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Posts tagged 21S

Tropical Cyclone Imelda (21S) Expected to Turn Back to Southward Trajectory

20.6S 52.7E

April 14th, 2013 Category: Tropical Cyclones

Tropical Storm Twenty One (21S) – April 13th, 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) should revert to a generally poleward trajectory by TAU 12 after it rounds the southern edge of the steering ridge. After TAU 48, a building subtropical ridge to the south will begin to deflect the cyclone westward.

The system will peak intensity over the next 12 hours as good outflow persists; afterwards, increasing vertical wind shear (VWS) will gradually weaken and eventually dissipate the system by TAU 72. The available numeric guidance is widely spread and lends low confidence to the track forecast. Maximum significant wave height is 25 feet.

Tropical Cyclone Imelda (21S) Restrengthens, Continues Towards Madagascar – April 14th, 2013

10.7S 53.4E

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

Tropical Storm Twenty One (21S) – April 13th, 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 315 nm northeast of La Reunion, has tracked west-southwestward at 03 knots over the past six hours.

Animated infrared satellite imagery shows a deepened central dense overcast feature over a well-defined low-level circulation center. This is also evident on an SSMI-S microwave pass. The initial position is based on closely grouped agency fixes and on the above microwave image with high confidence. The initial intensity is an average of subjective Dvorak estimates from PGTW, KNES, and FIMP.

Upper level analysis (ULA) indicates the system is between two ridge axes – one to the north and another to south in an area of low (05-10 knot) vertical wind shear (VWS). TC 21S has unpredictably tracked west-southwestward over the past 12 hours. From the same ULA, the steering ridge to the east was not significantly weakened by a passing mid-latitude trough as previously forecast. Instead, the ridge was deformed into a kidney-shaped anticyclone along a meridional axis. In response to this transformation to the steering ridge, 21S jogged westward. This recent development necessitated a slight change in the JTWC track forecast philosophy.

Tropical Cyclone Imelda (21S) Dissipating

12S 54.8E

April 13th, 2013 Category: Tropical Cyclones

Tropical Storm Twenty One (21S) – April 12th, 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) is expected to struggle in maintaining its structure but will continue to track along the southwestern periphery of a near-equatorial ridge to the northeast.

Vertical wind shear (VWS) values are expected to increase as the system tracks poleward, contributing to the gradual dissipation by TAU 36. There is a distinct possibility that the cyclone will dissipate sooner. As the system weakens, the steering flow will begin to shift to lower levels. Available numerical model shows a large spread in the tracks, therefore, there is low confidence in the current track forecast. Maximum significant wave height is 18 feet.

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.