TC 13S(EMNATI): moving south: extratropical transition within 24h// Invest 90S now Medium//Invest 99S & Invest 98P updates, 24/09utc

FORECAST REASONING. SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING. FORECAST DISCUSSION: TC 13S IS FORECAST TO CONTINUE TRACKING SOUTH TO SOUTHEASTWARD ALONG THE WESTERN PERIPHERY OF THE DEEP SUBTROPICAL RIDGE (STR) TO THE EAST THROUGH THE FORECAST PERIOD. AFTER THE END OF THE FORECAST PERIOD, THE REMAINING SUBTROPICAL LOW WILL LIKELY TURN POLEWARD ONCE MORE AS IT MOVES TOWARDS A WEAKNESS THAT DEVELOPS IN THE STR AHEAD OF AN APPROACHING MID-LATITUDE TROUGH. OVERALL, NO SIGNIFICANT CHANGE IN THE INTENSITY IS FORECAST THROUGH THE FORECAST PERIOD, WITH THE CONTINUED ROBUST POLEWARD OUTFLOW BEING OFFSET BY VERY LOW OHC, DECREASING SEA SURFACE TEMPERATURES, PROGRESSIVELY INCREASING VERTICAL WIND SHEAR AND MORE SIGNIFICANT DRY AIR ENTRAINMENT. THE SYSTEM WILL BEGIN LOSING ITS TROPICAL CHARACTERISTICS BY 12H AND IS FORECAST TO COMPLETE TRANSITION TO A STRONG GALE-FORCE SUBTROPICAL LOW NO LATER THAN 36H.

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN ILL-DEFINED AND BROAD CIRCULATION SOUTHWEST OF MADAGASCAR, WITH AT LEAST THREE DISTINCT VORTICES ROTATING ABOUT THE LARGER CENTER, WHICH IS DEVOID OF ANY SIGNIFICANT ORGANIZED CONVECTIVE ACTIVITY. THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE, PLACED IN THE CENTROID OF THE TRIO OF VORTICES, AS THERE IS NO CLEAR-CUT CENTER PER SE. THE INITIAL INTENSITY OF 45 KNOTS IS ASSESSED WITH HIGH CONFIDENCE, WELL ABOVE THE PGTW AND ADT DVORAK ESTIMATES, BASED ON A 240236Z SMAP PASS WHICH INDICATED AN ARC OF 45-50 KNOT WINDS EXTENDING ACROSS MOST OF THE SOUTHERN HEMISPHERE OF THE CIRCULATION, WITH A RADIUS OF MAXIMUM WINDS OF ROUGHLY 150KNM. WHILE WATER VAPOR IMAGERY SUGGESTS THE SYSTEM REMAINS RELATIVELY ISOLATED FROM THE DRY AIR ENSCONCED OVER THE MOZAMBIQUE CHANNEL, TOTAL PRECIPITABLE WATER (TPW) DATA INDICATES A NARROW SWATH OF VERY DRY AIR IS BEING ENTRAINED FROM MADAGASCAR DIRECTLY INTO THE CORE OF THE SYSTEM. POLEWARD OUTFLOW REMAINS ROBUST, AND SHEAR RELATIVELY LOW, BUT THE LACK OF ANY SIGNIFICANT OCEAN HEAT CONTENT IS INHIBITING INTENSIFICATION.

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN TIGHT AGREEMENT ON THE FORECAST TRACK, WITH VERY LITTLE SPREAD THROUGH THE PERIOD OF THE FORECAST, LENDING HIGH CONFIDENCE TO THE JTWC TRACK. INTENSITY GUIDANCE IS MIXED, WITH THE HWRF AND COAMPS-TC SUGGESTING SOME MINOR INTENSIFICATION TO ABOUT 50 KNOTS BY 24H, WHILE THE REMAINDER OF THE GUIDANCE INDICATES STEADY OR WEAKENING INTENSITY FROM 00H. THE JTWC FORECAST LIES SLIGHTLY BELOW THE INTENSITY CONSENSUS MEAN WITH HIGH CONFIDENCE.

