An exceptional rainstorm affected the eastern coast of Peninsular Malaysia during 9–11 December 2004 as a result of a westward
propagating tropical disturbance known as the Borneo vortex. Rainfall totals near the storm center exceeded 600 mm and led
to flash floods, loss of life and severe damage in the area. This study presents the results of a numerical simulation of
this event using the fifth generation of the Penn State – NCAR Mesoscale Model (MM5). The model successfully simulated the
synoptic circulation and reproduced the episode with comparable spatial patterns and total accumulated amount of precipitation
to the observed.
Various sensitivity experiments showed that the local topography is decisive in shaping the rainfall distribution during the
storm episode. The role of the terrain elevation appears to be to block the westward progression of the system and inhibit
excessive rainfall in the inland areas of Peninsular Malaysia. To the north of the storm center where coastal terrain elevation
is relatively high, orography plays an important role in the rainfall by providing an additional forcing for moist air lifting.
An additional fake dry simulation suggested that latent heat release is crucial for the development of the storm. Without
latent heating, the vertical coupling of low-level convergence and upper level divergence is weakened and the vertical motion
associated with the storm is suppressed.