Storm May Drop Inch of Rain on L.A. Basin

The first substantial rain in almost a month should start falling in Southern California this morning as a brisk Arctic storm moves inland from the Pacific, forecasters say.

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 tenth 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.

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--should 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 should 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.

The high temperature at the Los Angeles Civic Center on Wednesday was 69 degrees after an overnight low of 60. The rainfall total for the season was 23.84 inches, compared to a normal season’s total for the date of 13.06 inches.