Mid-April through October is known as the “low season”. The seasons are the opposite on the east coast (the Gulf of Siam side.) Ko Samui for example, has its high season during Phuket’s low season.
This is a transitional month. The previously Northeastern monsoon winds gradually start moving around to eventually come from the Southwest. LIght winds from different directions. Somewhere around the middle of the month, you can count on some heavy showers. The second half of the month is usually pretty wet. Frequent thunder showers occur. They are often quite heavy and often brief. It seldom rains all day. Warm weather rain or shine … the monsoon is nothing to avoid.
April and May are the hottest months of the year. The temperatures range from 80ºF to 95ºF (27ºC to 36ºC.) In the middle of the month, the Thais celebrate Songkran, the water festival. Everyone splashes and throws water on everyone else. It’s certainly something to see.
On the aesthetic side, low season storms can be absolutely stunning. The colors are often breathtaking. The sunsets are ineffable. The power of nature is at its best.
The middle of the month is the official end of high season. Negotiating prices becomes easier. Many species of orchids are in bloom in both saltwater and freshwater areas. Most of the migratory birds are gone, but Thailand hosts plenty of resident birds. You’ll still see plenty!