Nothing seasonal is running in Kuala Lumpur & the west coast in September — which is its own answer. Pack for the temperature.
Going in August
Nothing seasonal is running in August.
Packing for Kuala Lumpur & the west coast?
Build a packing plan around the season you are actually travelling in.
Seasonal ranges are typical, not guarantees — a monsoon can arrive early and a dry season can break. Check the forecast before you seal the bag.