Heavy rain, flash flooding forecast for Southern California and Arizona Friday

Heavy rainfall will continue to drench Southern California through Friday, with flash flooding expected in both the Los Angeles and San Diego metro areas, according to the National Weather Service. The rain will move into Arizona throughout the day, and could also cause flooding in the Phoenix area, forecasters said.

"This storm will deliver a month's worth of rain in about 24 hours to portions of Southern California and Arizona," AccuWeather meteorologist Bernie Rayno said. Some mountainous areas near Los Angeles could pick up as much as 20 inches of rain by the time the storm winds down, AccuWeather said.

In all, 26 million people in California and the Desert Southwest live where a flood watch is in effect as of Friday morning.

The rain will lead to delays for both land and air travelers, the weather service said. "Impacts to air travel can be expected across much of the western U.S., with significant delays possible."

SoCal storms could lead to mudslides, debris flows

In Southern California, rain was expected to fall across the region's coastal ranges Friday, with the possibility of urban flash flooding, including areas of the Los Angeles Basin. The weather service said mudslides and debris flows are possible in areas that have been burned by wildfires, as well as areas of steep terrain.

The region saw isolated thunderstorms with a few lightning strikes on Thursday night due to the El Niño-influenced storm. Ventura County cities Oxnard and Port Hueneme issued evacuation orders for some coastal areas with at least one shelter opened for fleeing residents.

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Arizona to see flooding

Heavy to excessive rainfall impacting Southern California will head toward central and southern Arizona Friday through early Saturday, the weather service forecasts. These regions could also see isolated to scattered areas of flash flooding.

"Some of the local slot canyons and burn scars closer to areas of steep terrain, and also the dry washes/arroyos over the deserts, will be vulnerable to seeing these runoff impacts. However, generally much of the rainfall across these areas should be beneficial," the weather service said.

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Rain and snow will hit the Pacific Northwest

To the north, a cold front will move across the Pacific Northwest and Intermountain West through Friday and Saturday, according to the weather service. The regions will see rainfall at lower elevations but snowfall at higher up.

The Washington Cascades should see the heaviest snowfall, but snow will also fall over the northern and central Rockies.

Colorado's San Juan mountains will see snowfall amounts of 1 to 2 feet through Saturday due to the related storm system.

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This article originally appeared on USA TODAY: Southern California weather forecast: Rain, flash flooding risk remain