000
WTPZ41 KNHC 140853
TCDEP1
 
TROPICAL STORM IRWIN DISCUSSION NUMBER  33
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
200 AM PDT FRI OCT 14 2011

AFTER BRIEFLY DIMINISHING IN THE LATE EVENING...VIGOROUS CONVECTION
HAS RE-FIRED NEAR THE CENTER OF THE TROPICAL STORM.  THERE IS NO
NEW SHIP DATA...BUT IT WOULD NOT BE SURPRISING IF THE SYSTEM WERE A
BIT STRONGER THAN EARLIER.  SATELLITE ESTIMATES ARE UNCHANGED FROM
SIX HOURS AGO...HOWEVER...SO THE INITIAL WIND SPEED REMAINS 40 KT. 

NIGHT VISIBLE IMAGES INDICATE THAT THE CYCLONE IS DRIFTING TO THE
SOUTH-SOUTHEAST.  IRWIN SHOULD MOVE FASTER TO THE SOUTH AND
SOUTH-SOUTHWEST BY EARLY SATURDAY AS A MID-LEVEL RIDGE BUILDS OVER
MEXICO...RESULTING IN AN UNUSUAL TRACK FOR AN EASTERN PACIFIC
TROPICAL CYCLONE.  HOWEVER THE GLOBAL MODELS ARE IN GOOD AGREEMENT
ON THIS COURSE AND ONLY A SMALL SOUTHWESTWARD ADJUSTMENT WAS MADE
TO THE PREVIOUS FORECAST DURING THE FIRST COUPLE OF DAYS.  IN THE
MEDIUM RANGE...THE GLOBAL MODELS DIVERGE ON HOW MUCH WESTWARD
PROGRESS THE STORM WILL MAKE.  THE DIFFERENCES APPEAR TO BE DUE TO
WHETHER IRWIN IS STEERED MORE BY THE WESTERLY LOW-LEVEL FLOW OR THE
MID-LEVEL RIDGE.  SINCE IRWIN IS FORECAST TO BE RELATIVELY WEAK AT
THAT TIME...THE NEW FORECAST TRACK LEANS ON THE SLOWER SOLUTIONS
HAVING IRWIN BEING CAUGHT UP IN THE ITCZ...AND SHOWS THE CYCLONE
STATIONARY AFTER DAY 3.  

THE STORM IS CURRENTLY EXPERIENCING LOW SHEAR CONDITIONS OVER WARM
WATERS...BUT THIS IS PROBABLY THE MOST FAVORABLE ENVIRONMENT IT
WILL ENCOUNTER FOR THE FORESEEABLE FUTURE.  WATER VAPOR IMAGES
INDICATE THAT MID/UPPER-LEVEL DRY AIR IS APPROACHING FROM THE
NORTH.  WHILE IRWIN IS FORECAST TO MOVE FASTER TO THE SOUTH...WHICH
COULD LIMIT THE INFLUENCE OF THE DRY AIR...THE FORECAST TRACK MOVES
OVER THE SIGNIFICANT COLD WAKE OF JOVA JUST AFTER 24 HOURS.  IN
ADDITION... MOST OF THE GLOBAL MODELS SHOW AN INCREASE IN
NORTHEASTERLY SHEAR TO ABOUT 20 KT IN A DAY OR TWO...AND EVEN
STRONGER AT LONG RANGE.  THESE FACTORS SHOULD PROMOTE WEAKENING OF
IRWIN OVER THE WEEKEND...AND THUS THE OFFICIAL FORECAST IS REDUCED
FROM THE PREVIOUS ONE.  IT IS WORTH NOTING THAT MUCH OF THE
GUIDANCE IS HIGHER THAN THE NEW FORECAST...HOWEVER THESE MODELS ARE
UNAWARE OF THE COLD WAKE OF JOVA.  SINCE THE NEW FORECAST STALLS
IRWIN OVER THE COOLER WATERS WITH STRONG EASTERLY SHEAR
PRESENT...REMNANT LOW STATUS IS INDICATED AT 120 HOURS...AND IT
WOULD NOT BE SURPRISING IF IT HAPPENED SOONER THAN THAT TIME.  
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  14/0900Z 18.9N 106.8W   40 KT  45 MPH
 12H  14/1800Z 18.4N 106.7W   40 KT  45 MPH
 24H  15/0600Z 17.2N 106.8W   40 KT  45 MPH
 36H  15/1800Z 15.7N 107.0W   35 KT  40 MPH
 48H  16/0600Z 14.7N 107.6W   35 KT  40 MPH
 72H  17/0600Z 15.0N 109.0W   30 KT  35 MPH
 96H  18/0600Z 15.0N 109.0W   30 KT  35 MPH
120H  19/0600Z 15.0N 109.0W   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
FORECASTER BLAKE