New Delhi, June 2 -- By Olivia Le Poidevin

GENEVA, June 2 (Reuters) - The United Nations weather agency forecast on Tuesday a moderate or possibly a strong El Nino that could drive up global temperatures and increase the risk of extreme weather over the coming months.

El Nino is a periodic warming of sea surface temperatures in the central and eastern Pacific Ocean, ​which typically lasts between nine and 12 months, according ​to the World Meteorological Organization.

The WMO said warm ocean waters were driving El Nino's development and predicted above-average temperatures in most parts of the world from June to August. The WMO said it is likely El Nino will continue until November.

It also said it remained uncertain how s...