More rain and thunderstorms are forecast through to May 13 across the northern, northeastern and central regions, according to the Thai Meteorological Department (TMD).
In a statement, the TMD said the rain, caused by a cold mass over southern China and upper Vietnam, will alleviate the heat for residents in the three regions, but people have been warned to brace themselves for summer storms.
For the southern region, the eastern wind is expected to gain strength and bring heavy rain to some areas. People there have been urged to closely follow the TMD's weather warnings.
According to the TMD, a change of wind in the Andaman Sea is expected from May 14-16, which could signal a change of season. However, more factors are needed to determine when the rainy season begins.
Earlier, the department forecasted that the rainy season would start in the third or fourth week of this month.
Prime Minister Srettha Thavisin has instructed ministries and security agencies to mobilise their resources to assist people still affected by drought, according to government spokesman Chai Wacharonke.
He said Mr Srettha, alarmed by the long absence of rain during his visits upcountry, told relevant agencies to act to find ways to provide water to drought-hit areas.
He said the prime minister also told the Ministry of Public Health, the Ministry of Interior and the Ministry of Defence to make necessary preparations for summer storms.
Hospitals and medical facilities in the provinces where storms are forecast are told to stay alert and be ready to provide services.
In Ubon Ratchathani, one girl was killed and her sister injured when a summer storm wreaked havoc on Monday, local officials said on Tuesday.
The storm caused extensive damage across the province, especially in Muang Samsip district.
A house wall collapsed and injured the two sisters, 10 and 8, during the strong winds, and one of them succumbed to her injuries.
According to officials, more than 1,000 houses were damaged by the storm, with 100 of them in Muang Samsip district.
Earlier, the Office of the National Water Resources predicted that intense heat and summer storms would prevail over the upper North and upper Northeast from now until Thursday.