Roy Matsuda, lead forecaster said the calculation is based on 15 minutes of heavy rain at the tunnel although only one to two inches have actually fallen. Other areas in the mountain range could also see those high numbers tonight, Matsuda said.

"The amount so far are not that great but it looks like the rainfall is going to stick there," he said. "It looks like it will continue for the rest of the night."

Matsuda said a combination of wind direction, low pressure and instability in the atmosphere is causing the heavy downpour. The winds are blowing parallel to the mountains in a southeasterly direction and that is a dangerous indication, he said. It causes a rainy situation and the low pressure enhances that.

"It will make the clouds bigger and the rainfall rate heavier," Matsuda said. "The most heaviest will be on the mountain."

The winds may change tomorrow and that will bring relief, he said, adding that more heavy rain is expected later this week as a low pressure area moves in from Kaua'i.