The National Weather Service has posted a flash flood watch for Oahu and Kauai County starting Wednesday afternoon.

An approaching storm system could bring heavy rain through Thursday.

Southeast winds ahead of a weather front will carry warm, moist air over the islands while another system is bringing colder temperatures to the upper atmosphere.

“The moisture and instability will combine to produce a threat of heavy rain,” forecasters said. “The threat of heavy rain will be greatest over Kauai County starting Wednesday evening. The heavy rain threat will spread to Oahu later Wednesday night.”

A flash flood watch means that conditions may develop that lead to flash flooding.