South Indian: 21S(GUAMBE) forecast to intensify to US/Category 2 by 48hours, 18/09utc update

21S(GUAMBE). 18/07UTC. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A STEADILY IMPROVING CONVECTIVE SIGNATURE WITH WEAK CYCLONIC BANDING FEATURES BECOMING EVIDENT WITHIN THE CENTRAL DENSE OVERCAST (CDO) WHICH IS OBSCURING THE LOW LEVEL CIRCUATION CENTER (LLCC).

21S(GUAMBE). WARNING 2 ISSUED AT 18/09UTC. ENVIRONMENTAL ANALYSIS INDICATES FAVORABLE CONDITIONS WITH A DEVELOPING POINT SOURCE DIRECTLY OVER THE LLOW LEVEL CENTER PROVIDING LOW (5-10 KTS) WIND SHEAR, MODERATE TO STRONG, NEARLY RADIAL OUTFLOW AND WARM (29- 30C) SSTS. TC 21S IS CURRENTLY ENSCONCED BETWEEN A BUILDING SUBTROPICAL RIDGE (STR) CENTER OVER NORTHERN MADACASGAR AND A SECONDARY STR CENTER OVER NORTHERN SOUTH AFRICA, RESULTING IN SLOW MOVEMENT SOUTHWARD. OVER THE NEXT 12-24 HOURS, THE EASTERN STR IS EXPECTED TO BUILD SUFFICIENTLY TO BECOME THE DOMINATE STEERING MECHANISM AND TURN TC 21S TO A WEST-SOUTHWESTWARD TRACK. AS THE STR MOVES SOUTH-SOUTHEASTWARD THROUGH 48H, TC 21S WILL ACCELERATE SOUTH THEN SOUTHEASTWARD AS IT ROUNDS THE RIDGE AXIS. AFTER 72H THE SYSTEM RAPIDLY ACCELERATES SOUTHEASTWARD ALONG THE BAROCLINIC ZONE AS IT BEGINS TO INTERACT WITH STRONG MID-LATITUDE WESTERLY FLOW ALOFT. TC 21S IS EXPECTED TO STEADILY INTENSIFY UNDER FAVORABLE CONDITIONS TO A PEAK OF 85 KNOTS/CATEGORY 2 BY 48H, THEN MAINTAIN INTENSITY THROUGH 72H. THEREAFTER, INCREASING WIND SHEAR WILL OFFSET THE STRONG POLEWARD OUTFLOW, LEADING TO SLOW WEAKENING. TC 21S WILL BEGIN EXTRATROPICAL TRANSITION BY 96H, AND BECOME A STORM-FORCE EXTRATROPICAL SYSTEM BY 120H.

ALL MEMBERS OF THE CONSENSUS CONCUR ON THE OVERALL FORECAST TRACK SCENARIO BUT DISPLAY SIGNIFICANT CROSS TRACK UNCERTAINTY THOUGH 48H, WITH NAVGEM AND THE COAMPS-TC TAKING THE SYSTEM MUCH CLOSER TO THE COAST OF MOZAMBIQUE, AND THE GFS AND ECMWF KEEPING THE SYSTEM WELL OFFSHORE IN THE MOZAMBIQUE CHANNEL. AFTER 72H, THE PACE AND STRENGTH OF THE INTERACTION WITH THE WESTERLIES RESULT IN EXTREMELY LARGE ALONG-TRACK UNCERTAINTY, WITH THE GFS, UKMET AND ECMWF WELL OUT AHEAD (1480KM) OF THE REMAINING MEMBERS OF THE CONSENSUS. THE JTWC FORECAST TRACKS CLOSE TO THE ECMWF THROUGH 48H, BUT CLOSER TO THE NAVGEM THROUGH 120H. OVERALL THE JTWC FORECAST IS MUCH FASTER AFTER 72H THAN THE PREVIOUS FORECAST. DUE TO THE VERY HIGH UNCERTAINTY IN THE LATER PORTIONS OF THE FORECAST, THERE IS OVERALL LOW CONFIDENCE IN THE JTWC FORECAST TRACK.

21S(GUAMBE). 18/08UTC. INTENSIFYING SYSTEM. Eumetsat. Enhanced by Patrick Hoareau.

18/06UTC. JTWC IS ISSUING 6HOURLY WARNINGS ON 01W AND 12HOURLY WARNINGS ON 21S. 3 HOURLY SATELLITE BULLETINS ARE ISSUED FOR BOTH SYSTEMS.