First Storm of the Season Is Expected : Weather: A half-inch of moisture is predicted. Organizers of a valleywide graffiti cleanup led by the mayor say the event will take place, rain or shine.

The first substantial storm of the season began moving into Southern California on Friday night, bringing predictions of half an inch of rain before the system moves out Sunday.

Rain should start falling here before dawn today, said James McCutcheon, a meteorologist with WeatherData Inc., which provides forecasts for The Times. Widespread showers should continue in the Los Angeles Basin throughout the day.

But organizers vowed that a valleywide graffiti cleanup led by Mayor Richard Riordan, City Councilwoman Laura Chick and Los Angeles Deputy Police Chief Mark A. Kroeker will take place today, rain or shine.

Operation Sparkle cleanups are set for Encino, Van Nuys, North Hollywood, Pacoima, Northridge and Reseda.

Riordan, Chick and Kroeker are scheduled to participate in the Franklin Field cleanup in Encino this morning.

“They will all be there even if we’re standing under umbrellas,” said Kevin Koga, a spokesman for Kaiser Permanente Hospital in Woodland Hills, which is co-sponsoring the event.

Likewise, a two-day youth art festival scheduled at Peter Strauss Ranch in the Santa Monica Mountains National Recreation Area also will be held regardless of the rain, park officials said.

Forecasters said about half an inch should fall in the Los Angeles area, with perhaps twice that much in the foothills and mountains, before the storm moves east sometime Sunday afternoon.

It’s expected to be a relatively warm storm system, born over the mid-Pacific near Hawaii, so there won’t be the sort of mountain snowfalls associated with the chilly systems that often arrive later in the fall from the Gulf of Alaska.

McCutcheon said a large stream of subtropical moisture was being pulled into the Southland by winds circulating counter-clockwise around another, smaller storm system to the north.

The condition is known as the “pineapple connection,” in which tropical moisture is carried from Hawaii by low-pressure systems.

“We are expecting rain, not just coastal drizzle,” McCutcheon said. “There is a good chance of rain or light showers for most of the Southland for the next 24 hours with a clearing trend seen by Sunday,” he said.

He said the rain should begin tapering off by tonight, with a few lingering showers through Sunday morning.

Temperatures are expected to remain generally comfortable through the weekend, with overnight lows in the upper 50s and low 60s and highs ranging from the upper 60s along the coast to the mid-70s in the inland valleys.

Forecasters said it will be mostly sunny on Monday, Tuesday and Wednesday, except for some fog and low clouds along the coast in the late night and early morning hours. Temperatures are expected to increase gradually during the early part of next week.

The high temperature at the Los Angeles Civic Center on Friday was 76 degrees, following an overnight low of 64. The rainfall total for the weather season--which runs from July 1 through June 30--stands at .16 of an inch.