Thunderstorm warning: hail, flooding rain expected for South Island

Severe thunderstorms - with marble-sized hail and flooding rainfall - are expected to hit the upper South Island today, MetService warns.

It's advising people that if severe weather does strike, people should take shelter immediately.

"Afternoon heating and cold conditions in the upper atmosphere over the northern South Island are expected to combine to create convection and thunderstorms this afternoon and early evening," MetService said in a thunderstorm warning issued this morning.

"Some of these thunderstorms may become severe due to localised downpours and large hail.

"These storms are capable of producing marble sized hail and flooding rainfall.

"Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous. Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips."