Rainfall Totals Up to 6 1/2 Inches Predicted : Storms: Forecasters say back-to-back weather systems will drench the Southland through Thursday. Flooding possible.

Light showers began falling here Tuesday night as the first of two subtropical storms started moving inland over already sodden Southern California, bringing the promise of heavy rain by tonight and the threat of more flooding and mudslides.

Forecasters for the National Weather Service and WeatherData Inc. said that as much as 6 1/2 inches of rain could fall by Thursday night on foothill slopes that last month’s downpours have left especially susceptible to slides and flooding.

The weather service also intensified its winter storm watch advisory, saying that more than two feet of snow could fall in the San Gabriel and San Bernardino mountains before the weekend.

Dean Jones, a meteorologist with WeatherData, said the first storm will be moderate in terms of moisture and intensity, with about half an inch of rain or less expected in the Los Angeles area, and perhaps twice that much in the foothills and mountains, before the system starts to move to the east sometime this afternoon.

“That first storm is just a tease, just a taste of what’s coming,” Dean said. “The second storm, the one that’s right behind it, has a lot more moisture and it’s a lot stronger.

“By midday Thursday, there could be another four inches of rain along the coast, with up to six inches in the foothills,” he said. “Because of the saturated ground, the potential for flooding, rockslides and mudslides is very high.”

Jones said the main force of the second storm should move out by Thursday night, leaving mostly cloudy weather and scattered showers on Friday. After a brief break in the stormy weather on Saturday, he said, there could be more rain on Sunday from an Arctic system expected to head south along the coast.

Los Angeles officials ordered the evacuation of three small apartment buildings in the El Sereno district at the base of a rain-soaked hillside that appeared about to give way on Tuesday.

David Hsu, an engineer with the city Department of Building and Safety, said a six-inch crack that developed in the hillside last week above the buildings on Waldo Court had widened to about two feet by Tuesday morning, “and that’s good evidence of some moving going on.”

The number of tenants evacuated was not immediately available.

In Orange County, Anaheim city officials were worried about what may happen tonight and Thursday morning if a predicted three to five inches of rain hits the Anaheim Hills, where 46 homes already have been undermined by mudslides.

“To quote one of the geologists, when you put that much water into this hillside, the results probably are not going to be good,” said Bret Colson, a spokesman for the city. “We don’t know what (the effect) will be, but it’s not going to be for the better.”

About 150 residents of a 25-acre area of Anaheim Hills have been ordered from their homes since the ground began slipping during last month’s heavy rains.

In Agoura Hills, residents were concerned about additional damage where a sliding hillside has forced the evacuation of two homes. And in the hilly neighborhoods of the northeast San Fernando Valley, residents said they were bracing for the storms.

The Sunland-Tujunga area has not seen flooding so far this year, but in the past, it has been hit hard. “Usually, all hell breaks loose here,” said J. Sylvia Gross, chairwoman of the board of the Sunland-Tujunga Assn. “It could still happen.”

In Sylmar, which also has been the scene of serious flooding in years past, firefighters said residents were “about as well-prepared for flooding as we’ve ever seen.”

“We’ve given out thousands of sandbags in the past few weeks,” said Capt. William Sommers at City Fire Station 96. “I think everyone who wants them has them.”

To the north, up to four feet of snow was predicted for the High Sierra by the weekend. The California office of emergency services has warned of avalanches, and Caltrans said that major delays could be expected on most mountain roads, including the two most heavily traveled routes across the Sierra--Interstate 80 and U.S. 50.

The Sierra snowpack, which is the principal source of water for urban Californians, is well above normal for the date, pushing officials ever closer to declaring an end to the state’s prolonged drought.

Although sprinkles fell in foothill communities shortly after noon on Tuesday, there was no measurable rain at the Los Angeles Civic Center by 3 p.m. The season’s rain total stood at 19.82 inches, more than double the normal total for the date of 9.6 inches.

Times staff writer Jim Herron Zamora contributed to this story.