Rain Makes Return Visit; More Due Saturday : Storms: Errant polar jet stream brings weather. ‘It may be wet into early April,’ forecaster says.

The leading edge of a brisk arctic storm moved inland from the Pacific Ocean on Thursday, bringing Southern California its first substantial rain in almost a month.

Heavy rain began pounding Los Angeles shortly after sundown Thursday and the National Weather Service warned that there could be urban flooding during the night and into this morning. Forecasters said as much as two inches of rain could fall in some parts of the Los Angeles Basin by this afternoon.

Traffic slowed to a crawl during the night as heavy squalls left standing water on several freeways. Officials closed roads leading into the Sepulveda Flood Control Basin as runoff waters began to rise.

The rains will tail off tonight and Saturday, but a new, less powerful storm with could arrive Saturday evening, said Steve Burback, a meteorologist with WeatherData Inc., which provides forecasts for The Times.

“It may be wet into early April,” Burback said. “I don’t see an end in sight.”

The return of the rains results from fluctuations of the polar jet stream, a high-altitude current of air that flows from west to east across North America, forecasters said.

“In January and February, the stream swung south across the

main body of the United States,” said Dean Jones, another WeatherData forecaster. “That pulled storm systems from the Pacific Ocean in across Southern California.”

Then, around March 1, “the jet stream swung farther north,” he said. “High pressure started to build over the West Coast, and that blocked the storms and kept them from moving into Southern California.”

Jones said the region is in an uncertain transition period during which the path of the jet stream probably will be swinging back and forth. It is pretty far south now, and that is why Southern California will be getting rain.

The snow level is down to 5,000 feet, with eight inches or more expected in parts of the San Gabriel and San Bernardino mountains.

As the rains approached Los Angeles, more than 160 county workers spent Thursday sandbagging behind two homes in Agoura Hills.

Residents from about half a dozen homes in Studio City were temporarily evacuated Tuesday when a 300-foot stretch of hillside began moving behind three houses.

At the Los Angeles Civic Center, 0.11 of an inch of rain had fallen by 3 p.m. Thursday. Later readings were not available, but the National Weather Service said that considerably more rain fell there later in the day. As of 3 p.m., the season’s rainfall total for downtown Los Angeles was 23.95 inches, compared to a normal season’s total for the date of 13.13 inches.

Times staff writer Julie Tamaki contributed to this story.