SOUTH INDIAN: 26S(SEROJA) forecast to intensify and absorb TC 27S by 96hours, Invest 91S now monitored to the East of 90°Est, 07/09utc update

07/08UTC. 9H LOOP. FROM RIGHT TO LEFT: TC 26S(SEROJA). TC 27S AND INVEST 91S. IF NEEDED CLICK TO ANIMATE.

27S(SEROJA). WARNING 12 ISSUED AT 07/09UTC. THE SYSTEM REMAINS COCOONED IN A MARGINALLY SUPPORTIVE ENVIRONMENT CHARACTERIZED BY MODERATE (15-20 KTS) EASTERLY VERTICAL WIND SHEAR(VWS) OFFSET BY DIFFLUENT FLOW ALOFT, THOUGH THERE IS NO DISTINCT OUTFLOW CHANNEL AT PRESENT. TC 26S IS FORECAST TO CONTINUE TRACKING SOUTHWESTWARD ALONG THE NORTHWEST PERIPHERY OF A DEEP-LAYER SUBTROPICAL RIDGE (STR) CENTERED OVER WESTERN AUSTRALIA THROUGH 72H, BEFORE ROUNDING THE RIDGE AXIS NEAR 72H, AND THEN ACCELERATING SOUTHEASTWARD THROUGH 120H. THE SYSTEM IS EXPECTED TO MAKE LANDFALL SOUTH OF SHARK BAY NEAR 108H. SLOW INTENSIFICATION IS FORECAST OVER THE NEXT 36 HOURS AS THE SYSTEM MOVES SLOWLY INTO A GENERALLY MORE FAVORABLE ENVIRONMENT WITH INCREASED MOISTURE AND DECREASED VWS. TC 26S IS FORECAST TO RAPIDLY INTENSIFY, FROM 60 KNOTS TO 110 KNOTS/US CAT 3 BETWEEN 48 AND 72H, AS IT APPROACHES THE RIDGE AXIS, VWS DROPS TO THE 5-10 KNOT RANGE AND A COMPACT POINT SOURCE DEVELOPS ALOFT OVER TOP OF THE SYSTEM. AFTER ROUNDING THE RIDGE AND ACCELERATING SOUTHEASTWARD, INCREASING SHEAR, COOLER WATERS AND ULTIMATELY LAND INTERACTION WILL LEAD TO RAPID WEAKENING THROUGH THE REMAINDER OF THE FORECAST. THE SYSTEM IS EXPECTED TO BEGIN EXTRA- TROPICAL TRANSITION AFTER LANDFALL AS IT MOVES EAST OF PERTH. AT THIS POINT, THE BINARY INTERACTION WITH TC 27S IS NOT EXPECTED TO IMPACT THE TRACK FORECAST, BUT THERE REMAINS A LOW PROBABILITY OF A SLIGHT WOBBLE FURTHER WEST IN THE MID-RANGE PORTION OF THE FORECAST AS THE TWO SYSTEMS BEGIN TO INTERACT WITH ONE ANOTHER.

26S(SEROJA). DYNAMIC MODEL GUIDANCE IS IN OVERALL GOOD AGREEMENT ON THE OVERALL SCENARIO, WITH SOME DIFFERENCES IN THE TIMING AND SHARPNESS OF THE RECURVE STARTING TO EMERGE. IN PARTICULAR, THE ECMWF HAS SHIFTED MUCH FURTHER TO THE WEST IN THE LATEST RUN AND IS NO THE WESTERN-MOST OUTLIER AMONGST THE CONSENSUS MEMBERS. THE JTWC FORECAST LIES ALONG THE WESTERN EDGE OF THE GUIDANCE ENVELOPE AS A NOD TOWARDS THE ECMWF, BUT REMAINS CONSISTENT WITH THE PREVIOUS FORECAST TRACK. OVERALL CONFIDENCE REMAINS MODERATE IN LIGHT OF THE UNCERTAINTY ASSOCIATED WITH THE BINARY INTERACTION WITH TC 27S.

26S(SEROJA). 07/0920UTC. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS WELL-DEFINED LOW LEVEL CLOUD BANDS WRAPPING INTO A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) POSITIONED ON THE EASTERN EDGE OF FLARING, SHEARED CONVECTION.

26S(SEROJA). 07/0820UTC. MICROWAVE IMAGE SHOWED THE LOWER LEVEL CLOUD BANDS WRAPPING INTO A WELL DEFINED LLCC TO GOOD EFFECT AND LENT HIGH CONFIDENCE TO THE INITIAL POSITION.

