Residents are urged to use “;extreme caution”; as heavy rain, flooding and flash flooding could hit parts of the state through the weekend, authorities warned.

Oahu and Kauai were expected to be the main areas affected today, said National Weather Service forecaster Maureen Ballard, “;but something could still pop up over Maui County and the Big Island.”;

A flood watch is in effect until tomorrow night.

Heavy rain and thunderstorms were brewing offshore and were expected to come ashore as the day wore on and over the weekend, with some areas getting more rain than others, Ballard said, joining the city Department of Emergency Management in urging residents to be prepared for possible flooding.

The city said Oahu's Windward coast could experience storm conditions resembling those of a March 2006 storm — except for duration — that caused road closures, flooding and minor landslides.

In 2006 it rained for more than 40 days. Rainwater breached Kauai's Ka Loko Dam, killing seven people. On Oahu it led to a massive sewage spill in Waikiki that sent millions of gallons of raw sewage into the Ala Wai Canal.

While urging residents to use extreme caution, the city Department of Emergency Management also recommended that people who have experienced flood conditions in the past acquire sandbags and plastic tarps.

 

  Weather forecast

 

  Residents get ready for what could be major flooding. 

 

[  Watch  ]

 

 

 

 

The brewing storm was caused by the unusual mixing of two common events: a cold front to the west of Kauai and a southeasterly stream of air bringing moisture over the state.

The two conditions together create an unstable atmosphere.

It is like “;putting water on top of oil in a container,”; said Vladimir Ryshko, a meteorologist at the National Weather Service. “;Same thing happens.”;

A flash flood watch means conditions might develop that lead to flash flooding, which could be deadly. But forecasters do not know how much precipitation might fall.

“;Flooding rain is possible, but we can't really pin things down,”; Ryshko said. “;The potential exists for heavy rain and damaging flooding.”;

He said narrow canyons and deep gorges would be dangerous places.