Tropical Storm IRWIN

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TROPICAL STORM IRWIN DISCUSSION NUMBER  19
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 PM PDT MON OCT 10 2011
 
CONVECTION ASSOCIATED WITH IRWIN REMAINS LIMITED AND DISORGANIZED.
THERE IS A RAGGED PATCH REMOVED FROM THE CENTER ON THE SOUTH SIDE
OF THE CIRCULATION...WITH SPOTTY AND SOMEWHAT SHALLOW CONVECTION IN
A NORTHERN BAND. DVORAK T-NUMBERS ARE UNCHANGED FROM 1200 UTC...SO
THE INITIAL INTENSITY IS HELD AT 35 KT.  AN EASTERLY SHEAR
ENVIRONMENT CONTINUES TO PREVAIL OVER THE TROPICAL CYCLONE...WITH A
MORE STABLE AIR MASS CONTINUING TO WRAP AROUND THE WEST SIDE OF THE
CIRCULATION. WHILE THE EFFECT OF THE NEARBY STRATOCUMULUS FIELD
MAY LESSEN AS IRWIN MOVES FARTHER AWAY FROM IT IN THE NEXT DAY OR
TWO...THE SHIPS MODEL OUTPUT INDICATES NO RELENTING OF THE SHEAR.
IN FACT...SOME INCREASE IN SHEAR IS SEEN TOWARD THE END OF THE
FORECAST PERIOD...AND THERE IS SOME CHANCE THAT IRWIN COULD
DEGENERATE INTO A SHALLOW CYCLONE BY THAT TIME. THE NHC INTENSITY
FORECAST HAS BEEN LOWERED FROM THE PREVIOUS ONE AND CONTINUES TO
SIDE WITH THE STATISTICAL-DYNAMICAL GUIDANCE.
 
AS IS THE CASE WITH WEAK SYSTEMS...THE CENTER HAS BEEN DIFFICULT TO
LOCATE. VISIBLE SATELLITE IMAGES SHED SOME LIGHT ON THE CENTER
LOCATION AND SUGGEST THAT IRWIN IS MOVING 085/06. THE CYCLONE
SHOULD BE CARRIED EASTWARD AND THEN NORTHEASTWARD WITH SOME
INCREASE IN FORWARD SPEED DURING THE NEXT 2-3 DAYS AS IT BENDS
CYCLONICALLY AROUND A MID-TROPOSPHERIC LOW CUTTING OFF SOUTHWEST OF
BAJA CALIFORNIA. AFTER ABOUT 4 DAYS...IRWIN IS EXPECTED TO TURN
EASTWARD AND EAST-SOUTHEASTWARD AS IT BECOMES EMBEDDED IN THE
STRENGTHENING LOW-LEVEL WESTERLY FLOW ASSOCIATED WITH A NORTHWARD
SHIFT OF THE PACIFIC ITCZ OVER MEXICO. ALTHOUGH GLOBAL MODELS SHOW
SOME MID-LEVEL RIDGING BUILDING NORTH OF IRWIN BY THAT TIME...THE
SHALLOW CYCLONE WILL PROBABLY NOT BE ABLE TO ESCAPE FROM THE
LOW-LEVEL WESTERLIES. SOME LARGE DIFFERENCES ARE SEEN IN THE TRACK
GUIDANCE AT DAYS 4-5...LIKELY DUE TO MODEL DIFFERENCES IN THE
VORTEX DEPTH. THE NEW OFFICIAL TRACK FORECAST HAS BEEN LARGELY
UNCHANGED FROM THE PREVIOUS ONE...AND IS BETWEEN THE MULTI-MODEL
CONSENSUS AND THE SHALLOW BAM TRACK.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  10/2100Z 14.9N 116.9W   35 KT  40 MPH
 12H  11/0600Z 14.9N 115.7W   30 KT  35 MPH
 24H  11/1800Z 14.9N 114.1W   30 KT  35 MPH
 36H  12/0600Z 15.3N 111.9W   30 KT  35 MPH
 48H  12/1800Z 16.1N 109.7W   30 KT  35 MPH
 72H  13/1800Z 17.3N 106.2W   30 KT  35 MPH
 96H  14/1800Z 17.0N 104.0W   30 KT  35 MPH
120H  15/1800Z 16.5N 103.0W   30 KT  35 MPH
 
$$
FORECASTER KIMBERLAIN
 
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