The first winter storm of the season is expected to dump rain throughout the Islands until the weekend and may produce floods, a small-craft advisory and even snow.

The National Weather Service warned yesterday that an evolving system had the potential to bring heavy rain and thunderstorms.

A flash flood watch is in effect until late tomorrow night, although the National Weather Service last night canceled flood advisories — a higher level of alert — for Hawaii and Oahu counties. The flood advisory for Maui County was kept in effect until at least 1:15 a.m.

A disturbance in the upper atmosphere northeast of the Islands will move southwest, positioning itself just west of Kauai, said Tom Birchard, lead forecaster. It will remain there from tomorrow until it weakens, he said.

"We're looking at a significant event ... expecting potential for heavy showers and thunderstorms ... on and off into the weekend," Birchard said. "I'm not committed to an amount but I wouldn't be surprised if certain areas saw 5, 10, 20 inches of rain over the course of several days."

Birchard added that winter weather is also predicted for the summits and upper elevations, with a possibility of snow on the Big Island peaks.

Yesterday, Big Island and Oahu county workers were preparing for the possibility of flooding, but city spokesman Bill Brennan said crews work throughout the year to keep streams and drainage canals free of debris.

"We don't wait until a storm is at our doorstep," Brennan said in an e-mail. "They have also been checking the city's storm drain systems and clearing those systems as needed, and with the heavy showers forecast, city crews are now double-checking."

Yesterday, crews were clearing ditches at Lunalilo Home Road, Pacific Palisades and Dole Road, Brennan said. Sand berms were lowered at outlets for Niu and Ulehawa streams as well as outlets from Kaaawa to Läie. Storm drains were cleared or checked in Kahaluu, Käneohe, Waialua and Waianae, he said.