26S. WARNING 10 ISSUED AT 07/09UTC. TC 27S IS ENSCONCED IN A MARGINAL ENVIRONMENT WITH A MODERATE (15-20 KTS) EASTERLY VERTICAL WIND SHEAR(VWS) INDUCED FROM THE GENERAL EAST-NORTHEASTERLY ENVIRONMENTAL FLOW ALOFT AS WELL AS OUTFLOW FROM TC 26S TO THE NORTHEAST. WHILE SHEAR IS MODERATE, IT IS BEING OFFSET SOMEWHAT BY ROBUST POLEWARD OUTFLOW AND WARM SSTS. TC 27S IS FORECAST TO CONTINUE TRACKING SLOWLY NORTHWARD OVER THE NEXT 24 HOURS UNDER GENERALLY WEAK STEERING CONDITIONS, BUT WILL TURN EAST BY 36H AS IT MOVES WITHIN 650KM OF TC 26S AND IS CAPTURED BY THE LARGER SYSTEM. ONCE CAPTURED, TC 27S WILL BEGIN BINARY INTERACTION WITH TC 26S, RAPIDLY ACCELERATING AROUND TO A SOUTHEASTWARD AND THEN SOUTHWARD TRACK BETWEEN 48H THROUGH 96H. THE TWO SYSTEMS WILL MOVE TO WITHIN 315KM OF ONE ANOTHER BY 72H AND TC 27S WILL BEGIN TO BECOME ABSORBED BY TC 26S, WITH FULL MERGER EXPECTED NO LATER THAN 96H. CURRENT ENVIRONMENTAL CONDITIONS ARE EXPECTED TO PERSIST OVER THE NEXT 24 HOURS, WITH LITTLE TO NO CHANGE IN INTENSITY EXPECTED BEFORE 24H. AS THE SYSTEM BEGINS TO MOVE NORTH OF TC 26S IT WILL MOVE INTO AN AREA OF IMPROVED OUTFLOW AND DECREASED VWS, ALLOWING FOR A BRIEF PERIOD OF INTENSIFICATION, TO A PEAK OF 50 KNOTS AT 48H. AFTER THIS POINT, THE COMBINATION OF INCREASING VWS AND DISRUPTION OF THE VORTEX DUE TO INTERACTION WITH TC 26S WILL LEAD TO RAPID WEAKENING AFTER 72H.

26S. DYNAMIC MODEL GUIDANCE IS IN POOR AGREEMENT, AND THE JTWC FORECAST TRACK HAS SHIFTED SLIGHTLY TO THE WEST THAN THE PREVIOUS FORECAST, CLOSE TO THE EGRR SOLUTION. THE GFS, NAVGEM AND ECMWF SOLUTIONS MOVED FURTHER EAST IN THE MID-RANGE OF THE FORECAST BUT ALIGN CLOSELY WITH THE JTWC FORECAST BY 96H. CONFIDENCE IN THE TRACK REMAINS LOW DUE TO THE UNCERTAINTY ASSOCIATED WITH THE BINARY INTERACTION WITH TC 26S.

26S. MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A VERY WELL-DEFINED, COMPACT AND PARTIALLY EXPOSED LOW LEVEL VORTEX DISPLACED NORTHEAST OF DEEP FLARING CONVECTION.

INVEST 91S. INVEST 91S IS IN A MARGINAL ENVIRONMENT FOR TROPICAL DEVELOPMENT WITH GOOD POLEWARD OUTFLOW ALOFT AND WARM (29-30C) SEA SURFACE TEMPERATURES OFFSET BY STRONG (20-25KTS) VERTICAL WIND SHEAR (VWS). GLOBAL MODELS ARE IN GOOD AGREEMENT THAT INVEST 91S WILL CONSOLIDATE AND INTENSIFY OVER THE NEXT 24-48 HOURS AS IT TRACKS TO THE SOUTHEAST UNDER AN AREA OF LOW (10-15KTS) VWS.

07/10UTC. JTWC IS ISSUING 6HOURLY WARNINGS ON 26S(SEROJA) AND 27S. 3HOURLY SATELLITE BULLETINS ARE ISSUED FOR BOTH SYSTEMS. INVEST 91S IS NOW ON THE MAP WITH CURRENTLY LOW CHANCES OF REACHING 35KNOTS WITHIN 24HOURS.