The Northeast will be the first region to be hit by gusting winds and hail before storms spread to other regions, the department said.
People in the upper part of Thailand should be cautious outdoors, especially in areas near large trees and unsecured billboards, the department said.
It also urged farmers to make efforts to prevent crop damage.
The department also reported that temperatures in Nong Khai’s Muang district and Mukdahan's Muang district reached 41.3 Celsius on Saturday.
Two other districts saw temperatures surpass 40 degrees Celsius: Petchabun's Muang district (41.2C) and Lopburi's Chai Badan district (40.2C).
The seven hottest districts on Saturday were: