Heavy rain and thunderstorms swept across the state Friday, causing widespread flooding and triggering a series of power outages that affected approximately 60,000 Hawaiian Electric Co. customers on Oahu.

The wet weather was generated by an unstable air mass that accompanied a low-pressure system eastward across the state, according to the National Weather Service.

HECO spokesman Darren Pai said weather conditions, including lightning strikes, likely contributed to a 120-megawatt loss of power generation at the Kalaeloa Power Plant. That led to three generating units at the Kahe Power Plant (with a combined capacity of 314 megawatts) to also shut down, followed by a 40-megawatt loss of power at the HPOWER plant.

Kalaeloa Power Plant is operated by Kalaeloa Partners, an independent power producer contracted by HECO.

“The combined loss of all of this generation forced us to initiate some emergency outages,” Pai said. “The system is designed to automatically respond this way to make sure we have a balance of power in the system, and this is to prevent even more widespread outages or bigger problems.

“We understand that this was a big disruption for our customers, and we worked as quickly as we could to get the power back on line,” Pai said.

The outages affected customers in Iwilei, Kakaako, Pauoa, Waipahu, Waialua, Nuuanu, Moanalua, Aiea, Wahiawa, Makaha, Kaneohe, Salt Lake, Pearl City, Kailua, Kahaluu, Punaluu, Hauula, Kahuku and Kapalama, as well as along Kapiolani Boulevard.

Power to most areas was restored by 12:35 p.m.

As of Friday evening HECO personnel were still investigating the specific cause of the initial loss of power generation. However, Pai said HECO crews did initiate repairs to equipment believed to have been affected by lightning strikes.

HECO asked customers to limit electricity use Friday during the peak evening hours of 6 to 8 p.m. to ensure sufficient power availability.

The stormy weather hit Kauai first, starting late Thursday evening.

In the 24-hour period ending at 8 a.m. Friday, nearly 10 inches of rain fell at the North Wailua Ditch. Heavy rain activity was also recorded at Mount Waialeale (7.7 inches) and Princeville Airport (6 inches).

The Red Cross briefly opened Hanalei Elementary School as an emergency shelter.

The heavy rain caused the Wailua River to overflow, and water covering Kuamoo Road from the river forced tourists to evacuate a vehicle early Friday morning, a county official told the National Weather Service.

Flooding and pooling prompted police to close Kuhio Highway near the Hanalei Bridge. Flooding was also reported in the Waimea Theater building.

The state Department of Land and Natural Resources closed the Kalalau Trail.

The state Health Department issued a brown-water warning for Kauai and advised the public to stay out of floodwaters and runoff due to possible contamination.

In addition, the Kauai County Department of Water issued an immediate water conservation request for all Hanapepe and Eleele residents due to a power outage at a well and tank site.

According to department officials, severe weather conditions and high-river levels prevented personnel from accessing the site.

By midmorning the stormy weather had shifted to Oahu, where radar detected rain falling at a rate of 1 to 2 inches per hour over West Oahu from Kalaeloa to Waianae.

The Weather Service responded with a flood advisory for the island. The advisory was canceled at 1 p.m.

The storm continued eastward through the early afternoon, with heavy activity reported on Maui along the windward slopes of Haleakala between Paia and Kipahulu just after 1 p.m. By 2:19 p.m. radar indicated that shower activity had increased to a rate of 3 to 4 inches per hour in the Twin Falls and Hamoa areas.

Meanwhile Hawaii island remained under a flash-flood warning and flood advisory for most of the afternoon as heavy rain hit Highway 19 between Papaikou and Laupahoehoe, and Mamalahoa Highway between Kalaoa and the Waikoloa junction.

By 5:21 p.m. Friday rain in North Hilo and along the Hamakua Coast had dissipated, but heavy storm activity was still being reported along portions of the North Kona and South Kohala districts.

The Weather Service said there was a possibility of snow showers on Hawaii island summits as the upper-level trough moved through from Friday to today.

Forecasters predict that a drier air mass could move back over the islands from today to Sunday, followed by a weakening cold front from the northwest Monday.

A weak high-pressure system is expected to build over the island during the middle of the coming week. A stronger area of high pressure will build to the north of the state, heralding the return of tradewinds toward the end of the week, according to forecasters.