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Tropical Cyclone Twenty One (21S) Expected to Intensify – April 8th, 2013

11.1S 59.0E

April 8th, 2013 Category: Image of the day, Tropical Storms

Tropical Storm Twenty One (21S) – April 8th, 2013

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Track of Tropical Storm Twenty One (21S) - April 8th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Twenty One (TC 21S) will continue to track west-southwestward through TAU 48 under the influence of the deep layered sub-tropical ridge (STR) to the south.

A gradual decrease in vertical wind shear (VWS) and fair outflow will allow for intensification. Additionally, a pool of higher ocean heat content located along the forecast track will aid in this intensification. After TAU 72, a deep layer mid-latitude trough will move in from the west and begin to reorient the STR in the north-south direction, allowing for TC 21S to turn poleward.

The forecast calls for a peak intensity of 95 knots by TAU 96. Though dynamical model guidance is in generally good agreement with the poleward deflection in the extended forecast, the timing and sharpness in the deflection are still unclear. This is further evidenced by increasing spread noted in ensemble model products. There is high confidence in the three day forecast, but low confidence in the extended portion.

Tropical Cyclone Twenty One (21S) Tracking West-southwestward

11.8S 60.4E

April 8th, 2013 Category: Tropical Storms

Tropical Storm Twenty One (21S) – April 7th, 2013

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Track of Tropical Storm Twenty One (21S) - April 8th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Twenty One (TC 21S), located approximately 335 nm southwest of Diego Garcia, has tracked west-southwestward at 11 knots over the past six hours. Maximum significant wave height is 15 feet.

Animated infrared satellite imagery depicts a partially-exposed low-level circulation center (LLCC) located east of persistent deep convection. A SSMIS 37 ghz image shows good lower-level banding structure over the western semicircle but very fragmented and weak bands to the east.
An ASCAT pass depicted 35-40 knot winds over the western portion of the LLCC which are wrapping into the center.

The current position is based on the aforementioned imagery with fair confidence. The current intensity is based on the timely ASCAT pass. Upper-level analysis indicates TC 21S is approximately five degrees equatorward of the subtropical ridge (STR) axis. Vertical wind shear (VWS) remains moderate (15-20 knots), causing the deep convection to be sheared to the west. Animated water vapor imagery depicts moderate poleward outflow; however, equatorward outflow is limited due to troughing.

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.

Tropical Cyclone Gino (15S) Forecast to Begin Extra Tropical Transitioning

19.5S 75.9E

February 15th, 2013 Category: Tropical Cyclones

Tropical Cyclone Gino (15S) – February 13th, 2013

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Track of Tropical Cyclone Gino (15S) - February 13th, 2013 © Univ. of Wisconsin

Track of TC 15S

Tropical Cyclone Gino (15S), located approximately 1110 nm south-southeast of Diego Garcia, has tracked south-southeastward at 12 knots over the past six hours. Maximum significant wave height is 21 feet.

Animated multispectral satellite imagery depicts a partially-exposed low-level circulation center (LLCC) with deep convection sheared over the southeast quadrant of the LLCC. Despite the recent weakening trend, an SSMIS image indicates deep convective banding wrapping from the east quadrant into the southwest quadrant and tightly-curved, shallow banding elsewhere.

The initial intensity is assessed at 70 knots based on the lower end of Dvorak current intensity estimates ranging from 65 to 90 knots. TC 15S is forecast to track southeastward along the southwest periphery of the subtropical ridge and will encounter strong (40 to 60 knots) vertical wind shear (VWS) and cooler SSTs (24 to 25C), which will serve to rapidly weaken the system.

Gino is forecast to begin extra-tropical transition (ETT) by TAU 12 and should complete ETT by TAU 36. The dynamic models indicate a weak ETT with the remnant low becoming quasi-stationary near TAU 36 and turning northwestward as a strong high builds in to the south. The model guidance is in good agreement with this scenario, therefore, there is high confidence in the JTWC forecast track. Due to the strong VWS and cooler SST, there is a possibility that the system will weaken below 35 knots prior to completing ETT.

Tropical Cyclone Gino (15S) Shifts to South-southeastward Motion – February 14th, 2013

14.8S 75.9E

February 14th, 2013 Category: Image of the day, Tropical Cyclones

Tropical Cyclone Gino (15S) – February 13th, 2013

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Track of Tropical Cyclone Gino (15S) - February 13th, 2013 © Univ. of Wisconsin

Track of TC 15S

Tropical Cyclone Gino (15S) continues to track around a subtropical ridge (STR) located to the east, and has shifted from a southwestern track to a more south-southeastward motion over the past 12 hours. As the system continues to track along the southwestern quadrant of the STR, an approaching mid- latitude trough will quickly lead to strong VWS over the LLCC.

Sea surface temperatures are currently marginal and have been steadily declining over the past 12 hours, with further cooling expected through TAU 72. The steady weakening trend and increasing interaction with the mid-latitude trough will begin extra-tropical transition (ETT) by TAU 48. ETT will be complete by TAU 72 as VWS and SST values rapidly deteriorate. Model guidance is in fair agreement leading to high confidence in the JTWC forecast track.

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