HEAVY RAIN AND FLASH FLOODS

In Johor Baru, Johor on 18 April 2006

Prepared by

1.         Introduction

Heavy rain from localized thunderstorm activity between 3.30pm and 5.30pm caused flash floods in downtown Johor Baru on 18 April 2006.  Light variable winds near the earth’s surface and an unstable atmosphere which are charateristics of the intermonsoon season favors strong convective activity resulting in localized thundertorm and heavy rains in the late afternoon and early eveining, mainly over the west coast states of Peninsular Malaysia. In this report, analyses of the rainfall, satellite and radar imageries during the period of this heavy rain episode is presented.

2.         Satellite imageries on  18 April 2006

Imageris from MTSAT weather satellite are shown in Appendix A for the period from 2.30 pm to 6.300 pm on 18 April 2006 (Figure 1a, Figure 1b, Figure 1c, Figure 1d & Figure 1e).  Over Johor  Isolated convective cloud development started after 2.30pm mainly concentrated to the southern tip and between 3.30pm and 5.30pm a small patch of cloud intensified (red color echo) to produce thunderstorm and heavy rainfall that fell mainly over the urban areas of Johor Baru. 

3.         Weather Radar imageries on 18 April 2006 

Weather radar imageries from the network of radar stations in Peninsular Malaysia (Appendix B - (Figure 2a, Figure 2b, Figure 2c, Figure 2d, Figure 2e, Figure 2f, Figure 2g, Figure 2h, Figure 2i, Figure 2j, Figure 2k & Figure 2l) shows in more detail the development and occurence of thunderstorm and heavy rainfall.  The sequence of imageries at intervals of half hour or 20 minutes is a composite of the six weather radar stations in Peninsular Malaysia starting at 1.00pm and ending at 6.00pm. In the early afternoon isolated convective cloud development is seen over the west coast states and southeast Pahang. By mid-afternoon it becomes more scattered and over the southern tip of Johore a small patch of thunder cloud started to develop. This thunder cloud intensified rapidly   (indicated by the red and yellow colors) that caused heavy rainfall between 3.00pm and 5.00pm. After that thunderstorm activity began to weaken and ove most places the rain began to subside.  

4.         Conclusion

The heavy rain and flash flood that occured over Johor Baru on the afternoon of 18 April 2006 is  a result of localised thunderstorm development. Very intense rain occured over a very short period that caused flash floods to occur over several places in the downtown area of Johor Baru.