The National Weather Service has issued a flood watch for Kauai County and Oahu, effective Tuesday morning through Thursday afternoon.

Forecasters say an unstable air mass is expected to move northward into the islands Tuesday through Thursday as a “Kona low” develops to the west of Kauai.

This will increase southerly winds across the state, bringing the potential for heavy rains, flash flooding and thunderstorms to Niihau, Kauai and Oahu beginning on Tuesday.

The potential for flash flooding and thunderstorms may expand to Maui County and Hawaii island on Wednesday and Thursday.

A flood watch means flooding caused by excessive rainfall is possible.

During a flood watch, the public should be aware that flood-prone roads and other low-lying areas may be closed due to elevated runoff and overflowing streams, and property damage may occur in urban areas.

Kauai County officials are urging the public to be vigilant ahead of the potential severe weather conditions.

“Officials advise caution and recommend avoiding non-essential travel and outdoor activities during inclement weather with heavy rain and possible strong thunderstorms in the forecast,” said Kauai Emergency Management Agency Administrator Elton Ushio in a news release. “In recent Flash Flood Warning events, multiple people had to be rescued when trying to cross flood-prone areas like roadways, rivers/streams, and low-lying areas.”

Kuhio Highway near Hanalei Bridge may need to be closed with little to no advance notice, they warned, and utilities may be disrupted.

On Oahu, crews with Honolulu’s Department of Facility Maintenance will be using heavy equipment Tuesday morning to move sand at Kaelepulu Stream at Kailua Beach Park to mitigate flooding ahead of the anticipated Kona low.

Last week, crews cleared stream mouths throughout East Honolulu, the Waianae area and the North Shore, according to city officials, while work is currently underway in the Pearl City district.

The NWS says the public should monitor later forecasts and be prepared to take action should flash flood warnings be issued.