Tropical Cyclone Narelle (08S) Expected to Intensify20.6S 112.5E
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.