Flash floods hit parts of southern Thailand as the country's rainy season fast approaches.

Roads in Nakhon Si Thammarat were inundated with 20-30 centimetre floods on May 11 following heavy downpours indirectly caused by Tropical Cyclone Asani off the coast of India.

Asani has strengthened the south and southeasterly winds prevailing over Thailand, but the country is also approaching its rainy season, which is expected to be declared in mid-May.

The Thai Meteorological Department said the rainy season will likely last until mid-October, with a temporary break between June and July because of the El Nino and La Nina phenomena.

Soaring temperatures reaching 35 degrees Celsius are often followed by powerful tropical storms with thunder, lightning, rain and flash floods which cause rivers to flow faster and become dangerous.

Neighbouring countries in Southeast Asia such as Malaysia, Indonesia and the Philippines are also reaching the start of their tropical monsoon rainy season. Many of the under-developed nations struggle to cope with the heavy rain due to under-investment infrastructure.