Hawaii's flash flood season, October through April, has begun with moderate to severe drought across most of the state, says a National Weather Service senior hydrologist.

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Above-normal rainfall is not forecast for Hawaii until early next year, according to the Climate Prediction Center of the National Oceanic and Atmospheric Administration, said Kevin Kodama.

Meanwhile, more than 75 percent of the state is suffering from worse drought conditions than last year, with serious impacts on agriculture and some water restrictions, he said.

Maui and the Big Island have been hit the hardest with widespread agricultural damage and some livestock deaths, he said. The worst conditions are in the Big Island's Kohala area, Waimanalo on Oahu and West Molokai, he said.

Rainfall is about 50 percent below normal in many parts of Oahu, he said. A mandatory 30 percent water restriction was imposed on irrigation in Waimanalo and the Board of Water Supply has asked for a voluntary 10 percent cutback in water usage.

In Hilo, the “;rain city,”; zeros are coming up every day on rain gauges, Kodama noted. “;It's very unusual in tradewind weather.”;

He provided the drought update during yesterday's “;wet season”; briefing on flash floods, which are the major cause of direct weather-related deaths in Hawaii.

The drought season normally ends in September with October and November racking up the most flash floods, Kodama said. November has been Hawaii's worst storm month over the years.

Flash flooding occurs most often from October through April, he said.

He offered a little bit of history: Hawaii had 461 flash flood events in 48 years, with 72 in November and 64 in October. March had 67, with the number inflated by the heavy rain of March 2006.

Notable events included:

» February-April 2006, with 28 floods statewide during an extended wet period, seven deaths and $50 million in property damage.

» October 2004, when 8.70 inches of rain fell in five hours in Manoa, causing $100 million worth of damage, mostly at the University of Hawaii.

» November 2000, when 37 inches of rain drenched the Big Island's southeast and east sides in 24 hours (22 inches in six hours). Damage was $70 million.

» December 1987 and January 1988, remembered as the “;New Year's flood,”; with 22 inches saturating East Oahu in 24 hours, causing $34 million in damage.

A flash flood is defined by the weather service as “;a flood caused by water which rises rapidly to inundation within six hours of the causative event.”;

In Hawaii, Kodama said, there are two main types of flash floods.

The first is a so-called “;wall of water,”; such as when Ka Loko Reservoir Dam breached on Kauai's North Shore March 14, 2006, sweeping away homes and families.

Hawaii has more than 120 dams, mostly 70 to 90 years old and built for agriculture, Kodama said. A flash-flood warning is issued if a dam break is imminent or has occurred but “;there is probably no lead time”; for it, he said.

But the second, and most common, cause of flash floods in the state is rapid stream rise — several feet in 30 minutes or less.

The weather service issues a flash flood “;watch”; when conditions pose a threat of flooding. A warning is issued when dangerous conditions are imminent or occurring. Residents should take preparatory action when a watch is issued and immediate action if there is a warning.