Heavy rain and gusty winds were expected to threaten Oahu for a second day this morning after storms and gusts driven by a passing cold front clobbered many areas of the island Monday.

West Oahu, and particularly the Waianae Coast, seemed to get the worst of Monday’s storm as Makaha Valley experienced at least 10 inches of rain, flooding streets and cutting a muddy river across Makaha Beach. Heavy wind apparently blew down a gas pump roof at a 76 gas station in Nanakuli.

In Waialae two people and a dog were swept away by floodwaters near Waialae Avenue. Honolulu Fire Department units responded, and they were safely accounted for on shore at the beach, makai of their entry point.

Gusty wind caused large sections of the island to experience blackouts throughout the day and night, including in mid- to late afternoon when the College Football Playoff championship was being played.

The National Weather Service even briefly issued a tornado warning for Molokai after radar spotted a severe thunderstorm near Kepuhi and moving north at 30 mph.

Meanwhile, two large Hawaiian Electric generating units at the Waiau Power Plant went offline Monday afternoon, leaving Oahu with a lack of backup capacity and prompting Hawaiian Electric to threaten rolling blackouts for the entire island.

The company laid out a schedule of half-hour blackouts for different areas Monday evening, from 5:30 to 9 p.m., and urged customers to cut back on power use.

For the first couple of hours, the blackouts were not necessary because demand for electricity was below normal, said Hawaiian Electric spokesman Darren Pai.

“Demand for electricity was lower than normal,” Pai said. “Whether that was from cooler weather or from our customers helping out, the blackouts weren’t needed because they were using less electricity.”

But that changed shortly before 8 p.m. Pai said the blackouts were launched starting with the Nuuanu and School Street area and Hawaii Kai and Waimanalo.

The company soon thereafter issued a new rolling blackout plan, forcing power outages for half-hour periods from Kaimuki to the Leeward side of Oahu through 11 p.m.

Pai said officials would continue to monitor the weather and electricity use through the night to determine whether any more blackouts are necessary.

While assessments and repairs were underway, he said, the Waiau generating units were not expected to be back online overnight. Making matters worse, the rainy and cloudy conditions reduced solar energy production across the island, he said, and battery energy storage systems could not charge to full capacity.

“We apologize for the need to take the extraordinary step of initiating outages and we appreciate the patience of everyone who is affected,” Jim Kelly, Hawaiian Electric vice president of government, community relations and corporate communications, said in a news release. “Faced with a potential generation shortfall, we want to give customers a heads up so they can plan around these brief outages and help conserve power at a critical time. On an island, we’re on our own and there’s no one to call on when our reserve generation runs short.”

Elsewhere, Hawaiian Electric crews responded to individual blackouts in Waipahu, Hawaii Kai, Makaha and Haleiwa, among other places.

The National Weather Service said the unstable air mass is expected to spread eastward to Maui and Hawaii island through today, and a flood watch remains in effect for the entire state until this afternoon.

Monday’s storm was blamed on a southeastward- moving cold front that pulled deep tropical moisture and unstable showers and thunderstorms across the islands. A second front is forecast to deliver another round of wet weather Thursday and Friday.

Southern slopes of the Big Island also will see enhanced rainfall into Wednesday.

Forecasters said models show wet and unstable weather spreading eastward across the state through today as the front moves across the islands, though the front is expected to slowly weaken as it travels down the island chain.

Strong winds might push downslope gusts along north and east slopes of island mountains. Observations show that winds remained below advisory level Monday, but forecasters said they will continue to monitor conditions.

The Big Island has the chance of experiencing thunderstorms through tonight, forecasters said. Strong winds and snowfall are possible on the summits of Mauna Kea and Mauna Loa. But the air mass remains too warm to support wintry weather at the Haleakala summit on Maui, they said.

Weather will briefly improve across each island after the cold front passes late today near Kauai and on Wednesday for Oahu and Maui County. The Big Island will see lingering showers Wednesday, with improvement forecast from Wednesday night into Thursday.

Another front is forecast to move into the islands from the northwest, with showers spreading down the island chain, from Thursday through Friday. Improving weather is forecast statewide from Friday night into the weekend.