THE
CENTER. THE LATEST DVORAK INTENSITY ESTIMATES ARE 30 AND 35 KT FROM
SAB AND TAFB...RESPECTIVELY. SINCE THE OVERALL STRUCTURE OF THE
CYCLONE HAS NOT CHANGED MUCH SINCE THE EARLIER ASCAT PASS WHICH
INDICATED WINDS OF ABOUT 30 KT...THE INITIAL INTENSITY IS HELD AT
THIS VALUE.

THE DEPRESSION IS FORECAST TO REMAIN IN A LOW SHEAR ENVIRONMENT FOR
MUCH OF THE FORECAST PERIOD. THEREFORE...THERMODYNAMICS WILL LIKELY
BE THE MAIN FACTOR THAT REGULATES THE INTENSITY OF THE TROPICAL
CYCLONE. THE SYSTEM IS FORECAST TO REMAIN OVER 27-28C WATERS DURING
THE NEXT 2-3 DAYS WHICH SHOULD ALLOW FOR GRADUAL STRENGTHENING.
AFTER THAT TIME...SEA SURFACE TEMPERATURES GRADUALLY DECREASE AND
FALL BELOW 26C AFTER 96 HOURS. THE CYCLONE IS ALSO EXPECTED TO
ENCOUNTER A DRIER AND MORE STABLE ENVIRONMENT LATE IN THE
PERIOD....WHICH ALONG WITH THE COOLER SSTS SHOULD CAUSE THE CYCLONE
TO WEAKEN. THE OFFICIAL INTENSITY FORECAST IS BETWEEN THE SHIPS AND
LGEM GUIDANCE...AND LOWER THAN THE MORE AGGRESSIVE DYNAMICAL
MODELS.

THE INITIAL MOTION ESTIMATE IS 280/11 KT. A DEEP-LAYER RIDGE
EXTENDING WESTWARD FROM MEXICO INTO THE CENTRAL PACIFIC IS EXPECTED
TO STEER THE CYCLONE WESTWARD TO WEST-NORTHWESTWARD DURING THE
FORECAST PERIOD. A COUPLE OF MID- TO UPPER-LEVEL TROUGHS WILL PASS
TO THE NORTH OF THE CYCLONE DURING THE PERIOD WHICH COULD BRIEFLY
WEAKEN THE RIDGE AND CAUSE THE SYSTEM TO TURN WEST-NORTHWESTWARD.
AFTER THE PASSAGE OF THE FIRST TROUGH IN 36-48 HOURS...THE RIDGE IS
FORECAST TO STRENGTHEN AND CAUSE THE CYCLONE TO MOVE ALMOST DUE
WESTWARD FOR A COUPLE OF DAYS. IN 96 TO 120 HOURS...THE SECOND
TROUGH IS EXPECTED TO WEAKEN THE RIDGE BETWEEN 120W AND 130W. AT
THAT TIME...THE MODELS BEGIN TO DIVERGE...WITH THE GFS SHOWING A
SLIGHTLY DEEPER TROPICAL CYCLONE MOVING MORE WEST-NORTHWESTWARD.
THE ECMWF DEPICTS A WEAKENING SYSTEM THAT TURNS WESTWARD IN THE
LOW-LEVEL FLOW. THE OFFICIAL FORECAST IS IN GOOD AGREEMENT WITH THE
TIGHTLY CLUSTERED GUIDANCE THROUGH 72 HOURS...AND LEANS TOWARD THE
ECMWF SOLUTION OF A WEAKER CYCLONE NEAR THE END OF THE PERIOD.
  
FORECAST POSITIONS AND MAX WINDS
 
INIT  20/1500Z 10.6N 104.7W   30 KT  35 MPH
 12H  21/0000Z 11.1N 106.6W   35 KT  40 MPH
 24H  21/1200Z 11.6N 109.1W   40 KT  45 MPH
 36H  22/0000Z 11.7N 111.2W   50 KT  60 MPH
 48H  22/1200Z 11.7N 113.4W   55 KT  65 MPH
 72H  23/1200Z 12.0N 117.8W   65 KT  75 MPH
 96H  24/1200Z 13.0N 121.3W   60 KT  70 MPH
120H  25/1200Z 13.8N 125.0W   50 KT  60 MPH
 
$$
FORECASTER BROWN
 
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