Storm Is Expected to Dump Inch of Rainfall : Weather: Despite a nearly monthlong dry spell, homeowners are still dealing with the threat of mudslides in Agoura Hills and Studio City.

The first substantial rain in almost a month is expected to start falling in Southern California this morning as a brisk Arctic storm moves inland from the Pacific, forecasters say, even as homeowners and city officials struggle with slides by sodden earth at both ends of the San Fernando Valley.

Dean Jones, a meteorologist with WeatherData Inc., said as much as an inch of rain could fall in the coastal valleys, with up to 1 1/2 inches in the foothills before the storm heads out to the east Friday afternoon.

It may seem a bit strange: almost 18 inches of rain in Los Angeles between Jan. 1 and Feb. 25, followed by less than a 10th of an inch through the first 3 1/2 weeks of March.

But Jones said there is a fairly simple explanation: fluctuations of the polar jet stream, a high-altitude current of air that flows from west to east across North America.

“In January and February, the stream swung south across the main body of the United States,” he said. “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 now, during which the path of the jet stream probably will be swinging back and forth. It is pretty far south now, and that’s why Southern California will be getting rain. Where it will be in a week--or two weeks--is uncertain.

In Studio City and Agoura Hills, homeowners faced the threat that more rain will loosen already weakened earth, causing mudslides and property damage.

In Agoura Hills, a small landslide caused about 10 feet of earth movement on a small hillside behind one home, but no one had been evacuated and authorities said Wednesday that no structures were in imminent danger.

Curious neighbors, who assembled across the street from the house in the 27300 block of Provident Road, said the earth had been slowly moving down the hillside for more than a week.

The owners of the endangered home declined to comment.

“I’m always nervous when I see this, and I think most people on these hills are nervous,” said neighbor John Lowell, 61.

Lowell and other neighbors said they expected the problem to get worse if rains washed through the hillside behind the house.

In Studio City, city officials warned Wednesday that several hillside homes and a nearby street may suffer more damage in the coming week because of a sliding hill.

Residents of a group of homes were evacuated Tuesday, all but one temporarily, because a 300-foot-wide chunk of hillside is moving behind six homes on Laurie Drive.

David Hsu, a geo-technical engineer with the Los Angeles Department of Building and Safety, said he found a patch of asphalt in the road “heaving up” Wednesday. The rising pavement is located in front of a house at 11258 Laurie Drive, which was ordered evacuated March 14.

“This is a very bad sign,” Hsu said, indicating the earth movement may run deeper than previously thought and that at some point the entire street may become undermined by the slide.

In a “worst-case scenario,” Hsu said, “we will see sometime next week the loss of the street.”

The leading edge of the current storm--which has been working its way slowly south along the coast, dropping heavy rain and snow in some parts of Oregon and Northern California--will probably arrive in the Los Angeles area by dawn today, with rain falling steadily until about noon, forecasters said.

Jones said there should be scattered showers and thundershowers, accompanied by cool, gusty winds, through Friday morning, with the possibility of a few lingering showers Friday afternoon. He said the snow level will drop to about 5,000 feet, with six to eight inches expected in the San Gabriel and San Bernardino mountains.

Saturday and Sunday will probably be cool, breezy and partly cloudy, Jones said, with high temperatures in the Los Angeles Basin in the 50s and low 60s and overnight lows that could dip into the upper 30s in the coldest areas of the San Fernando and San Gabriel valleys.

He said the approach of another storm will bring a good chance of additional rain on Monday.

“If the jet stream stays where it is, there could be more rain later in the week, but it’s still a little early to tell,” Jones said.

Times staff writer Julie Tamaki contributed to this story.