Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  13
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 AM PDT SUN OCT 09 2011
 
IRWIN CONTINUES TO LACK DEEP CONVECTION NEAR THE CORE...WITH ONLY A
LITTLE BIT IN A LOOSE BAND TO THE SOUTH OF THE CENTER.  AN ASCAT
PASS AT 0500Z SHOWED WINDS AS HIGH AS ABOUT 35 KT IN A BROAD BAND
ABOUT 45 N MI FROM THE CENTER...CONSISTENT WITH THE STRUCTURE
OBSERVED IN MICROWAVE IMAGERY FROM THE NRL WEB PAGE.  BASED ON
THESE OBSERVATIONS...THE INITIAL INTENSITY IS LOWERED TO 40 KT.
 
THE INITIAL MOTION IS 055/4. A PAIR OF TROUGHS TO THE NORTH OF THE
TROPICAL CYCLONE ARE EXPECTED TO GUIDE IRWIN GENERALLY EASTWARD
OVER THE NEXT TWO TO THREE DAYS IN THE WAKE OF HURRICANE JOVA.
TOWARDS THE END OF THE FORECAST PERIOD...BOTH THE GFS AND ECMWF ARE
SHOWING A BINARY INTERACTION BETWEEN IRWIN AND A PIECE OF THE
SECOND TROUGH...WHICH IMPARTS A TURN OF THE CYCLONE TO THE
NORTHEAST. THE OFFICIAL FORECAST ATTEMPTS TO BLEND THE OUTPUT OF
THESE TWO MODELS WITH THE PREVIOUS OFFICIAL TRACK...ALTHOUGH THERE
IS A LARGE SPREAD AMONG THE GUIDANCE MODELS AFTER THREE DAYS AND
THE LATTER PART OF THE FORECAST IS CONSIDERED TO BE OF LOW
CONFIDENCE.

IRWIN HAS NOT HAD THE REQUISITE ORGANIZED CONVECTION OF A TROPICAL
CYCLONE FOR AT LEAST 12 HOURS...AND IT IS NOT CLEAR THAT THE NEW
BAND IN THE SOUTHERN SEMICIRCLE WILL PERSIST.  IN ADDITION TO
EASTERLY SHEAR...IRWIN MAY BE INGESTING THE MORE STABLE AIR TO ITS
NORTH.  THE LARGER RMW ALSO DOES NOT FAVOR RE-INTENSIFICATION IN
THE SHORT TERM.  THE SHIPS AND LGEM MODELS SHOW A GRADUAL DECAY
OVER THE NEXT FEW DAYS...AND THE GFDL AND HWRF HAVE BACKED OFF
CONSIDERABLY FROM THEIR PREVIOUS AGGRESSIVE FORECASTS.  STILL...THE
UPPER WIND PATTERN IS EXPECTED TO GRADUALLY BECOME LESS HOSTILE AND
IRWIN IS FORECAST TO MOVE OVER WARMER WATERS.  THE OFFICIAL
FORECAST ASSUMES THAT IRWIN WILL NOT DEGENERATE TO A REMNANT LOW
AND SHOWS SOME RESTRENGTHENING AFTER 48 HOURS.
  
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  09/0900Z 14.9N 120.1W   40 KT  45 MPH
 12H  09/1800Z 15.3N 119.3W   35 KT  40 MPH
 24H  10/0600Z 15.7N 118.2W   35 KT  40 MPH
 36H  10/1800Z 15.9N 117.0W   35 KT  40 MPH
 48H  11/0600Z 16.0N 115.5W   35 KT  40 MPH
 72H  12/0600Z 15.5N 112.5W   40 KT  45 MPH
 96H  13/0600Z 17.0N 109.0W   45 KT  50 MPH
120H  14/0600Z 19.0N 107.0W   50 KT  60 MPH
 
$$
FORECASTER FRANKLIN
 
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