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Tropical Cyclone Narelle (08S) Reaches Peak Intensity – January 12th, 2013

19.9S 113.4E

January 12th, 2013 Category: Image of the day, Tropical Storms

Tropical Storm 08S – January 11th, 2013

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Track of Tropical Storm 08S - January 11th, 2013 © Univ. of Wisconsin

Track of TS 08S

Tropical Cyclone Narelle (08S), located approximately 335 nm north-northwest of Learmonth, Australia, has tracked west-southwestward at 06 knots over the past six hours. Maximum significant wave height is 46 feet.

Recent animated enhanced infrared satellite imagery shows an approximately 25 nm eye and an 111753z AMSU-B 89 ghz image depicts the deepest convection contained to the northern and western quadrants with a slight weakness in the southeastern quadrant of the eye wall. There is high confidence in the initial position given the eye.

Initial intensity was based on congruent Dvorak estimates of 115 knots from PGTW, KNES, and APRF. Upper-level analysis indicates the cyclone resides just equatorward of the subtropical ridge (STR) axis under light to moderate (10-20 knot) vertical wind shear (VWS). Recent animated water vapor imagery shows excellent poleward outflow enhanced by a connection into the mid-latitude westerlies and weaker equatorward outflow due to upper-level troughing well northwest of the system.

TC 08S is tracking along the northwestern periphery of a deep-layered STR anchored over south-central Australia. The cyclone is expected to round the western edge of this ridge over the next 72 hours before it re-curves southeastward as the ridge recedes in response to a mid-latitude trough approaching from the west. This re-curvature will coincide with the system getting absorbed into the baroclinic zone, becoming a cold core low by TAU 96.

Narelle has peaked in intensity and should gradually weaken mainly due to increasing VWS and cooling sea surface temperatures. After TAU 72, these harsh conditions will exponentially escalate; therefore, there is a distinct possibility that the system will dissipate before it becomes extra-tropical. Objective aid guidance continues to converge in the near term with the spread increasing beyond TAU 48. Currently, there is only a 70 nm spread between the vortex trackers at TAU 36, near the latitude of Learmonth. The extended forecast remains just inside of the model consensus to offset the western-most outliers (NOGAPS and GFS) and favors the ECMWF, which has so far been the top performing model. High confidence remains in the official JTWC track forecast.

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.

Tropical Cyclone Narelle (08S) Current Intensity 75 Knots

15.5S 119.7E

January 10th, 2013 Category: Tropical Storms

Tropical Storm 08S – January 8th, 2013

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Track of Tropical Storm 08S - January 9th, 2013 © Univ. of Wisconsin

Track of TS 08S

Tropical Cyclone Narelle (08S) located approximately 500 nm north-northeast of Learmonth, Australia, has tracked south- southwestward at 06 knots over the past six hours. Maximum significant wave height is 28 feet.

The current intensity of 75 knots is based on a consensus of Dvorak satellite intensity estimates from the same reporting agencies. Recent animated infrared satellite imagery shows deep convection flaring over a well-defined low-level circulation center (LLCC), particularly along the northwestern quadrant.

The system is tracking south-southwestward along the northwestern periphery of a subtropical ridge that extends across central Australia. This ridge is expected to remain the dominant steering mechanism throughout much of the forecast period. As the system tracks closer to the ridge axis over the next 12 to 24 hours, it is expected to track a bit further westward before making a poleward turn after TAU 48.

Passage over very warm water, low vertical wind shear, and strong outflow aloft will allow the system to intensify significantly over the next 48 hours. After TAU 72, passage over cooler water and entrainment of drier and cooler air during the extended forecast period will result in steady weakening.

The dynamic models are in good general agreement through TAU 36, but continue to diverge beyond that point. NOGAPS and the UKMET model show a steady turn around the ridge axis and track the system inland as it undergoes extra-tropical transition in the extended period. GFS and GFDN depict a nearly straight poleward track with steady weakening and extra-tropical transition of a remnant low after TAU 120. The ECMWF model, which has been very consistent over the past several runs, shows the system tracking more slowly and farther westward than the other models, with a rapidly dissipating circulation following the lower level flow.

The current forecast is consistent with the GFS and GFDN scenarios, given expectations that the cyclone will remain very strong through TAU 96. However, if the system dissipates more quickly during the extended period than currently forecast, TC 08S may follow a slower and more westward track similar to the ECMWF forecast. Due to the continued spread in dynamic guidance after TAU 48, there is low confidence in the JTWC forecast track.

Tropical Cyclone Narelle (08S) Expected to Intensify

12.4S 123.9E

January 9th, 2013 Category: Tropical Storms

Tropical Storm 08S – January 8th, 2013

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Track of Tropical Storm 08S - January 8th, 2013 © Univ. of Wisconsin

Track of TS 08S

Tropical Cyclone Narelle (08S) located approximately 610 nm north-northeast of Learmonth, Australia, has tracked westward at 7 knots over the past six hours. Maximum significant wave height is 20 feet.

Animated infrared (IR) satellite imagery indicates a rapidly consolidating low-level circulation center (LLCC) located under a central dense overcast feature with improved deep convective banding over the western semi-circle of the system. An AMSU-B microwave image shows deep convection surrounding a formative microwave eye feature. There is good confidence in initial position based upon the eye feature in the microwave imagery.

TC 08S is located equatorward of the upper-level ridge axis and is under weak to moderate (10 to 20 knots) east-northeasterly vertical wind shear with improved poleward outflow into a deep midlatitude trough positioned west of Australia. The initial intensity is assessed at 55 knots based on congruent Dvorak intensity estimates of 55 knots.

Narelle is forecast to track south to southwestward under the steering influence of the low- to mid-level subtropical ridge positioned southeast of the system. Dynamic model guidance is tight agreement with the exception of the GFDN, which tracks the system well west of the other models and at slower speeds. Overall, there is high confidence in the JTWC forecast track. TC 08S is expected to rapidly intensify under favorable environmental conditions and reach a peak intensity of 130 knots by TAU 72.