The National Weather Service issued a flash flood watch for Oahu, Kauai and Niihau, which took effect at noon today and runs through Wednesday afternoon.

A low-pressure system “will drift south this week and break down the ridge over the western islands, opening the door for deep unstable southerly winds to develop over Kauai and Oahu through Thursday,” the weather service said. “An upper level trough rotating around the low will produce moderate to heavy showers bands over these western islands with thunderstorms.”

Forecasters said heavy showers and isolated thunderstorms are expected to increase this afternoon through tonight. “Periods of heavy rainfall are possible over leeward and windward communities of Oahu and Kauai through Wednesday afternoon.”

“Flood prone roads and other low lying areas may be closed due to elevated runoff and overflowing streams. Urban areas may receive more significant flooding and property damage due to rapid runoff,” forecasters added.

The public is advised to monitor forecasts and prepare to take action should the weather service issue a flash flood warning.

Forecasters noted high pressure is expected to build in the north of the region by Friday, allowing easterly trade winds to spread across the region into the first half of next week.