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Tropical Cyclone Imelda (21S) Slowly Weakening

12.7S 54.8E

April 12th, 2013 Category: Tropical Cyclones

Tropical Storm Twenty One (21S) – April 11th, 2013

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Track of Tropical Storm Twenty One (21S) - April 11th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Imelda (TC 21S) located approximately 510 nm north-northeast of La Reunion, has tracked east-southeastward at 04 knots over the past six hours.

Animated multispectral satellite imagery shows convection near the center of the system has been slowly weakening over the past 12 hours with a partially exposed low level circulation center (LLCC). Upper level analysis indicates a developing mid-latitude trough to the southwest of the system has started to mitigate the equatorward outflow channel and is creating low to moderate vertical wind shear (VWS) along the western boundary of the LLCC.

Over the past 24 hours, this shift in upper level support has led to the observed weakening trend in Dvorak estimates. Model guidance indicates that the near equatorial ridge (NER), steering TC 21S, will build to the southeast as a deep-layered trough (DLT) to the southeast weakens and continues to move eastward. Based on the timing of this feature and the approach of a second DLT from the southwest, the forecast track will undergo a weakly defined stair-step path to the southeast as the NER re-orients under the influence of the transitory mid- latitude troughs.

The second DLT to the southwest will continue tracking eastward, allowing the equatorward outflow channel to re- establish itself, improving the overall upper level environment by TAU 24. Favorable sea surface conditions in association with improved outflow aloft will allow for the re-intensification of the system through TAU 72. Beyond TAU 72 increasing VWS and decreasing sea surface temperatures will stifle further development, leading to a weakening trend through the remainder of the forecast period.

Due to the current poor steering environment and model guidance persisting with a stronger southerly track for the past 24 hours, there is low confidence in the current track forecast. Maximum significant wave height is 28 feet.

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