Sumatra squall

They may occur at any time of the year, but are most common from April to November, coinciding with the southwest monsoon and inter-monsoon periods when the prevailing winds have a greater westerly component.

Typically, Sumatra squalls affect Malaysia and Singapore for one to two hours in the night or morning, producing heavy rains along with wind gusts of 40–80 km/h (25–50 mph).

Individual forecasters subjectively assessing the outputs of various regional- and global-scale models and analysing real-time observations can have moderately accurate results, though the process is slow and laborious.

[18] The presence of tropical cyclones over the South China Sea can serve to increase the frequency of Sumatra squalls by shifting the prevailing winds to the west or southwest.

[21] Several hypotheses exist to explain the development of the initial convective cells that later form the Sumatra squall: heating of air parcels at the surface by the warm waters of the Straits of Malacca, convergence of land breezes from the east coast of Sumatra and the west coast of Peninsular Malaysia,[22] or mountain waves generated by air flowing down the eastern slopes of the Barisan Mountains.

Once these thunderstorms develop, the presence of vertical wind shear, caused by contrasting westerlies in the lower troposphere below an altitude of 4 km (2.5 mi) and easterlies above that, is necessary for the individual cells to organise into and persist as a squall line.

[13] The squall forms a bow-shape on weather radar as it intensifies, but does not evolve further to a comma-shape as the turning effect of the Coriolis force near the equator is too insignificant to generate enough rotation.