Thailand's recent "rain bombs" have triggered concern about a repeat of the massive floods in 2011 that caused widespread losses, paralysed cities and stung the economy.

On May 25-27, Thailand experienced a repeat of this kind of weather, with torrential downpours that cause immediate flooding.

The rain bombs followed a series of unusual climate patterns that have occurred in the country since the start of this year.

May is considered an early start to the rainy season. Thailand's summer in April saw lower-than-average temperatures.

Assoc Prof Seree Supratid of Rangsit University said that La Niña, a pattern of cold ocean temperatures in the central and eastern tropical Pacific, has altered its pattern, cooling temperatures across the country in April.

The average rainfall for May was about 214mm, which is higher than the average over the past 30 years, he said.

However, it is too soon to predict massive flooding for this year.

He said downpours from June to July are expected to ease, and dry weather will return from August to September. Heavy rainfall should make a comeback in October.

Some risk-prone areas in the capital have installed additional water pumping systems to help increase drainage.

In other provinces, water has been released from 21 large reservoirs to cope with the rainfall.