Santa Monicas Gave a Lift to Storm’s Fury : Meteorology: Chain of mountains forced clouds to dump their moisture in the same way that rising heat can trigger summer flash floods.

The storm that hit with rare vengeance in some areas of Southern California this week while giving only a pleasant soaking in others was produced by a churning tropical weather system that got a boost from the coastal Santa Monica Mountains.

The storm would have dropped its rain anyway, but not so quickly and not in such isolated deluges, had it not come ashore over the mountains. They were the chief reason that the western San Fernando Valley got much more rain than other areas, leading the Los Angeles River to overflow its channel.

Fronting the sea from Santa Monica into Ventura County, the mountain chain sent more moisture spiraling up into the storm above, overloading a system that was already primed for action.

“Everything was just right,” said meteorologist Rea Strange of Pacific Weather Analysis in Montecito. “It (the storm) exploded and then it just died.”

In Monday’s sudden downpour, the community of Woodland Hills--hard against the north slope of the mountains--received 6.14 inches of rain. Other Valley areas measured four or more inches, while a few miles to the south, Los Angeles International Airport recorded less than a quarter-inch of rain from the same storm.

The powerful storm resulted from the confluence of two jet streams that pushed a warm, wet Pacific system over the coast.

The stage was set last week when a huge low pressure area in the upper atmosphere “parked itself” off the West Coast, said Rick Dittman, a meteorologist with WeatherData, of Wichita, Kan. The normal jet stream, which is like a river of air about 20,000 feet above the ground, bore down out of the Gulf of Alaska, creating “little disturbances that were spinning around” inside the low pressure area, he said.

Meanwhile, a southern branch of the jet stream moved into the area, and the two streams were converging as they neared the coast of California. On Monday, the real trouble began when the southern branch “took a flat track” toward Malibu, Dittman said.

The southern stream pumped tropical moisture into the storm that was building in the low pressure area, and the two streams carried the system toward the coast like a leaf floating down a river.

As the storm approached land, surface winds blasting in from the sea hit the Santa Monica Mountains and were deflected upward.

The rising air cooled, forming still more clouds and adding fury to the arriving storm. The mountains acted as a triggering mechanism, forcing the clouds to begin dumping their moisture.

“The mountains played a vital role,” Dittman said. “Without the mountains, you wouldn’t get such tremendous rain.”

Dan Cayan, a meteorologist with the Scripps Institution of Oceanography in San Diego, said the effect was similar to summer flash flooding in the desert, except in those storms heat instead of simple topography rules the day.

Flash floods occur when a mass of cold air in the upper atmosphere moves over the desert, allowing the warmer air near the surface to rise.

“It can’t all rise, so it usually rises in certain places and descends in other places,” Cayan said. The rain forms in areas where the air rises quite rapidly.

In the desert, heat in the form of “thermal lifts” carries the air skyward. The lifting effect provided this week by the Santa Monica Mountains was more mechanical than thermal, Dittman said.

When surface winds blow in off the sea, they hit the mountains that block the air, he said. Wind can either go around an obstruction or up, and in the case of this week’s storm the wind was deflected up. That provided a lifting mechanism that carried more moisture--the fuel for the storm--into the churning system.

The overloaded storm dumped its rain Monday in a fury that lasted only a few hours and then weakened into a more manageable but still strong shower as it moved across the Los Angeles area.

Flooding in the Sepulveda Dam Basin and other problems left in the storm’s wake did not occur because of the volume of rain, but because it was dumped so quickly.

“It’s not the total amount, but the rate,” Cayan said. “It just came out so fast it couldn’t drain away.”

Why It Rains / Where It Rains

A strong storm system moving over the coast of Southern California Monday flooded some areas and spared others partly because the storm was intensified by winds that carried moisture up from the Santa Monica Mountains.

1. Clouds rich in moisture had already been pushed over the California coast before the worst of the storm hit.

2. Surface winds swept over the beach, hit the coastal mountains and were deflected up.

3. The air cooled as it rose, creating even more clouds and probably causing parts of the overhead system to swirl.

4. The upwelling greatly enriched the moisture content of the storm system, causing heavy rainfall along the coast and in the San Fernando Valley.