The biggest storm to strike the Southland this rainy season is something of a meteorological double-whammy.

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The strong Eastern Pacific weather system — greatly bloated by an “athmospheric river” consisting of a subtropical plume of moisture — produced showers in Los Angeles County beginning Tuesday afternoon but won’t generate heavy rain in the county until late tonight, forecasters said.

The event is likely to produce 1 to 3 inches of rain in L.A. County Coastal and valley areas and between 2 and 5 inches in the mountains, said National Weather Service meteorologist Curt Kaplan, adding that rainfall expectations for L.A. County have been scaled down slightly since yesterday.

Voluntary and mandatory evacuation orders will go into effect in portions of Los Angeles County today amid fears of mud slides and torrents of debris down fire-denuded slopes.

A flash flood watch will be in effect from tonight through late Thursday night not only in burn areas of Los Angeles County but also in urban areas. It will be in effect in the San Gabriel and Santa Monica mountains; the San Gabriel, San Fernando and Santa Clarita Valleys; Los Angeles, including the coast, metropolitan Los Angeles, downtown, and the Hollywood Hills.

“In addition to the flash flooding and mud and debris flow risk in recent burn areas, there will be other flooding threats in non-burn areas due to the long duration and intensity of this storm,” according to an NWS statement. “Widespread urban roadway flooding is possible as well as rockslides and mudslides, especially near canyon roadways. As a result, there could be significant travel delays and road closures across the region between Tuesday and Thursday night.”

The rain likely will stop late Thursday or early Friday, according to the NWS.

The NWS said rainfall rates up to six-tenths of an inch per hour are possible late tonight, possibly increasing to three-quarters of an inch per hour or higher at times Thursday. “Isolated rainfall rates as high as one inch per hour cannot be ruled out,” it added.

“Rainfall of this intensity can produce dangerous mud and debris flows near recent burn areas,” forecasters said. “Flash flooding is a very dangerous situation.

“Southern California residents in or below the recently burned areas are urged to take the steps necessary to protect their property. Persons in the watch area should remain alert and follow directions of emergency preparedness officials,” it said.

The NWS forecast rain in L.A. County today and relatively mild temperatures, though they are expected to go down slightly amid more rain Thursday and remain in the 60s through at least next Tuesday, including on days forecast to be sunny.

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