A Sumatra squall (plural: Sumatra squalls or Sumatras) is a squall line—a line of thunderstorms—that develops over the Indonesian island of Sumatra and moves eastwards over the Straits of Malacca, producing heavy rain and gusty winds in Malaysia and Singapore. Sumatra squalls typically form in the predawn hours and early morning, and last for a few hours. 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. The passage of Sumatra squalls can result in fallen trees, flash floods, and property damage. Shipping along the Straits of Malacca may also be affected.