From Kinabalu blog:
To make your life easy, the higher the precipitation, the higher the amount of rainfall. If you look closely at the table (climate for Kota Kinabalu is for the year 2007), January to April are the driest months where the precipitation are the lowest (i.e. below 150mm per month). This is the best time to come here and climb Mount Kinabalu. However, seasonal variations can occur in any year.
Rain can occur at any time of the year. In Sabah the main rainy seasons are from October to January when the rains come with the north east monsoon, and from May to July, with the south west monsoon.
Continue reading (incl. pics) at: When is the best time to go to Mount Kinabalu?