State Department of Transportation officials planned to inspect the H-1 Freeway near School Street today to try to determine why soil washed onto the Ewa-bound lanes after a heavy rainstorm, prompting the closure of the Ewa-bound freeway for four hours and backing up traffic during the morning commute.

While skies are clearing over Oahu, the National Weather Service says the threat of more heavy rain still exists. Oahu, Molokai, Maui and Hawaii island remain under a flash flood watch until 6 p.m.

Dan Meisenzahl, the state Department of Transportation spokesman, said  an initial evaluation suggests the School Street closure was not a “mudslide”  or a collapse of a retaining wall, but debris, mainly top soil, that washed onto the road. If it had been a mudslide, there would have been rocks, boulders and other materials on the road, he said.

The soil on the roadway prompted the closure of the Ewa-bound freeway at the School Street off-ramp from about 3:30 a.m. to 7:20 a.m., backing up traffic on the freeway and on Pali Highway.

Meisenzahl said another closure on the Ewa-bound lanes under the Punahou Street overpass was due to the failure of a pump that is supposed to help drain the low area on the freeway.

Police closed H-1 Freeway lanes at Punahou Street from about 3:30 a.m. to 5 a.m. Two Ewa-bound lanes of the H-1 Freeway between the Alexander Street overpass and the Punahou Street overpass reopened after the flood waters drained.

The wet weather is also believed responsible for numerous stalled vehicles and minor traffic accidents from Mililani to Kalihi this morning.

Meisenzahl said road crews and Honolulu firefighters were able to clear the freeway at School Street by hauling away excess mud.  

He said the mud covered the entire ewa bound lanes of the freeway. State transportation crews, using a bobcat removed one drump truck load of mud from the road.

This morning’s heavy rains did not affect a $2.53 million project, completed in March 2010,  that repaired a retaining wall and added new drainage near School street to help control the flow of water from a natural spring, Meisenzahl said.

Forecasters say a moist and unstable air mass is creating conditions that could lead to more heavy showers and thunderstorms at any time and any place today.

The city opened its emergency management center at 2:45 this morning to monitor the situation, said John Cummings, spokesman for the City Emergency Services Department.

At 5:30 a.m., the National Weather Service lifted a flash flood warning for Oahu issued earlier because of heavy rains and thunderstorms stretching from Mililani to Kahala.

Seven inches of rain fell in Moanalua in the 24-hour period ending at 5 a.m. Upper Nuuanu recorded 6 inches and 5.4 inches fell in Manoa.

The weather service also issued a winter weather advisory for the summits of Mauna Kea and Mauna Loa and a high surf advisory for east-facing shores. A mixture of rain and snow is expected to fall on the Hawaii island summits today. Forecasters expect to lift the high surf advisory sometime today as winds and waves decline.