Second Storm Is Expected to Hit Southland Today : Weather: Heavy rain is likely, but partly cloudy skies are predicted by New Year’s Day.

The first of two year-end storms poured almost half an inch of rain on Los Angeles by Monday afternoon, and forecasters said the second should bring heavy rain and snow throughout Southern California today.

More precipitation is predicted for later in the week, but the forecasters said the rain should stop before Friday morning, with partly cloudy skies and cool temperatures expected for the New Year’s Day Rose Parade.

Rain and snowfall were heavy in the northern part of the state on Monday, and officials said the Sierra snowpack, which provides most of the water used by urban Californians, apparently was up to normal depths for the date. It was the first time the snowpack has been at or above normal this late in the year since the state’s prolonged drought began in 1987.

The inclement weather in the region now is the product of two distinct but interrelated storm systems, said Rick Dittmann, a meteorologist with WeatherData Inc., which provides weather information to The Times.

The first storm--a vast system of subtropical air from the central Pacific--swept in from the Southwest during the weekend. The second--a smaller but more intense cold front from the Gulf of Alaska--headed south through the state on Monday, drawing up moisture from the first system as the two merged over Southern California’s northern mountains Monday night.

“There’s a complex interaction between the two,” Dittmann said. “That makes it hard to say how much precipitation will fall, and exactly when.”

Precipitation from the first storm began falling here Sunday afternoon. By nightfall, roads and freeways in Southern California were dangerously slippery, tangling traffic for motorists headed home after the three-day holiday weekend.

Fifteen people were injured, one of them critically, when a van skidded out of control on Interstate 5 near Magic Mountain on Sunday night, setting off a collision involving five vehicles.

The rain continued off and on throughout the night, and despite lighter traffic than usual, Monday morning’s commute was a mess.

“There were a lot of spin-outs and minor collisions because people were driving too fast,” said Officer William Preciado of the California Highway Patrol.

The main force of the first storm had moved out to the east by noon Monday, but the more powerful and blustery second storm already was pummeling the northern half of the state and heading south.

Heavy rain fell in the Sacramento Valley all day on Monday, and forecasters predicted up to three feet of new snow during the night in the High Sierra.

Resort operators said that despite winds gusting up to 80 m.p.h. atop some of the highest runs, hundreds of skiers still flocked to the slopes around Lake Tahoe.

The heavy snow forced motorists to use tire chains on Interstate 80 and U.S. 50 over the summit of the Sierra. The CHP said a series of minor accidents on Interstate 80 was delaying traffic up to four hours in both directions in the Truckee area Monday afternoon.

Officials watching California’s prolonged drought were encouraged by the addition to the Sierra snowpack, which appeared to be at or above normal depths, although accurate measurements cannot be made until the storm passes.

“It’s looking much better,” Dee Davis, a spokesman for the state’s Drought Information Center, said Monday afternoon. “It’s been a long time since we’ve seen a storm like this. We expect heavy snow through Tuesday and scattered snow showers for the rest of the week.” The National Weather Service issued a winter storm warning for the mountains around Los Angeles on Monday afternoon as the powerful second storm began to move in. Snow started falling in the Tehachapi Mountains at nightfall, with winds up to 60 m.p.h. expected in some of the higher mountain mountain passes during the night.

Forecasters said as much as a foot of new snow could fall along the northern peaks of the San Gabriel and San Bernardino mountains by this afternoon, with six to eight inches along the southern ranges above 6,000 feet.

Dittmann said the rainfall will vary widely in the Los Angeles Basin, with as much as an inch expected in some areas.

“There will be showers on and off through Wednesday, with things starting to dry out on Thursday,” Dittmann said. “Right now, the Rose Parade looks pretty safe.”

A daily total of 0.46 of an inch of rain had fallen at the Los Angeles Civic Center by 4 p.m. Monday, raising the season’s total there to 3.73 inches. The normal season’s total for the date is 4.16 inches.

Other daily rainfall totals as of 4 p.m. Monday included 0.84 of an inch in San Bernardino, 0.76 in Monrovia, 0.61 in Torrance, 0.59 in Culver City, 0.53 in Woodland Hills and 0.49 in Anaheim.