Tropical Storm IRWIN
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TROPICAL STORM IRWIN DISCUSSION NUMBER 36
NWS NATIONAL HURRICANE CENTER MIAMI FL EP112011
800 PM PDT FRI OCT 14 2011
THE LAST FEW VISIBLE IMAGES OF THE DAY SHOWED LOW CLOUDS IN THE
SOUTHEASTERN QUADRANT OF IRWIN MOVING EASTWARD TOWARDS THE MEXICAN
COAST...INSTEAD OF TURNING CYCLONICALLY AROUND THE CENTER. THIS
FLOW PATTERN IS LIKELY PRODUCING LOW-LEVEL DIVERGENCE...WHICH BY
ITSELF WOULD NOT BE SUPPORTIVE OF CONVECTIVE DEVELOPMENT. AS A
POSSIBLE CONSEQUENCE...MUCH OF THE DEEP CONVECTION THAT DEVELOPED
WITH IRWIN EARLIER IN THE DAY HAS EVAPORATED. CURRENT INTENSITY
ESTIMATES FROM TAFB...SAB...AND THE UW-CIMSS ADT RANGE FROM 30 TO
50 KT...SO THE INITIAL INTENSITY IS MAINTAINED AT 40 KT.
THE LOW-LEVEL CENTER BECAME A LITTLE MORE APPARENT JUST BEFORE
SUNSET...AND WITH THE SUPPORT OF MICROWAVE DATA...IRWIN IS
ESTIMATED TO BE MOVING SOUTH-SOUTHEASTWARD WITH A 12-HR MOTION OF
165/6 KT. A MID-LEVEL HIGH OVER NORTHWESTERN MEXICO IS EXPECTED TO
EXPAND SOUTHWESTWARD OVER THE NEXT FEW DAYS AND SHOULD CAUSE IRWIN
TO TURN SOUTHWARD SOON AND WESTWARD BY 48 HOURS. THEREAFTER...BROAD
LOW/MID-LEVEL CYCLONIC FLOW NEAR CENTRAL AMERICA IS FORECAST TO
DRIFT WESTWARD JUST FAR ENOUGH TO CAUSE IRWIN TO SLOW DOWN AND MOVE
ERRATICALLY. MOST OF THE TRACK GUIDANCE AGREES ON THIS SCENARIO
AND EVEN SUGGEST THAT IRWIN COULD BECOME NEARLY STATIONARY ON DAY 4
AND TURN BACK TO THE SOUTHEAST BY DAY 5. THE NEW OFFICIAL FORECAST
HAS BEEN SHIFTED A LITTLE TO THE SOUTH AND WEST ON DAYS 4 AND 5 BUT
IS OTHERWISE VERY SIMILAR TO THE PREVIOUS FORECAST.
THE FORECAST TRACK TAKES IRWIN THROUGH A MARGINAL ENVIRONMENT FOR
INTENSIFICATION DURING THE NEXT FEW DAYS. NEGATIVE FACTORS INCLUDE
INCREASING VERTICAL SHEAR...THE COLD OCEANIC WAKE LEFT BY HURRICANE
JOVA...A DRY ENVIRONMENTAL AIR MASS...AND THE AFOREMENTIONED
LOW-LEVEL DIVERGENCE. HOWEVER AS IRWIN MOVES SOUTHWARD AWAY FROM
MEXICO...IT WILL BE MOVING INTO A MOISTER AIR MASS...AND LOW-LEVEL
CONVERGENCE SHOULD BE ABLE TO INCREASE. THESE FACTORS COULD FOSTER
RE-DEVELOPMENT OF DEEP CONVECTION. GIVEN THE CONFLICTING
SIGNALS...THE OFFICIAL FORECAST CONTINUES TO HOLD THE INTENSITY
STEADY FOR THE FIRST 3 DAYS...FOLLOWED BY GRADUAL WEAKENING AND
TRANSITION TO A REMNANT LOW. IT SHOULD BE NOTED THAT NEARLY ALL OF
THE INTENSITY MODELS INDICATE SOME STRENGTHENING DURING THE NEXT
COUPLE OF DAYS AND IS ANOTHER REASON TO NOT SHOW WEAKENING DURING
THE FIRST PART OF THE FORECAST PERIOD.
FORECAST POSITIONS AND MAX WINDS
INIT 15/0300Z 17.7N 106.4W 40 KT 45 MPH
12H 15/1200Z 16.8N 106.7W 40 KT 45 MPH
24H 16/0000Z 15.6N 107.2W 40 KT 45 MPH
36H 16/1200Z 14.8N 108.0W 40 KT 45 MPH
48H 17/0000Z 14.5N 108.9W 40 KT 45 MPH
72H 18/0000Z 15.0N 110.0W 40 KT 45 MPH
96H 19/0000Z 15.0N 110.5W 30 KT 35 MPH
120H 20/0000Z 14.5N 110.0W 30 KT 35 MPH...POST-TROP/REMNT LOW
$$
FORECASTER BERG
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