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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.

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