Tropical Depression IRWIN

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TROPICAL DEPRESSION IRWIN DISCUSSION NUMBER  21
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 AM PDT TUE OCT 11 2011
 
SATELLITE IMAGERY INDICATES SOME RECENT DEEP CONVECTION HAS BEGUN TO
DEVELOP OVER THE SOUTHEAST PORTION OF THE CYCLONE.  THUNDERSTORM
ACTIVITY OVER THE REMAINING QUADRANTS...HOWEVER...CONTINUES TO BE
QUITE LIMITED.  DVORAK SATELLITE INTENSITY ESTIMATES ARE 35 KT FROM
TAFB...AND 30 KT FROM SAB.  IT WOULD NOT TAKE MUCH MORE BANDING
DEVELOPMENT FOR IRWIN TO RETURN TO TROPICAL STORM STRENGTH...BUT I
HAVE ELECTED TO KEEP THE SYSTEM AS A DEPRESSION FOR THIS ADVISORY
BASED ON THE POSSIBILITY THAT THE NEW CONVECTION COULD BE
SHORT-LIVED AS HAS OCCURED OVER THE PAST COUPLE OF DAYS.
 
THE SHIPS MODEL AND UW-CIMSS SHEAR PRODUCT INDICATES FAVORABLE LIGHT
850-200 MB SHEAR OVER THE CYCLONE.  THE CIRA AMSU AREA-AVERAGED
WIND SHEARS PRODUCT...HOWEVER...INDICATES AN INCREASE IN MID-LEVEL
VERTICAL SHEAR...POSSIBLY UNDERCUTTING THE OUTFLOW ALOFT.  THIS
ALONG WITH A DRY/STABLE SURROUNDING ENVIRONMENT APPEARS TO BE
INHIBITING SIGNIFICANT DEVELOPMENT.  FURTHERMORE...STATISTICAL AND
DYNAMICAL MODELS INDICATE AN INCREASE IN NORTHEASTERLY SHEAR TOWARD
THE END OF THE FORECAST PERIOD.  THE INTENSITY FORECAST IS AN
UPDATE OF THE PREVIOUS ONE...BUT NOW INDICATES IRWIN AS A REMNANT
LOW AT DAY 5...CONSISTENT WITH THE MODEL GUIDANCE.
 
THE INITIAL MOTION IS 080/7.  IRWIN REMAINS EMBEDDED WITHIN THE
DEEP WESTERLY FLOW ASSOCIATED WITH A MAJOR SHORTWAVE TROUGH
SITUATED TO THE NORTHWEST OF THE CYCLONE.  THE DEPRESSION IS
EXPECTED TO CONTINUE ON A GENERAL EASTWARD TRACK DURING THE NEXT 24
HOURS.  FROM 36-72 HR...THE LARGE-SCALE MODELS SHOW A CUT-OFF LOW
DEVELOPING AT THE BASE OF THE SHORTWAVE TROUGH.  THIS MID- TO
UPPER-LEVEL FEATURE SHOULD INDUCE A NORTHEASTWARD TURN OF THE
CYCLONE.  FOR THE REMAINDER OF THE PERIOD...THE DYNAMICAL MODELS
GENERALLY AGREE WITH A GRADUAL TURN BACK TO THE EAST OR
EAST-SOUTHEAST AS THE VERTICALLY SHALLOW SYSTEM BEGINS TO TRACK
WITHIN THE STRENGTHENING NORTHWESTERLY LOW-LEVEL FLOW.  THE
OFFICIAL NHC FORECAST IS SIMILAR TO THE PREVIOUS ONE AND IS NUDGED
TO THE LEFT TO COINCIDE WITH NOGAPS AND THE TV15 HFIP CONSENSUS
MODEL WHICH ALSO INCLUDES THE GFS...ECMWF...AND UKMET GLOBAL
MODELS.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  11/0900Z 15.2N 115.5W   30 KT  35 MPH
 12H  11/1800Z 15.2N 114.3W   30 KT  35 MPH
 24H  12/0600Z 15.6N 112.3W   30 KT  35 MPH
 36H  12/1800Z 16.4N 110.0W   30 KT  35 MPH
 48H  13/0600Z 17.1N 107.9W   30 KT  35 MPH
 72H  14/0600Z 17.5N 105.1W   30 KT  35 MPH
 96H  15/0600Z 16.5N 103.5W   30 KT  35 MPH
120H  16/0600Z 16.0N 103.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER ROBERTS/BROWN
 
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