A winter storm expected to reach the Islands tomorrow will shower the state with periods of heavy rainfall throughout the weekend, posing a serious threat for flooding, especially on O'ahu and Kaua'i.

The National Weather Service today extended a statewide flash-flood watch from tomorrow night through 4 a.m. Sunday. "There's a threat for serious flooding across the Islands," National Weather Service forecaster Chris Brenchley said.

In Honolulu, the city's Department of Emergency Management today urged residents to exercise extreme caution.

"This looks to be the biggest rain producer of the winter season to date and we need to be prepared and aware of the hazards that floods and heavy rains can create," Mayor Mufi Hannemann said in a news release.

Emergency Management director Mel Kaku added: "Based on information we received from the National Weather Service this afternoon an area of unstable weather will develop off of Kaua'i creating an intense period of heavy rains and possible flooding. The weather service modeling of these conditions shows that O'ahu and Kaua'i are in the primary threat and hazard area. We may begin experiencing the effects of this storm system beginning Friday morning with the heaviest showers and increased potential for flooding on Saturday night through Sunday morning."

Residents on O'ahu's windward coast could be impacted by storm conditions very similar to those seen during the 2006 March floods to include road closures, flooding and minor landslides with the difference being a much shorter duration of activity than the 2006 storm, Kaku said.

"If you have experienced flood conditions in the past we highly recommend you consider some basic protective actions such as acquiring sandbags or plastic tarps," observed Peter Hirai, plans and operations officer for the Department of Emergency Management.

Southeasterly winds are bringing in a moist air mass from the deep tropics, according to the weather service. At the same time, a cold front is moving in and together they will destabilize the atmosphere, leading to periods of heavy rain.

Brenchley observed the lack of winds will also be a factor this weekend.