An approaching winter storm is poised to open an “atmospheric river” of subtropical rain clouds and dump a lot of precipitation onto Southern California beginning Tuesday night, forecasters said Sunday.

And while Monday will be sunny and warm, high clouds will begin to move into the Southland in advance of the anticipated storm system.

The three computer models used by the National Weather Service have different results, with one showing barely any rain. But two others show the subtropical moisture pipelines ” going great guns” over recent burn areas west of Los Angeles, and in the cities of L.A. County themselves.

Very early rainfall estimates for the three days starting Tuesday vary around 1-1/2 to 4 inches for the coast and valleys, and 3-6 inches in the foothills and mountains. Of more concern, peak rainfall rates may hit one inch per hour in some areas, where the mountain slopes that face the south will bear the onslaught of warm rain clouds rising up against the topography, the NWS said in a analysis.

“During the heaviest rains, it will be possible to see three-hour rainfall (of) up to 1 to 2 inches in some areas,” the NWS warned. “This storm will bring the potential for major mud and debris flows on the recent burn areas, including the Thomas, Whittier, La Tuna and Creek burn areas.

“Widespread urban and small stream flooding, and rockslides, are likely at times during this event.”

The storm will move into the L.A. area late Tuesday with a 50 percent chance of rain overnight, according to the NWS. Temperatures will drop to the lower 50 degrees with winds expected to gust around 15 miles per hour. In the San Fernando Valley rainfall was expected to begin after midnight. Santa Clarita has a 70 percent chance of rain beginning Tuesday night. And in Orange County the storm was predicted to hit overnight on Tuesday, according to the NWS.

The rain was expected to continue throughout the day and night until Thursday evening. After that the storm was expected to move east sometime late Thursday or Friday, the NWS predicted.

–City News Service

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