Oahu residents who awoke to peals of thunder early Tuesday morning would have been treated to quite a show had they peeled open their eyes.

About 350 lightning strikes lit up the sky between 3 and 4 a.m. from the Honolulu airport westward, said Jon Jelsema, lead forecaster at the National Weather Service in Honolulu.

It was “a pretty prolific lightning show,” he said.

The thunderstorm activity moved over the eastern side of Oahu between 5 and 6 a.m., pushing offshore shortly after 7 a.m.

It’s unusual to have an upper-level low-pressure system this time of year, Jelsema said. “This time of year typically we would be seeing a nice, breezy tradewind pattern. This is more reminiscent of a wintertime setup.”

The unusually strong low-pressure system and the abundant low-level moisture combined to produce an unstable air mass, which caused the lightning and showers, he said.

The more humidity, the more easily thunderstorms can develop, Jelsema said.

The weather service issued a flash flood warning from 6:40 to 12:30 a.m. Tuesday for Oahu, as radar showed thunderstorms hitting several areas of the island. Rain gauges showed torrential rain falling between 3 and 4 inches per hour in Oahu’s southeast and northwest.

The rainfall Kauai experienced Tuesday moved toward Oahu. The heavy rain is expected to continue on Oahu today.

A flash flood watch continues through this afternoon for Kauai County.

Maui County could see heavy rainfall as the system slowly moves east, Jelsema said.

Although the rain will end, the humidity will linger but will decrease by the weekend with a lighter tradewind pattern beginning to return tonight. By Thursday night tradewinds will flow across Oahu and Kauai.

A Hawaiian Electric Co. spokeswoman said the weather contributed to pockets of outages here and there. A 1,900-customer outage occurred Tuesday in Makakilo. Most were quickly restored by 6:30 a.m.