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Tropical Cyclone Eight (08S) Forms Off Coast of Australia – January 8th, 2013

15.2S 120.2E

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

Tropical Storm 08S – January 7th, 2013

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Track of Tropical Storm 08S - January 7th, 2013 © Univ. of Wisconsin

Track of TS 08S

Tropical Cyclone Eight (08S), located approximately 700nm north-northeast of Learmonth, Australia, has tracked west-southwestward at 7 knots over the past six hours   (click here for previous images). Maximum significant wave height is 12 feet.

Animated infrared satellite imagery shows a consolidating low level circulation center (LLCC), with improved convective bands wrapping around the LLCC and deepening central convection. An AMSU-B microwave image shows increasing organization around the LLCC as the LLCC has become better defined.

The initial position is based upon the aforementioned imagery with fair confidence. The initial intensity is slightly higher than congruent Dvorak intensity estimates of 30 knots from all agencies based upon the improved structure.

Upper level analysis indicates TC 08S is under a highly diffluent region of the subtropical ridge, providing good outflow and low (05-10 knot) vertical wind shear (VWS). Additionally, an upper-level low southwest of Australia is enhancing the poleward outflow. Sea surface temperatures (SSTs) are favorable for intensification at 30 to 31 degrees Celsius and remain favorable until south of 22S where they begin to rapidly decline.

TC 08S is currently tracking west- southwestward along a northwest periphery of a deep layered subtropical ridge to the southeast over Western Australia. The system will continue to track west-southwest and increasingly take a more southern track as the current steering ridge reacts to several transient mid-latitude troughs passing to the south.

TC 08S will increase to a peak of 125 knots by TAU 96 as favorable environmental conditions of low VWS, good outflow and high SSTs are forecast. Dynamic model guidance is in fair agreement with the southwestern track but disagree slightly on the extent of the poleward turn. The JTWC official forecast is close to multi-model consensus. Due to the spread in solutions among the models on the extent of the southern turn, forecast confidence is low.

Area of Convection Off Australia Coast Has High Probability of Becoming Tropical Cyclone

13.6S 118.3E

January 7th, 2013 Category: Tropical Storms

Area of Convection – January 6th, 2013

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Track of Area of Convection - January 6th, 2013 © Univ. of Wisconsin

Track of Area of Convection

The formation of a significant tropical cyclone is possible in the Timor Sea, off the coast of Australia, within 120 nm either side of a line from 11.1S 121.5E to 13.4S 118.2E within the next 12 to 24 hours.

Winds in the area are estimated to be 25 to 30 knots. METSAT imagery indicates that a circulation center is located near 11.4S 120.9E. The system is moving westward at 09 knots.

The area of convection previously located near 11.0S 123.5E  (click here for previous images) is now located near 11.4S 120.9E, approximately 590 nm west of Darwin, Australia. Recent multispectral and enhanced infrared satellite imagery shows a consolidation of convection, with fragmented bands wrapping around a low level circulation center (LLCC).

Upper level analysis indicates this area is approximately five degrees north of an anticyclone, providing good outflow and low (10 knots) vertical wind shear. Additionally, as the system moves southward, strong gradient-induced upper level winds moving into the southwestern region of Australia should further enhance the outflow over the next 24 hours.

Sea surface temperatures are a very favorable 30-31 degrees celsius. Maximum sustained surface winds are estimated at 25 to 30 knots. Minimum sea level pressure is estimated to be near 1001 mb. Due to increased consolidation of the LLCC, the potential for the development of a significant tropical cyclone within the next 24 hours is high.

Tropical Low Off Coast of Australia

14.1S 122.5E

January 7th, 2013 Category: Tropical Storms

Area of Convection – January 6th, 2013

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Track of Area of Convection - January 6th, 2013 © Univ. of Wisconsin

Track of Area of Convection

On 5 January, TCWC Darwin reported that a weak tropical low had formed within the Timor Sea about 160 km (100 mi) to the southeast of Dili in Timor-leste. The low has a medium chance of becoming a tropical cyclone and is being closely monitored.

Tropical Cyclone Narelle (08S) Rapidly Decaying

26.7S 112.3E

January 15th, 2013 Category: Tropical Storms

Tropical Storm 08S – January 14th, 2013

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Track of Tropical Storm 08S - January 14th, 2013 © Univ. of Wisconsin

Track of TS 08S

Animated enhanced infrared satellite imagery shows Tropical Cyclone Narelle (08S)‘s low level circulation center (LLCC) remains fully exposed and detached from the main convection that is frayed and rapidly decaying. Additionally, the same animation shows a wide band of cold air stratocumulus clouds advecting from the southwest.

These recent developments indicate Narelle is now embedded in the baroclinic zone and has transformed into a cold core low. An AMSU-B radial/height cross section reveals that the warm core anomaly has disappeared, corroborating the above satellite observation. TC 08S is expected to continue accelerating southeastward deeper into the baroclinic zone.

Tropical Cyclone Narelle (08S) Expected to Intensify

20.6S 112.5E

January 11th, 2013 Category: Tropical Storms

Tropical Storm 08S – January 10th, 2013

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Track of Tropical Storm 08S - January 10th, 2013 © Univ. of Wisconsin

Track of TS 08S

Tropical Cyclone Narelle (08S), located approximately 300 nm north of Learmonth, Australia, has tracked southwestward at 08 knots over the past six hours. Maximum significant wave height is 40 feet.

Animated multispectral satellite imagery shows that an eye is beginning to form over the low-level circulation center (LLCC). The current position is based on the nascent eye along with a 110020z SSMIS 37 ghz image with high confidence.

The current intensity is based on the range of Dvorak estimates from 90 to 102 knots. Upper-level analysis indicates that Narelle is approximately 10 degrees southwest of the subtropical ridge (STR) center and is under an extension of the STR axis.

Vertical wind shear (VWS) is weak to moderate at 10 to 15 knots from the east and maximum divergence resides just south of the system due to enhanced poleward outflow into the mid-latitude westerlies. Recent animated water vapor imagery shows expanding radial outflow as upper-level conditions have improved during the past six hours.

The cyclone is tracking along the northwestern periphery of a deep-layered STR anchored to the southeast over south-central Australia. It should continue to track along the western periphery of this ridge for the remainder of the forecast period.

A small extension of the ridge axis has been able to reside just south of the system and can account for the more southwest movement vice south-southwest. Not until TAU 72 does it seem that this STR extension finally will become eroded by the approaching deep mid-latitude trough from the west. This trough will enable the cyclone to then track more south, and eventually southeastward as it becomes embedded in the westerly flow and completes extra-tropical transition by TAU 120.

Peak intensity is expected by TAU 24, at 110 knots, which should carry over into TAU 36. However, beyond that time TC 08S should experience rapid weakening due to increasing VWS and very cool sea surface temperatures. The forecast track has been shifted more west of the previous forecast due to some rebesting of previous positions and the expected influence of the STR extension south of the system. The current forecast does open the CPA to Learmonth considerably and is consistent with model trends over the past 24 hours. The JTWC official forecast favors the ECMWF and model consensus but is slightly inside and faster during the 3-5 day forecast to account for known model tendencies in recurving scenarios. Due to the continued spread in tracker guidance track forecast confidence remains low.