Brunei Darussalam: Brunei Darussalam is set to transition from the inter-monsoon phase to the Southwest Monsoon beginning June 2025, with expectations for the monsoon to last until September. This period is characterized by more stable atmospheric conditions, leading to a decrease in thunderstorms compared to the inter-monsoon phase. Winds during this monsoon are anticipated to predominantly come from the southwest direction.

According to Radio Television Brunei, the Southwest Monsoon is also known for the active development of tropical storms and typhoon systems over the Northwest Pacific Ocean and the South China Sea. Although these weather systems usually form at a significant distance from the region, they can still impact local weather patterns. The presence of nearby tropical storms or typhoons may result in the development of squall lines. These are fast-moving bands of thunderstorms that align in a long, narrow formation, potentially bringing unstable weather conditions such as gusty winds and occasional thunderstorms.