Tropical cyclone Herbie developed on 17 May in a convective cloud cluster which had drifted slowly eastward from the central Indian Ocean during the previous 3 to 4 days. Apart from a few ship reports from the periphery of the storm the main observational evidence for the development of the system to cyclone intensity came from a brief period of gales at Cocos Island.
Despite the observations from Cocos Is, the analyses of the system's intensity and track are almost totally dependent on the interpretation of High Resolution GMS imagery. The dense cirriform cloud cover and the poor organisation of the system limited the confidence which could be attaihed to the satellite interpretation, particularly while the centre was north of about 16°S. Given these difficulties, it is possible to imagine different analysts deriving different tracks.
During 17 to 18 May the developing low moved southeast and probably reached cyclone intensity at about 0000 UTC 18 May. Twelve hours later the cyclone attained its maximum intensity with a central pressure of about 990 hPa near 10.5°S, 95.8°E.
The cyclone continued on a southeasterly track and by 0000 UTC 19 May was located near 12.3°S, 96.5°E, approximately 65 km west-southwest of Cocos Island where the maximum recorded wind gust was 87 km/h and the minimum recorded pressure was 995 hPa at 0000 UTC. No damage was reported from the island group.
The visual GMS image at 0300 UTC 19 May could be interpreted as indicating that the low-level circulation was divorced from the strongly convective area associated with the cyclone, and that a trough line had developed southward from the low centre. The NOAA-10 visual imagery at 0840 UTC showed two small circular cloud patterns in this trough. One of these patterns may have been the remnants of Herbie and the other a new development some 315 km to the south.
By 1200 UTC 19 May Herbie was no longer identifiable as a tropical cyclone and successive three-hourly GMS images showed an increase in the curvature of cloud features near the new low, signalling an intensification of the system. This intensification is thought to have occurred via an extratropical process, in which the low's primary kinetic energy source was enhanced by baroclinic conversion of potential energy, generated by an increase in the thermal gradient over the ocean area to the southwest of the system. This occurred as the result of a northward intrusion of cold air into low latitudes behind a cold front which traversed the Indian Ocean to the south of the system in the preceding twelve hours. A process similar to this has been noted by Brand and Guard (1978) in which they discuss extratropical storm evolution from tropical cyclones in the northwestern Pacific Ocean.
The deepening extratropical low accelerated as it moved southeastward along the northern side of the strengthening baroclinic zone and by 0000 UTC 20 May was located near 19.0°S, 101.2°E approximately 1500 km northwest of Shark Bay. Visible imagery at 0300 UTC showed a cloud signature with marked similarities to a developing mid-latitude baroclinic wave. The storm continued to accelerate and deepen, and crossed the coast near the town of Denham at 2230 UTC 20 May. At this time it was moving at 75 km/h and had a central pressure of 980 hPa.
Satellite imagery at 0000 UTC 21 May 1988 shows the typical signature of an extratropical cyclone with an almost total absence of convection near the centre and the major rain echoes displaced to the southern quadrants.
No wind recordings are available from Denham but the station barograph trace was made in Carnarvon, 100 km to the north, reporting a maximum wind gust of 120 km/h. The winds caused some structural damage along the coastal strip from Carnarvon to Denham, some crop losses in Carnarvon and a 30/000 tonne freighter the Korean Star ran aground and split in two near Cape Cuvier. A storm surge occurred within the semi-enclosed waters of Shark Bay, and caused inundation to parts of Denham.
The low continued in a southeasterly direction at 75 km/h through central Western Australia and where it amalgamated with a Southern Ocean passage of the extratropical low at 2230 UTC 21 cold front and was lost as a discrete system on satellite imagery.
Track and intensity
All times in WST - subtract 8 hours to convert to UTC. This does not include the portion of the track that moved to the southeast passing over Shark Bay on 21 May.