A powerful Kona low storm bearing down on Hawaii is expected to pound the islands for the next couple of days.

Forecasters said Thursday the storm will produce heavy rain and strong wind that could lead to dangerous flash flooding, felled trees and power outages.

While officials cautioned everyone to prepare for potential impact, forecasters said the greatest threat for excessive rainfall and flash flooding will be in the Hilo, Puna and Kau districts of the Big Island and, to a lesser extent, East Maui.

A flood watch was in effect through Saturday afternoon for all islands, and a winter storm warning was called until 6 p.m. Saturday for the Big Island summits.

The National Weather Service said up to 20 inches of rainfall could drench the islands over the next couple of days.

Officials across the islands have been warning about this storm for days, and on Thursday morning Hawaii County Mayor Mitch Roth signed an emergency proclamation allowing for a streamlined response and coordination of resources in the event of a disaster.

All county and state emergency operation centers were at least partially activated to monitor the storm.

Adam Weintraub, spokesman for the Hawaii Emergency Management Agency, said the storm comes at a time when a lot of Hawaii’s ground is already saturated from rain, which means flooding could develop much more quickly, and powerful wind also could knock down trees more easily.

“As usual, we’re hoping for the best but preparing for the worst,” Weintraub said.

Hawaiian Electric on Thursday morning was urging its customers to prepare for the storm and said its crews are ready to respond to storm-related outages.

“In some areas, power restoration may be delayed due to accessibility issues caused by fallen trees, debris or flooding,” the company said in a statement.

Hawaii is ordinarily dominated by the tradewinds that blow in from the northeast. But the counterclockwise flow around a Kona low-pressure area turns things around, bringing southwesterly wind sweeping over the islands from the Kona, or leeward, side.

Drawing moisture from tropical waters to the south, these storms are notorious for unleashing heavy rain, hailstorms, flash floods, landslides, high winds, large surf, waterspouts, thunderstorms and blizzards atop Hawaii’s tallest mountains.

On Oahu, officials announced that the Royal Hawaiian Band concert scheduled for noon to 1 p.m. today on Iolani Palace grounds will be moved indoors to the Honolulu Hale Courtyard, 530 S. King St.

On Maui, Mayor Richard Bissen said the county crews were busy doing pre-storm activities, such as clearing storm drain pipes and culverts.

“Along with the county’s Emergency Management Agency, our departments are readying their operations in order to support our community,” the mayor said in a news release.

Elsewhere, the Maui County Department of Water Supply issued a countywide request for customers to conserve water and reduce consumption through Sunday. Officials said heavy rain and the possibility of power outages could disrupt water pumping and distribution.

Also on Maui, the Kipahulu District of Haleakala National Park will be closed this morning due to the risk of flash flooding, road closures, landslides and road washouts in East Maui. All camping reservations for Kipahulu Campground today were canceled.

Kauai Emergency Management Agency Administrator Elton Ushio advised caution during the inclement weather and avoiding all nonessential travel, especially through flood-prone areas. Kuhio Highway, in the vicinity of the Hanalei Bridge, may need to be closed with short to no advance notice, he said.

On Hawaii island, officials said road crews, utilities, fire and police personnel were put on alert Thursday, and shelters were identified in the event they become necessary.

Waipio Valley Access Road was closed for everyone except Waipio Valley residents and farmers Thursday. Officials said they expect more closures in the coming days.