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Tropical Storm Twenty One (21S) Tracking Westward Towards Madagascar

13.2S 63.2E

April 6th, 2013 Category: Tropical Storms

Tropical Storm Twenty One (21S) – April 6th, 2013

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Tropical Cyclone 21S, located approximately 200 nm south- southwest of Diego Garcia, has tracked westward at 05 knots during the past six hours. Maximum significant wave height is 11 feet.

Animated infrared satellite imagery depicts persistent convection, which becomes sheared to the west by moderate vertical wind shear (VWS) leaving the eastern portion of the low- level circulation center (LLCC) partially-exposed. The initial intensity is assessed at 35 knots based on Dvorak current intensity.
Upper-level analysis indicates TC 21S is approximately 05 degrees equatorward of the subtropical ridge (STR) axis and moderate 20-knot VWS continues to hamper the convective development. Animated water vapor imagery indicates good poleward outflow, however equatorward outflow is limited due to troughing north and east of Madagascar.

TC 21S will continue to track west-southwestward under the influence of the deep layered STR to the south. By TAU 48 an upper-level trough will re-orientate the current west-east orientated STR to a northwest-southeast orientation. This will allow VWS to relax as the system nears the STR axis. In addition, a warmer pool of sea surface temperatures and ocean heat content during this time will allow TC 21S to intensify somewhat rapidly.

Further intensification is expected thru TAUs 72 – 96 as poleward outflow increases in response to a different and deeper upper-level trough approaching from the west. This trough will further break down the steering STR and allow for TC 21S to dip poleward towards the mid-latitudes. The forecast still calls for a peak of 100 knots by TAU 96 but this may need to be adjusted as the influence of favorable factors begins to take their effects. Model guidance is in a general agreement with the poleward deflection in the extended forecast however the timing and sharpness in the deflection are still unclear. There is high confidence in the three day forecast but low confidence in the extended portion.

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