Don’t let those bright blue skies over Honolulu fool you.

Oahu, Kauai and Niihau remain under a flash flood watch until 6 p.m. today, thanks to a lingering unstable weather system that lit up the skies last night.

"Although more widespread thunderstorm activity from last night has subsided, a nearby upper low southwest of the islands will keep the airmass unstable, with a potential for more localized heavy showers or thunderstorms developing this afternoon," National Weather Service forecasters said today.

The flash flood watch covers the rest of the state through Friday morning, forecasters said.

The forecast calls for possible heavy showers and scattered thunderstorms through Friday, with the weather system slowly diminishing from west to east. Light winds, afternoon showers, and clear and cool nights, are predicted for the weekend.

The summits of Mauna Loa and Mauna Kea above 8,000 feet, meanwhile, are under a winter storm warning through noon Friday. "Combined with the cold temperatures aloft, snow is expected to fall with total snow accumulations around 6 inches. Thunderstorms could produce heavy bursts of snow and frequent cloud-to-ground lightning from time to time. Conditions should improve on Friday as the upper trough weakens," the weather service said.

The same storm system brought hail, thunderstorms, and  flooding throughout Oahu and Maui last night and early this morning.

On Maui, lightning and thunder put on a "spectacular show," said Rod Antoine, Maui County spokesman. Heavy rains caused minor flooding in Apia this morning, closing Baldwin Beach Park, he said.

Kauai Awai-Dickson, Maui Electric Co. spokeswoman, said lightning strikes were reported in Hana and Kihei, causing brief power outages.

Maui County officials reported that lightning struck a tree near Wailuku Elementary School just before 6 a.m. today, setting a branch on fire.

The branch hit a power line on High Street fronting the school, knocking out power for 1,100 Wailuku residents for about an hour, Awai-Dickson said.

Forty-four customers in Hana also have been without power since 7 p.m. Wednesday.

A mudslide and fallen trees blocked Kalanianaole Highway just mauka of Kapaa Quarry Road for several hours early this morning, The road is now open, police said.

For the 24 hours ending at 8 a.m. today, the weather service reported that 2.74 inches of rain fell in Olomana, 3.28 inches in Maunawili, and 1.83 inches in Hawaii Kai.

Last night, a lighting strike blew a hole in the steeple of a Kaneohe church. The lightning bolt struck at Our Lady of Mount Carmel Church, at 48-422 Kamehameha Highway, before 6:50 p.m. and left a 10 square-foot hole in a wooden steeple above the church, said Honolulu fire Capt. Terry Seelig.

He said some people who were at the church were either in the main building or another building on the property when they heard a loud thunderclap. They went outside and saw the hole with a lot of debris on the ground.

Firefighters responded and climbed the roof to confirm there was no fire and found there was apparently no structural damage to the roof besides the steeple, Seelig said.

Firefighters used tarps from the church to patch the hole.

It wasn’t raining at the time, and no one was hurt, Seelig said.

Also last night, lightning disabled the outboard engine of a 22-foot boat about two to three miles off Portlock, stranding its three occupants.

Seelig said he didn’t know if the outboard engine was actually struck by lightning, but the "voltage in the air probably fried the motor," Seelig said today.

No was injured and the department’s rescue boat towed the disabled vessel to the Hawaii Kai boat ramp almost two hours later after the alarm was called in at 9:30 p.m.

Lightning may have been the reason 1,000 Hawaiian Electric customers in the Kaiser High School area of Hawaii Kai lost their power about 6:15 p.m. 

Forecasters received a report of hail in Kunia yesterday afternoon.

Moist air and low pressure in the upper atmosphere is creating unstable weather conditions with the possibility of more heavy rainfall and thundershowers, forecasters say.

Forecaster Derek Wroe said Wednesday that cold air in the upper atmosphere mixing with warmer air below sets up a condition ripe for occasional hail.

"Whenever you have cold air on top of warmer air … it creates an unstable condition," Wroe said. "And the air mass on the upper level trough appears to be a bit cooler than normal."

On Oahu, hail was last reported in Nanakuli in February 2009, Wroe said. Hail was also reported on the Kona side of the Big Island in 2008.