THE AREA OF CONVECTION (INVEST 90S) PREVIOUSLY LOCATED NEAR 12.8S 96.8E IS NOW LOCATED NEAR 13.0S 96.1E, APPROXIMATELY 120 KM SOUTHWEST OF COCOS ISLANDS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS DEEP CONVECTION CONSOLIDATNG OVER A PARTIALLY EXPOSED AND WELL DEFINED LOW LEVEL CIRCULATION CENTER (LLCC). EARLIER SCATTEROMETER DATA INDICATED A VERY SMALL CIRCULATION WITH 25 TO 30 KNOT WINDS SURROUNDING THE CENTER. THE ENVIRONMENT IS FAVORABLE FOR TROPICAL CYCLONE DEVELOPMENT WITH LOW TO MODERATE(15- 20KTS) EAST-NORTHEASTERLY VERTICAL WIND SHEAR, WEAK OUTFLOW ALOFT, AND WARM (28-29C) SEA SURFACE TEMPERATURES. GLOBAL MODELS ARE IN AGREEMENT THAT INVEST 90S WILL TRACK WESTWARD AS IT CONSOLIDATES AND CONTINUE TO INTENSIFY. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 20 TO 25 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.

GLOBAL MODELS ARE IN AGREEMENT THAT INVEST 90S WILL TRACK WESTWARD AS IT CONSOLIDATES AND CONTINUE TO INTENSIFY.

AN AREA OF CONVECTION (INVEST 99S) HAS PERSISTED NEAR 12.7S 128.4E, APPROXIMATELY 265 KM WEST OF DARWIN, AUSTRALIA.. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY AND A 231030Z SSMIS 91GHZ IMAGE REVEAL A FLARING, DEEP CONVECTIVE BURST OF A SLOW, ANTICYCLONICALLY TURNING MASS. ENVIRONMENTAL ANALYSIS REVEALS THAT THE INVEST IS IN A MARGINALLY FAVORABLE ENVIRONMENT FOR DEVELOPMENT DEFINED BY PRONOUNCED EQUATORWARD OUTFLOW, FAVORABLE (10-15KTS) VERTICAL WIND SHEAR, AND WARM (30-31) SEA SURFACE TEMPERATURES. SURFACE OBSERVATIONS FROM PT. FAWCETT IN THE TIMOR SEA RECORDED SUSTAINED 18KT NORTHERLY WINDS. GLOBAL MODELS ARE IN AGREEMENT REGARDING THE INTENSIFICATION AND GENERALLY QUASISTATIONARY MOVEMENT OF 99S OVER THE NEXT 24-48 HOURS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 15 TO 20 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1006 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS REMAINS LOW.

GLOBAL MODELS ARE IN AGREEMENT REGARDING THE INTENSIFICATION AND GENERALLY QUASISTATIONARY MOVEMENT OF 99S OVER THE NEXT 24-48 HOURS.

AN AREA OF CONVECTION (INVEST 98P) PREVIOUSLY LOCATED NEAR 10.5S 139.7E IS NOW LOCATED NEAR 10.9S 140.2E, APPROXIMATELY 195 KM NORTHWEST OF WEIPA, AUSTRALIA. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A LOW LEVEL CIRCULATION (LLC) WITH FLARING CONVECTION. ENVIRONMENTAL ANALYSIS INDICATES OVERALL FAVORABLE DEVELOPMENT FOR TROPICAL CYCLONE FORMATION WITH LOW TO MODERATE (15- 20KT) VERTICAL WIND SHEAR, STRONG POLEWARD OUTFLOW, AND VERY WARM (30-31C) SEA SURFACE TEMPERATURES. GLOBAL MODELS ARE IN AGREEMENT THAT THE SYSTEM WILL CONTINUE TO TRACK EASTWARD AND DEVELOP WITHIN THE NEXT 48-72HRS. MAXIMUM SUSTAINED SURFACE WINDS ARE ESTIMATED AT 15 TO 20 KNOTS. MINIMUM SEA LEVEL PRESSURE IS ESTIMATED TO BE NEAR 1004 MB. THE POTENTIAL FOR THE DEVELOPMENT OF A SIGNIFICANT TROPICAL CYCLONE WITHIN THE NEXT 24 HOURS IS LOW.

GLOBAL MODELS ARE IN AGREEMENT THAT THE SYSTEM WILL CONTINUE TO TRACK EASTWARD AND DEVELOP WITHIN THE NEXT 48-72HRS.