The seasons in Thailand is influenced by the Monsoon winds. The south of Thailand where Khao Lak is located has three primary seasons to which accommodation prices are directly linked. Tropical weather patterns of course makes rainfall difficult to predict far in advance, but if you prefer a generally dry and hot period, mid December to Mid April sees the least rain. However, the occasional thunder shower cannot be excluded. This period will naturally be the best time to visit if scuba diving is on the agenda since sea conditions and under-water visibility is more conducive for this type of activity. April to end October is when most rain and cooler conditions will be experienced.
Though Khao Lak can occasionally see a three day down-pour, these are fairly rare. You can typically expect an hour of rain in the afternoon and a good proper down-pour later during the night. From November things start to dry out and temperatures increase. This is the shoulder season and sees an increase in accommodation prices from the preceding April to October period. Mid December to mid January is peak tourist season and hotel prices at their highest. This brief period is not necessarily the most expensive because the weather is at its best, but primarily because it sees the highest levels of hotel occupancy. February and March is probably the best time to go to Khao Lak if hotel prices, sea and weather conditions are all considered.