But in terms of rainfall, last night's thunderstorm over East Honolulu was unspectacular: 1.54 inches at Hawai'i Kai Golf Course topped all O'ahu gauges, said National Weather Service lead forecaster Roy Matsuda.

More rain was expected today over much of the state and into tomorrow, clearing by Wednesday, Matsuda said.

O'ahu was under a flash-flood watch that began yesterday afternoon but was expected to be canceled by midday, Matsuda said. Maui County was under a flash-flood watch that was expected to last through 4 p.m.

"It's all shifting to Maui County, where it has been raining quite nicely," Matsuda said.

The rain came from an upper level system very similar to the one a week ago that drenched North Shore residents.

The gloomy clouds and thunderstorms are rare for May, Matsuda said.

"If you asked me what a May (storm system) looks like I would never have said this one or the one a week ago," he said. "There are pockets of instability but you don't expect this stuff. It is very much a winter type of system."