Updated: June 24, 1:02 p.m.

The Oahu emergency management agency wants residents and tourists to prepare for the possibility of heavy rains and flash floods that could begin as early as Monday afternoon. A flash flood watch remains in effect for Oahu, Kauai and Niihau from 6 p.m. Monday through Wednesday afternoon, according to the National Weather Service.

"The National Weather Service advised us this morning that an upper level low and trough west of the state will bring an increased chance of severe weather, heavy rain and possible thunderstorms, which could begin affecting us as early as this afternoon and lasting through Wednesday," said Hiro Toiya, director of the city's Department of Emergency Management.

Toiya said the island could see heavy rainfall in parts of Oahu not usually prone to flooding, including the 'Ewa plains, Kapolei and the Leeward coast. Floods could prompt evacuations, road closures and possible utility outages, he said.

An upper low moving into the area could bring heavy showers and thunderstorms, the weather service said.

"Showers could become heavy at times, potentially leading to flash flooding," the weather service said. A flash flood watch means conditions are favorable for quick-moving flooding.

The weather service advises motorists and pedestrians against crossing fast-flowing water and to move to higher ground immediately in rising waters.

A flood warning, once issued, means a flooding threat is imminent or occurring and immediate steps should be taken to get to higher ground.

The city says to take such flood precautions as: