The 1998/1999 tropical cyclone (TC) season over northwest Australia was characterized by an unusually large number of severe TCs, especially those making landfall (three category 5 TCs made landfall in December, March and April). In a companion paper at this conference, the unusual activity is attributed to a combination of favorable broad-scale, synoptic and mesoscale precursors acting on time scales ranging from five months prior to the TC season, through to time scales of a few days. Among the precursors identified were: below normal SLP values; abnormally warm SST and ocean sub-surface temperatures; above average relative humidity in the low- to mid-troposphere levels; anomalous low-level cyclonic vorticity; and weak vertical wind shears in the genesis region under study.

In this paper the focus is on the three category 5 TCs, Thelma, Vance and Gwenda, in the 96 hour period leading up to nearing and making landfall. Each of the three storms exhibited different behavior at landfall. Thelma was a slow-moving TC, with an erratic track, and made landfall as a category 5 TC before weakening over land. On the other hand, TC Vance was a faster moving TC that intensified to category 5 status well before landfall and maintained its circulation intensity until well after landfall. Finally, TC Gwenda weakened dramatically just before landfall as a result of a combination of unfavorable SSTs and strong vertical wind shears.

A major program of numerical simulation of the three storms has been carried out. The modeling focuses on the short-range prediction of the tracks, intensity and intensity changes of the three storms. The model predicted all the tracks very well. Apart from Thelma, the skill was not too surprising as they were steered by well-defined upper troposphere level anticyclonic winds. The intensity and intensity changes at landfall also were predicted well as the contributing factors were again well-defined, in the model initial states. These were: positive SST anomalies, diffluent upper tropospheric level flow and weak vertical wind shears for Thelma and Vance; and negative near-coastal SST anomalies and strong vertical wind shears for Gwenda. The importance of these factors has been confirmed and quantified in a series of adjoint sensitivity studies, the details of which will be presented.