Update: Cyclone Season 2012: Tropical Cyclone Giovanna (Southern Indian Ocean) 17 February 2012 20h00 SAST

Image: NOAA18 13H49 SAST (Nic Cronje, Nelspruit) Click on image for larger view.

171500Z POSITION NEAR 25.8S 43.2E.
TROPICAL CYCLONE 12S (GIOVANNA), LOCATED APPROXIMATELY 445 NM SOUTH-SOUTHWEST OF ANTANANARIVO, MADAGASCAR, HAD TRACKED EAST- SOUTH EASTWARD AT 02 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS A WELL-DEFINED LOW-LEVEL CIRCULATION CENTER (LLCC) WITH BUILDING CONVECTION SPIRALING INWARDS AROUND THE EASTERN AND SOUTHERN QUADRANTS. A 171125Z AMSU-B 89 GHZ IMAGE DEPICTS THE DEEP CONVECTION WRAPPING INTO THE SOUTHWESTERN PORTION OF THE LLCC. THE CURRENT POSITION IS BASED ON THE AMSU-B IMAGE WITH GOOD CONFIDENCE. THE CURRENT INTENSITY IS BASED ON THE HIGHER SIDE OF DVORAK ESTIMATES RANGING FROM 45-55 KNOTS FROM KNES/FMEE AND PGTW RESPECTIVELY, DUE TO THE CURRENT CONVECTIVE SIGNATURE IN MICROWAVE DATA. UPPER-LEVEL ANALYSIS INDICATES THE SYSTEM IS JUST TO THE SOUTHEAST OF A POINT-SOURCE OF DIFFLUENCE UNDER WEAK (05-10 KNOT) WESTERLY VERTICAL WIND SHEAR (VWS). HOWEVER, SPEED DIVERGENCE IS ENHANCED DOWNSTREAM OF THE LLCC DUE TO A POLEWARD TAP INTO THE MID-LATITUDE WESTERLIES. ANIMATED WATER VAPOR IMAGERY CLEARLY SHOWS THE CIRRUS BLOW-OFF QUICKLY STREAMING POLEWARD. ADDITIONALLY, THE STR CAN BE SEEN BUILDING OUT OVER SOUTHERN AFRICA. TC 12S IS CURRENTLY TRACKING SLOWLY ALONG THE SOUTHWEST PERIPHERY OF A WEAK EXTENSION OF THE SUBTROPICAL RIDGE, BUILT OVER NORTHERN MADAGASCAR. BY TAU 24, THE SYSTEM SHOULD COME UNDER THE INFLUENCE OF THE SOUTHEASTERN PERIPHERY OF THE STR OVER SOUTHERN AFRICA CAUSING IT TO TRACK OVER SOUTHERN MADAGASCAR BY TAU 48. TC 12S SHOULD DISSIPATE OVERLAND BY TAU 72 DUE TO LAND INTERACTION. NUMERICAL MODEL GUIDANCE CONTINUES TO COLLECTIVELY SHOW A GENERALLY EASTWARDS TRACK BUT SUCCESSIVE MODEL RUNS ARE HINTING TOWARDS THE LLCC TRACKING SOUTH OF MADAGASCAR AND DISSIPATING FURTHER OUT OVER THE SOUTHWESTERN INDIAN OCEAN. THE OFFICIAL
FORECAST FAVORS THE ECMWF, WHICH IS THE LEFT MOST OBJECTIVE AID AND TAKES THE LLCC OVER SOUTHERN MADAGASCAR DUE TO THE EXPECTED INFLUENCE OF THE STR TO THE WEST. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 171200Z IS 15 FEET.

- Joint Typhoon Warning Centre (JTWC)