Surprise Storm Blows Into L.A. : Weather: The rain was expected to be confined to the mountains and deserts. Rescuers pluck two teen-agers from floodwaters.

Rain was expected Wednesday over mountains and desert areas of Southern California, but shifting wind blew the downpour into the Los Angeles Basin.

A 15-year-old girl and a 15-year-old boy were caught in raging floodwaters in Rosemead, County Fire Department Inspector Barry Nugent said.

Rescuers got the girl quickly, but the boy just missed a line thrown to him and was swept along a wash into the Rio Hondo River. He was pulled from the water downstream in El Monte.

The teen-agers were taken to County-USC Medical Center. Their names and conditions were not immediately released.

The National Weather Service issued a flood advisory for Los Angeles County, warning residents that even brief cloudbursts could flood streets because the ground is saturated from previous storms.

Wednesday’s widely scattered storms arrived to the accompaniment of thunder and lightning that briefly interrupted radio and television signals over a broad swath of Southern California. Small hail was reported in El Toro at noon and heavy snow was reported at the 6,000-foot level in Big Bear, the weather service said.

The rains are coming from the same upper-level low-pressure system that has caused a forecast of showers for the last week, said Rick Dittmann, a meteorologist with WeatherData Inc., which provides forecasts for The Times.

“The system has slid a little farther south and east,” he said. “As a result, east-northeast winds developed. The showers and thundershowers in the desert were expected, but the winds blew the showers into the Los Angeles Basin.”

The Los Angeles Civic Center received just over 0.2 of an inch of rain by midafternoon Wednesday. More of the same may be in store today, rDittmann said, with about a 30% chance of showers with a few thundershowers mixed in.

“The weekend looks dry, but not tremendously warm,” Dittmann said. High temperatures will range from the mid-60s to the mid-70s along the coast, with inland areas approaching 80, he said.

Jerry McDuffie, meteorologist at the National Weather Service office in Los Angeles, said 20.63 inches of rain has been measured at the Civic Center since last July 1. Normal for this date is 13.42 inches, he said.

A total of 7.12 inches of rain was recorded at the Civic Center last month, almost equal to the 7.96 inches that fell downtown during February, McDuffie said.

He said March is usually “a little unstable. In April, it’s not uncommon at all to have some of these upper lows drop along our coastline and over Southern California. April is what we call a transitional month, going from a winter to a summer pattern.”