'Atmospheric Rivers' Bring Snow, Rain, Flood Threat to California | NBC Southern California

'Atmospheric Rivers' Bring Snow, Rain, Flood Threat to California

More rainfall is expected this weekend in Southern California, but the storm's biggest impact will likely be in Northern California

Rain caused crashes across Southern California. Tony Shin reports for the NBC4 News on Thursday, Jan. 5, 2017. (Published Thursday, Jan. 5, 2017)

The first of two atmospheric rivers, a long stretch of moisture that builds up from the tropics, brought rain to Southern California, snow to the Sierra Nevada region and the threat of flooding in Yosemite National Park and other parts of the state.

The express train of wet and snowy conditions slammed the state Thursday morning, and another system is expected this weekend and possibly into next week. That should improve the state's snowpack, which provides about a third of California's water in normal years for drinking, farming and wildlife when it melts in warm, dry months and flows into state reservoirs. 

"This is that long stretch of tropical moisture, so we tend to see mostly rain in Southern California," said NBC4 forecaster Crystal Egger. "But it is great news for our snowpack in the state. We've already had several feet and more on the way across the high Sierra. Then next week, it keeps snowing. We're in a very active pattern." 

The first manual survey of the snowpack this year revealed Tuesday that it holds about half as much water as normal, casting a shadow on the state that's hoping for an end to five straight years of drought. Four to five feet of snow is expected through Thursday in areas above 4,500 feet in Northern and Central California, while mountain areas below that could get two to three feet.

Snow isn't likely in the Southern California mountains due to warmer temperatures.

The Mammoth Mountain ski resort in the Eastern Sierra about 300 miles north of Los Angeles said Thursday it has received 40 inches to 7 feet of snow in a day and a half. The resort said snow is expected to continue throughout the day and the powerful storm may put chairlifts on hold.

But with the persistent rain comes the threat of flooding. Yosemite National Park could close because of flooding over the next several days and through the weekend. Heavy rain is expected in the park, pushing the Merced River well above flood stage.

The park flooded in January 1997, which caused extensive damage to park roads, campgrounds, lodging, and utilities. The park was closed for two months due to extensive damage to the park's infrastructure.

During the closure, there was no running water and electricity was intermittent. Since the 1997 flood, the park has made significant improvements to park roads and facilities.

Park officials continue to monitor the weather forecast and will make decisions in the next day or two based on the forecast, and the ability of the park to safely accommodate visitors and employees.

Flooding is possible in much of Northern California and the Sierra Nevada. Flash flood watches were issued for most of the Sierra along the Nevada-California line, where the National Weather Service said the potential for another 6 to 12 inches of rain would create serious flooding concerns into the weekend.

Light to moderate rain moved into Southern California overnight, marking another round of showers after Los Angeles' wettest month in six years. The most significant rainfall was overnight and early Thursday during the morning drive.

Crashes were reported on the 5 Freeway in Burbank, where at least two lanes were closed. There were no immediate reports of serious injuries.

In Boyle Heights, a big rig crashed onto an embankment on the eastboud 60 Freeway. The CHP said the big rig collided with a small car and rain was likely a factor in the crash.

In Redondo Beach, police advised drivers to avoid Ford Avenue at Dixon Street until at least 11:30 a.m. due to a sinkhole.

Skies should clear by the afternoon.

What are atmospheric rivers? Here's some of the science behind this weather event that can impact California.
Photo credit: NOAA

Friday looks to be dry before the next potentially much wetter atmospheric river takes aim on Central California Saturday afternoon through Monday. That system also is expected to deliver rain Sunday, possibly during the Golden Globes Awards ceremony in Beverly Hills, and into Monday in Southern California.

Atmospheric rivers are long plumes of moisture from the tropics or subtropics that move into higher latitudes. They're capable of unleashing a firehose on the state and creating hazardous conditions, including flooding and landslides, due to such a large amount of rain over a short period of time. 

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