All islands are under a flash flood watch from tomorrow through late Friday night, the National Weather Service said tonight.

Kauai and Niihau are under the watch starting tomorrow morning, followed by the other islands in the evening.

A “potentially dangerous weather situation will start to develop on Thursday,” they said. “Unstable conditions ahead of an approaching cold front will reach Kauai on Thursday, bringing the threat of flooding rainfall. The threat for heavy rain and flooding will spread to the remainder of the state Thursday night and will persist through at least Friday night.”

The entire state should expect locally heavy showers and thunderstorms today through late Friday night. 

The weather system leaves the islands before the Honolulu Marathon Sunday morning, forecasters said. Runners can expect normal tradewind weather for the event, and may expect some clouds and showers.

A flash flood watch means that conditions may develop that lead to flash flooding, and the public should pay attention to the latest forecasts, the weather service said.

The Big Island summits may see a mixture of snow and rain as early as tonight and last through Saturday. Precipitation may be heavy at times.