The weather office warns that summer storms and hail are likely in areas of the Northeast, the North, the Central Plain and the East from Wednesday through Friday.
Wanchai Sakudomchai, director-general of the Meteorological Department, said on Monday that thunderstorms and hailstorms were possible in the four regions, starting in the Northeast on March 7.
They could move into other regions, including Bangkok, on following days.
People should stay away from outdoor areas, big trees and insecure billboards during the storms, and farmers should be prepared to cope with damage to crops and infrastructure, he said.
Mr Wanchai said the weather was being brought by a high pressure system from China that would move into upper Thailand, beginning with the Northeast on Wednesday, and the South China Sea.
Chayapol Thitisak, director-general of the Department of Disaster Prevention and Mitigation, said people affected by the storm could seek help via the department's hotline number 1784 around the clock.