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Tropical Depression IRWIN Forecast Discussion Number 40

Issued at 800 PM PDT SAT OCT 15 2011

000
WTPZ41 KNHC 160235
TCDEP1
 
TROPICAL DEPRESSION IRWIN DISCUSSION NUMBER  40
NWS NATIONAL HURRICANE CENTER MIAMI FL       EP112011
800 PM PDT SAT OCT 15 2011
 
THE STRUCTURE OF IRWIN CONTINUES TO DETERIORATE.  THE LOW-LEVEL
CENTER RE-FORMED A LITTLE CLOSER TO THE CONVECTION THIS EVENING...
BUT THE CONVECTION HAS BEEN SHRINKING WITH ONLY A SMALL AREA OF
-50C CLOUD TOPS REMAINING.  A DELAYED 1708 UTC ASCAT PASS RECEIVED
JUST A FEW HOURS AGO SHOWED MAXIMUM WINDS OF ONLY 20-25 KT IN THE
SOUTHWESTERN QUADRANT...AND THE INTENSITY IS CONSERVATIVELY LOWERED
TO 30 KT.  IRWIN IS NOW MOVING AWAY FROM THE COLD WAKE LEFT BEHIND
FROM JOVA TOWARD WARMER WATER...BUT THE DAMAGE MAY ALREADY BE DONE. 
EAST-NORTHEASTERLY TO EASTERLY SHEAR IS EXPECTED TO INCREASE OVER
THE NEXT COUPLE OF DAYS...AND GIVEN THE CURRENT STRUCTURE OF THE
DEPRESSION...RE-INTENSIFICATION IS NOT EXPECTED.  THE NEW OFFICIAL
FORECAST NOW MAKES IRWIN A REMNANT LOW WITHIN 48 HOURS...BUT THIS
COULD OCCUR AS EARLY AS SUNDAY IF SUFFICIENT DEEP CONVECTION DOES
NOT RE-DEVELOP.  THE ECMWF IS THE ONLY MODEL TO DISSIPATE THE
SYSTEM...WHICH IT DOES BY DAY 3...SO THE FORECAST CONTINUES TO HOLD
OFF DISSIPATION UNTIL DAY 5.

IRWIN IS MOVING 200 DEGREES AT 8 KT...CAUGHT BETWEEN A LOW-LEVEL
RIDGE THAT EXTENDS FROM NORTHWESTERN MEXICO SOUTHWESTWARD ACROSS
THE PACIFIC...AND BROAD CYCLONIC FLOW OVER CENTRAL AMERICA.  THE
LOW-LEVEL STEERING IS EXPECTED TO COLLAPSE OVER THE NEXT COUPLE OF
DAYS...AND IRWIN IS THEREFORE FORECAST TO BECOME NEARLY STATIONARY
BY 48 HOURS.  THE REMNANT LOW COULD THEN BEGIN TO MOVE A LITTLE
FASTER TOWARD THE SOUTHEAST BY DAY 4 AS IT BECOMES INFLUENCED BY
STRENGTHENING SOUTHWESTERLY CROSS-EQUATORIAL FLOW.  THE NEW
OFFICIAL FORECAST IS RELATIVELY UNCHANGED FROM THE PREVIOUS ONE
THROUGH 48 HOURS BUT THEN IS A LITTLE FARTHER SOUTHEAST ON DAYS 3
AND 4. 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  16/0300Z 14.8N 107.3W   30 KT  35 MPH
 12H  16/1200Z 14.2N 108.0W   25 KT  30 MPH
 24H  17/0000Z 14.0N 108.8W   25 KT  30 MPH
 36H  17/1200Z 14.1N 109.5W   25 KT  30 MPH
 48H  18/0000Z 14.2N 109.8W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  19/0000Z 13.5N 109.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  20/0000Z 13.0N 108.5W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  21/0000Z...DISSIPATED
 
$$
FORECASTER BERG
 

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