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Tropical Cyclone Imelda (21S) with 55-Knot Winds

20.8S 52.7E

April 16th, 2013 Category: Tropical Cyclones

Tropical Storm Twenty One (21S) – April 14th, 2013

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Track of Tropical Storm Twenty One (21S) - April 14th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Imelda (TC 21S), located approximately 370 nm east of La Reunion, has tracked south-southeastward at 05 knots over the past six hours.

Animated infrared satellite imagery depicts rapidly decaying deep convection, now sheared about 70 nm east of the system center. An SSMIS image depicts well-defined, tightly-wrapped shallow convective banding wrapping into the low- level circulation center with an isolated area of deep convection near 20S 64E.

There is good confidence in the initial position based on this image as well as an AMSU composite image. The initial intensity is assessed at 55 knots based on Dvorak intensity estimates ranging from 45 to 65 knots.

Tropical Cyclone Imelda (21S) Tracking Southeastward at 14 Knots

20.1S 52.0E

April 15th, 2013 Category: Tropical Cyclones

Tropical Storm Twenty One (21S) – April 14th, 2013

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Track of Tropical Storm Twenty One (21S) - April 14th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Imelda (TC 21S), located approximately 340 nm east-northeast of La Reunion, has tracked southeastward at 14 knots over the past six hours. Maximum significant wave height is 32 feet.

Animated infrared satellite imagery indicates the system has significantly deepened. Additionally, an eye has opened then closed in the past 12 hours. The initial intensity is based on closely-grouped agency fixes and lined up well with a central warm spot on an SSMI-S microwave pass.

The initial intensity is based on a consensus of subjective Dvorak estimates from PGTW, KNES, and FIMP. Upper level analysis indicates the system is south of a ridge axis in an area of moderate (15-20 knot) northwesterly vertical wind shear (VWS); however, poleward outflow, which is enhancing the convection, remains robust, as evidenced on animated water vapor imagery.

Tropical Cyclone Imelda (21S) with Shallower Convection

11.8S 53.4E

April 13th, 2013 Category: Tropical Cyclones

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

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Track of Tropical Storm Twenty One (21S) - April 12th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Imelda (TC 21S), located approximately 405 nm north-northeast of La Reunion, has tracked south-southeastward at 08 knots over the past six hours.

Animated infrared satellite imagery depicts the associated convection has become more shallow and is now displaced 60 nm southeast of a fully exposed low level circulation center. The initial position is based on the above animation and on an SSMI-S microwave pass with fair confidence. The initial intensity is based on congruent subjective Dvorak estimates from PGTW, KNES, and FIMP.

Upper level analysis indicates the system is 10 degrees south of a ridge axis in an area of low to moderate (05-10 knot) vertical wind shear (VWS). Along-track sea surface temperatures remain favorable at 28-30 degrees Celsius. These dynamics are, for now, sustaining the convection; however, a narrow mid-latitude trough to the west is beginning to erode the western rim of the system.

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.

Tropical Cyclone Imelda (21S) Northeast of Madagascar – April 11th, 2013

11.1S 56.2E

April 11th, 2013 Category: Image of the day, Tropical Cyclones

Tropical Storm Twenty One (21S) – April 10th, 2013

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Track of Tropical Storm Twenty One (21S) - April 10th, 2013 © Univ. of Wisconsin

Track of TC 21S

Tropical Cyclone Imelda (TC 21S) located approximately 515 nm north-northeast of La Reunion, has tracked southward at 02 knots over the past six hours.

Animated multispectral satellite imagery depicts tightly curved spiral banding wrapping into a low level circulation center. An SSMIS 37ghz microwave image depicts deep convection favoring the eastern quadrants in response to slightly increased vertical wind shear (VWS), however the center remains well defined.

The system is now quasi-stationary as the steering ridge to the southeast is weakening and becoming re- oriented in response to an approaching deep-layer trough (DLT). The initial position is extrapolated from the aforementioned SSMIS image with fair confidence.

The initial intensity is assessed at 75 knots based on a range of Dvorak current intensity estimates from 65 to 77 knots. Upper level analysis indicates the system has a point source overhead providing moderate radial outflow, with VWS increasing slightly to 10 to 15 knots.

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