The overnight rain in Shanghai may strengthen this afternoon and turn into a big thunderstorm, local forecasters warned.

The Shanghai Meteorological Bureau issued an orange rainstorm alert, the second highest of a four-level warning system, at 12:27pm, predicting a torrential downpour in the next three hours with a precipitation of 50 millimeters.

The bureau also issued a yellow thunder alert, lowest of a three-level system, for the next six hours as churning rain clouds are likely to unleash a lot of thunderbolts.

The rainstorm will last from today to tomorrow as the rain belt moves northward. More rain will fall in the south part of Shanghai, the bureau said.

The rain should diminish on Thursday but will make a comeback at the weekend, the bureau forecasted.