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Tropical Cyclone Eighteen (18S) Expected to Struggle at Moderate Gale Intensity

13.9S 99.8E

February 27th, 2013 Category: Tropical Cyclones

Tropical Cyclone Eighteen (18S) – February 26th, 2013

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Track of Tropical Cyclone Eighteen (18S) - February 26th, 2013 © Univ. of Wisconsin

Track of TC 17S

Tropical Cyclone Eighteen (18S) is expected to struggle at moderate gale intensity over the next 48 hours with a distinct possibility of dissipation. After TAU 48, should the cyclone survive, decreasing vertical wind shear will contribute to a gradual intensification. The available numerical guidance are in overall agreement in eventually tracking the vortex eastward but with significant differences in track speed. Given the uncertainty in the steering mechanism, there is low confidence in the JTWC track and intensity forecasts. Maximum significant wave height is 17 feet.

Tropical Cyclone Eighteen (18S) in Weak Steering Environment

15S 101.9E

February 26th, 2013 Category: Tropical Cyclones

Tropical Cyclone Eighteen (18S) – February 26th, 2013

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Track of Tropical Cyclone Eighteen (18S) - February 26th, 2013 © Univ. of Wisconsin

Track of TC 17S

Tropical Cyclone Eighteen (18S), located approximately 1000 nm west-northwest of Learmonth, Australia, has tracked south-southeastward at 04 knots over the past six hours.

Animated multispectral satellite imagery shows the main convection remains detached from a fully exposed low level circulation that has become more elongated and irregular, making it difficult to pinpoint the center. The initial position is based on the above animation with poor confidence. The initial intensity, assessed higher than multi-agency subjective Dvorak estimates, is extrapolated from recent scatterometer data. Upper level analysis indicates the system is in an area of strong (30-40 knot) easterly vertical wind shear (VWS).

The cyclone is currently in a weak steering environment between the near-equatorial ridge (NER) to the north and a subtropical ridge (STR) to the south resulting in a very slow eastward drift. It is expected to remain in this state over the next 48 hours until the STR recedes to the east. At that stage, the NER will rebuild and resume steering and track the cyclone faster eastward.

Tropical Cyclone Eighteen (18S) Tracking Erratically

13S 100.5E

February 26th, 2013 Category: Tropical Cyclones

Tropical Cyclone Eighteen (18S) – February 25th, 2013

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Track of Tropical Cyclone Eighteen (18S) - February 26th, 2013 © Univ. of Wisconsin

Track of TC 17S

Tropical Cyclone Eighteen (18S), located approximately 180 nm south- southeast of Cocos Island, Australia, has tracked west-southwestward at 06 knots over the past six hours.

Animated multispectral satellite imagery (MSI) depicts a broad low-level circulation center (LLCC) with deep convection sheared west of the center. An SSMIS image depicts weak, shallow convective banding wrapping loosely into a defined LLCC.

TC 18S has tracked erratically over the past 12 hours. However, recent MSI indicates that the system is now tracking slowly eastward. Due to the broad nature of the LLCC and lack of visible imagery, there has been low confidence in the initial position over the past 12 hours; however, there is increased confidence in the current position based on the MSI, which showed the center was located about 80 nm west of the 25/12z TAU 12 forecasted position.

TC 18S is located within a competing steering environment between a strong high to the south and a near-equatorial ridge (NER) to the north. TC 18 is forecast to track slowly eastward through TAU 48 due to the complex steering environment and is expected to accelerate eastward after TAU 48 as the subtropical high to the south weakens and the NER becomes the dominant steering influence.

Dynamic model guidance has trended toward a much slower overall track with GFS and GFDN indicating excessively fast tracks, therefore, this forecast is positioned slower than the multi-model consensus favoring the ECMWF tracker. There is low confidence in the JTWC forecast track.

TC 18S is forecast to maintain at 35 knots through TAU 48 then will gradually intensify to a peak intensity of 55 knots by TAU 120 as vertical wind shear decreases. There is low confidence in the intensity forecast. Maximum significant wave height is 15 feet.

Tropical Cyclone Eighteen (18S) Becoming Better Organized

15.7S 101.2E

February 25th, 2013 Category: Tropical Cyclones

Tropical Cyclone Eighteen (18S) – February 25th, 2013

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Track of Tropical Cyclone Eighteen (18S) - February 25th, 2013 © Univ. of Wisconsin

Track of TC 17S

Tropical Cyclone Eighteen (18S), located approximately 120 nm southeast of Cocos Island, Australia, has tracked south-southeastward at 06 knots over the past six hours. Maximum significant wave height is 11 feet.

Animated multispectral satellite imagery depicts an exposed but well-defined low-level circulation center with deep convection sheared about 45 nm west of the center due to strong easterly vertical wind shear (VWS).

TC 18S has continued to struggle due to the poor upper-level environment, however, an AMSU image indicates better organized shallow banding wrapping into a well-defined LLCC. An ASCAT image showed 40 to 45 knot winds over the north quadrant and surface observations from Cocos Island have remained steady at 30 to 35 knots sustained with gusts to 40 to 50 knots.

The initial intensity is assessed at 40 knots, slightly higher than Ddvorak intensity estimates of 35 knots, based on the ASCAT image. There is good confidence in the initial position based on the exposed center in visible imagery.

TC 18S is forecast to track eastward under the steering influence of the near- equatorial ridge through the forecast period. Dynamic model guidance is in fair agreement, therefore, the JTWC forecast is positioned close to the multi-model consensus. TC 18S is expected to slowly intensify through TAU 120 due to persistent moderate to strong VWS. Due to the marginal environment, there is uncertainty in the extended intensity forecast, which may be significantly lower than the 60 knots currently forecast to occur near TAU 120. There is low confidence in the JTWC forecast track due to the sheared nature of the system and large differences in model track speed throughout the forecast.