Hawaii and Maui counties are under a flash flood watch through Thursday afternoon.

The National Weather Service also said Oahu will also be under a flash flood watch from Wednesday evening through Thursday afternoon. And the Big Island summits will be under a winter storm watch starting Wednesday with snow accumulation of up to 3 inches.

The weather service says residents can expect widespread moderate to heavy rain moving slowly, and isolated thunderstorms.

A lot of moisture from the east is moving over the islands and will combine with an upper level of low atmospheric pressure to bring possible flooding conditions.

The heavy rain will affect urban areas in lower elevations, more susceptible to flooding. Heavy rain will also hit higher elevations, likely producing flash flooding of streams.

The heavy rains will move from Maui County and Hawaii island before advancing to Oahu late Wednesday.

A flash flood watch means conditions may develop that lead to flash flooding, the Weather Service says.

A winter storm watch for Hawaii island summits begins Wednesday morning through Thursday afternoon.

The watch, which is for 8,000-foot elevation level and higher, warns of potential heavy snow, icy conditions, wind gusts as high as 70 mph, and possible road closures. 

A high wind warning also will be in effect on the summits, starting at 6 a.m. Wednesday.

Temperatures will be in the 20s and 30s, forecasters said.