UPDATE: 3:30 p.m.
The flood advisory for Oahu and Kauai has expired.
The flood watch for Kauai County and Oahu remains in effect through Thursday afternoon.
The flood watch for Maui County and the Big Island is in effect from Wednesday morning through Thursday afternoon.
A flood advisory has been posted for the island of Kauai through 1 p.m.
Radar at 9:52 a.m. showed rain spreading over Kauai from the south with heavier showers measuring in at rates up to 2 inches per hour.
A flood advisory is in effect for Oahu through 12:45 p.m. today.
At 9:40 a.m., radar showed heavy rain moving over East Oahu from the south. Rainfall was measured falling at a rate of up to 2 inches per hour.
Some locations that will experience flooding include Honolulu, Waipio, Waiahole, Waikane, Kahaluu, Ahuimanu, Aiea, Pearl City, Halawa, Moanalua, Kaaawa, Salt Lake, Kaneohe, Mililani, Kalihi, Waikele, Punaluu, Wahiawa, Waipahu and Kaneohe Marine Base, according to the National Weather Service.
Residents and visitors are advised to stay away from streams, drainage ditches and low-lying areas prone to flooding.
A flash flood watch is now in effect for Oahu and Kauai County through Thursday afternoon and will be expanded to cover all main Hawaiian islands Wednesday morning through Thursday afternoon.
The National Weather Service said flash flooding caused by excessive rainfall will be possible.
Forecasters say an unstable air mass is moving northward over the islands today through Thursday as a “Kona low” west of Kauai is increasing the potential for heavy rain and thunderstorms for Kauai and Oahu, and later spreading this potential over the remaining islands over the next few days.
This will increase southerly winds across the state, bringing the potential for heavy rains, flash flooding and thunderstorms to Niihau, Kauai and Oahu beginning today.
Showers are expected to gradually ease Friday through the weekend although the potential for locally heavy showers may continue as the Kona low lingers.
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 this 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.