About 48,000 Hawaiian Electric customers on four islands should start preparing for power shutoffs possibly lasting several hours to a few days under a new effort to prevent wildfires during high-risk weather conditions.

Hawaii’s largest electric utility on Wednesday announced parameters for an initial phase of its public safety power shutoff program scheduled to become active July 1.

Maui has the most customers subject to the program, about 24,800. About 19,300 customers on Hawaii island, 2,700 on Oahu and 1,300 on Molokai also are in coverage areas.

The program’s initial phase, which covers about 10% of the utility’s roughly 472,000 residential and commercial customers, focuses on places with the highest wildfire risk conditions, including West Maui, Central Maui, Upcountry Maui, West Oahu and West Hawaii.

Expansion of the program, which was developed in collaboration with public emergency response agencies, is anticipated.

Company officials and government leaders urged customers within program coverage areas to start preparing now for possible outages triggered by combinations of strong winds, low relative humidity, dry vegetation and other conditions.

“This is an action that is the last line of defense,” Jim Kelly, vice president of government and community relations for Hawaiian Electric, said during a media briefing at the company’s Ward Avenue headquarters in Honolulu. “We know it will be disruptive and will create many challenges in the affected communities for individuals, for businesses and for public facilities.”

Ideally, such shutoffs will come with 24 to 48 hours of advance notice, but perhaps less or no warning depending on the speed of shifting weather.

Utilities in California and other wildfire-prone areas of the country have implemented similar plans after wildfire disasters.

Hawaiian Electric has been working for the past several months on developing its program in the wake of the Aug. 8 wildfire that killed 101 people in Lahaina and destroyed most of the West Maui town.

The disaster occurred amid gale-force winds that blew down numerous utility poles. Though an official cause of the fire has yet to be determined, many survivors have alleged in several hundred lawsuits that a fallen Hawaiian Electric line ignited dry vegetation and led to the wind-whipped blaze that caused an estimated $5.6 billion in damage and became the deadliest U.S. wildfire in over a century.

Before the Lahaina fire but after deadly California wildfires in recent years, Hawaiian Electric had considered and decided against a power safety shutoff program, in part because the company did not expect the same catastrophic level of wildfires experienced in California and fueled by dry forests.

“In addition,” the company said in a January 2023 wildfire mitigation plan, “a lot of the Hawaiian Electric distribution circuits meander through non-­wildfire areas and then through potential wildfire areas. Thus, preemptively turning off circuits would impact customers that may not be in potential wildfire areas.”

After the Lahaina fire, the utility was criticized for not having such a power shutoff plan.

The scope of the program being activated July 1 was determined by factors that include fire history, historical wind data, vegetation levels, proximity to communities and escape routes.

Colton Ching, Hawaiian Electric senior vice president of planning and technology, said shutoffs won’t necessarily coincide with “red flag” warnings issued by the National Weather Service.

Ching said other factors will be considered as well, including data from company weather stations and spotters.

Red flag warnings are issued when forecast weather conditions meet three criteria — at least 15 mph winds sustained for several hours, relative humidity under 25% for several hours and less than 9% moisture in certain kinds of vegetation. Sometimes such warnings are issued for leeward areas of all islands for multiple days.

Ching said wind speed levels for triggering power shutoffs will likely be 45 to 50 mph, possibly in combination with other conditions.

The future frequency and duration of such events can’t reasonably be predicted. Kelly said the number of red flag warnings annually over the past decade have ranged from zero to five.

Two such weather warnings were issued in November. And since the Lahaina disaster, the nonprofit customer-owned utility serving Kauai implemented public safety power shutoffs twice.

Ching said that power shutoffs won’t necessarily happen for entire areas where the program applies, and that more precise shutoffs within an area can be limited to one or more circuits in the area depending on conditions.

On Maui, there are 45 circuits within all coverage areas. There are also 34 circuits within the program for Hawaii island, 15 for Oahu and two for Molokai.

Kelly said a triggered shutoff would probably last at least six to eight hours, in part because after weather conditions improve equipment will need to be inspected before being re-energized. It’s also possible that a shutdown lasts a day or two or three, he said.

Because of such potentially long power outages, the utility and public safety officials urge customers in program areas, and even some outside program areas, to prepare for outages that can affect the water supply and are especially critical for people who use medical devices relying on power.

“The time to prepare is today,” Jim Ireland, Honolulu Emergency Services Department director, said during Wednesday’s briefing.

Ireland said that people who need to power medical devices should have accessory power or a relocation plan. Ireland also said that his department is prepared to increase ambulance service to West Oahu and move people with medical needs by bus if necessary during wildfire prevention outages.

Brendan Bailey, Hawaiian Electric vice president of customer service, encouraged customers to stay informed about public safety outage plans by subscribing to the company’s mobile app and opting to receive notifications. Information about such outages will also be distributed through company social media accounts, its website and news media.

Honolulu Board of Water Supply chief engineer and manager Ernest Lau said that parts of leeward Oahu beyond Hawaiian Electric’s West Oahu public safety shutoff area could run out of water if an outage lasts long enough, though the BWS system includes big storage tanks that supply water via gravity and mobile generators can be deployed to power some pumps.

Assistant Honolulu Police Chief Darren Chun noted that HPD will be prepared for traffic control work and increasing officer presence for security during program power shutoffs.

Hiro Toiya, director of the Honolulu Department of Emergency Management, said wildfire preparedness is a collective responsibility for everyone.

“It’s the state, it’s the counties, it’s our nonprofit organizations, individuals and families,” he said. “And we see Hawaiian Electric taking responsibility to really reduce wildfire risk as much as possible.”