INVEST 91S under watch// 1809utc

JTWC IS ISSUING 3 HOURLY SATELLITE BULLETINS ON INVEST 91S.

THE AREA OF CONVECTION (INVEST 91S) PREVIOUSLY LOCATED NEAR 12.0S 98.9E IS NOW LOCATED NEAR 12.8S 98.1E, APPROXIMATELY 86 NM SOUTHEAST OF COCOS ISLAND. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) WITH DEEP FLARING CONVECTION TO THE SOUTHWEST, FORMING INTO A DISTINCT MID-LEVEL ROTATION. A 180330Z ASCAT-C PASS REVEALS 20-25 KNOTS OF WIND BEGINNING TO WRAP INTO THE LLCC FROM THE SOUTH AND 15-20 KNOT WINDS EXTENDING TO THE NORTH OF THE CIRCULATION. A 180851Z SURFACE OBSERVATION FROM COCOS ISLAND INTERNATIONAL AIRPORT SHOWS WEST-SOUTHWESTERLY 17 KNOT WINDS WITH GUSTS OF 27 KNOTS AND MSLP STEADILY DROPPING OVER THE LAST 4 HOURS FROM 1006MB TO 1003MB. ENVIRONMENTAL ANALYSIS INDICATES THAT INVEST 91S IS IN A FAVORABLE ENVIRONMENT FOR FURTHER DEVELOPMENT DUE TO WARM (29-30C) SEA SURFACE TEMPERATURES, LOW (LESS THAN 15 KNOTS) VERTICAL WIND SHEAR, AND GOOD POLEWARD OUTFLOW ALOFT. GLOBAL MODELS ARE IN AGREEMENT THAT 91S WILL CONTINUE TO SLOWLY DEVELOP. HOWEVER, BOTH THE GFS AND ECMWF ENSEMBLES AND DETERMINISTIC MODELS ARE IN POOR AGREEMENT WITH THE OVERALL TRACK MOTION OVER THE NEXT 24 HOURS AS THE GEFS-GFS IN PARTICULAR SHOW DEVELOPMENT OF A SECONDARY TROPICAL DISTURBANCE TO THE EAST-SOUTHEAST OF 91S AND WHICH ULTIMATELY MERGES WITH 91S OVER THE NEXT FOUR DAYS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 23 TO 28 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1003 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS UPGRADED TO MEDIUM.