Sumatras or Sumatra Squall Lines (SSL) is a term used in Singapore and Malaysia to describe squall lines that develop over Sumatra at night usually between April and November and then steered towards the west coast of Peninsular Malaysia and Singapore by the southwesterly winds of the southwest monsoon. The squall usually arrives during the pre dawn and early morning with strong wind gusts and thundery showers.
During the southwest monsoon, southwesterly winds pass over the mountains of Sumatra and creates mountain waves on the leeward side of the mountain thus creating an unstable atmosphere. As the atmosphere becomes highly unstable, the thunderstorms begin to merge into each other forming a squall line. This squall line is then steered by the southwesterly winds causing the storm to move northeastwards and moving into the Straits of Malacca. When this squall line moves offshore to the Straits of Malacca, it is usually night and thus, warmer air rising over the Straits of Malacca would cause the squall line to intensify. As the abundance of moisture over the Straits of Malacca condenses, it releases huge amounts of latent heat of condensation and this helps the squall line maintain its strength and intensify even more. This squall line then moves towards the coast of Peninsular Malaysia or Singapore. As this squall line moves inland, it is deprived of latent heat of condensation and hence, dissipates rapidly.