Rain swept across the Islands yesterday, with spotty but occasionally heavy showers mostly falling on O'ahu, the National Weather Service said.

More rain was forecast for today, and as of last night, O'ahu and Kaua'i were under a flash-flood watch through this afternoon. The entire state was under a flash-flood watch much of yesterday, but it was lifted for Maui County and the Big Island at 3:30 p.m.

There were no reports of serious floods, but O'ahu Civil Defense volunteers on the North Shore and city road crew supervisors islandwide patrolled areas where heavy rain and debris normally cause problems — culverts, bridges and stream mouths.

The weather service issued a flood advisory for O'ahu at 2:29 p.m. after heavy rain near Ma'ili created ponding on area roads. The Wai'anae Coast began receiving heavy rain about 12:30 p.m. That advisory remained in place until 6:30 p.m. as rain on west O'ahu subsided but increased over central O'ahu and North Shore areas.

Flooding Sunday night, when heavy rain first began falling over Windward O'ahu, closed Kamehameha Highway near Sacred Falls for two hours because of a blocked opening at Pokiwai Stream in Hau'ula. The highway was reopened to traffic at 11:30 p.m. Sunday after city crews unplugged the sand-clogged stream mouth with a backhoe, said Larry Leopardi, director of the city's Division of Road Maintenance.

A city crew also unplugged the mouth of nearby Waipuhi Stream as a precaution, he said.

"If we don't do that, there is a potential for water backing up," he said. "The streams would have an impact on housing if we didn't do that."

The soggy weather is the result of a kona low northwest of Kaua'i. The low-pressure system, which can be very difficult to predict, will make the atmosphere unstable as it draws moisture from the southeast, said Pete Donaldson, a weather service forecaster.

Thunder rumbled and lightning flashed over O'ahu yesterday from Waimanalo to Wai'anae and could erupt today in other areas as the showers continue.

"If it stays like this, it will be OK, it will run off fine," O'ahu Civil Defense spokesman John Cummings said of the steady but relatively light rains. "If we get a deluge, we get flash flooding. ... It has been pretty dry recently, though, so we don't have a lot of moisture saturating the ground."

Reach Mike Gordon at mgordon@honoluluadvertiser.com.