Some areas already felt the impact Tuesday, with the 15 Freeway near the California-Nevada line shut down overnight because of dangerous conditions, leaving drivers stuck in their cars for hours.
In Big Bear, snow was already falling Wednesday afternoon and people were stocking up on food and supplies to hunker down for days. Drivers were stopping to put on chains as Caltrans prepared to have crews out 24 hours a day to keep the roads clear.
The National Weather Service was forecasting the storm could bring six feet or more of snow to Big Bear and Wrightwood. Mount Baldy could potentially get as much as eight feet over the course of the storm.
In Pasadena, light hail fell from the skies Wednesday. One local teacher filmed students - some of them in shorts and T-shirts - playing outdoors as the small drops of ice fell.
Pasadena students play in hail falling from skies
The National Weather Service has issued a blizzard alert for the mountains of Los Angeles County for the first time since 1989, the agency said.
The warning is scheduled to take effect at 4 a.m. Friday and will last until 4 p.m. Saturday. The NWS predicts from 2 to 5 feet of snow could accumulate in the mountains above 4,000 feet, falling even as heavy winds gust up to 75 mph.
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Below that, at elevations of 2,000 to 4,000 feet, about 6-12 inches of snow are expected.
The last such warning for the area was issued on Feb. 4, 1989.
"Even if this is not our 1st, this is a dangerous storm. Do not travel in the mountains," on Friday and Saturday, the agency said on Twitter.
Visibility at that time is expected to be very low and travel is not advised through those areas.
Snow is expected to fall as low as 1,000 to 1,500 feet, meaning areas like Santa Clarita and lower-lying areas of the Inland Empire will see a rare coat of powder. Lancaster already saw some snow on Wednesday.
Passes like the Grapevine and the Cajon Pass are likely to also see dangerous driving conditions. Drivers are advised to bring chains and a full tank of gas and be prepared for difficult weather and road closures.
"They're expecting snow to drop as low as 1,000 feet," said Mark Bishoff with Caltrans. "The top of the Grapevine is a little over 4,000 feet, so they're expecting it to be impacted by snow."
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Blizzard warning issued for SoCal mountains, with snow also expected at unusually low elevations
"If you can stay home, then stay home. That's the best choice that you can make. If you do have to go, then make sure your car is prepped. Make sure the tires are inflated properly, your windshield (wipers) and your headlights are working."
Bishoff added California law requires headlights to be on when windshield wipers are in use.
He also recommends drivers have a kit with them in their car if they do need to hit the road: a blanket, cellphone, charger, water and snacks.
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The storm front was sweeping down from the Canadian Rockies and the Yukon, with the leading edge arriving late Tuesday. But the brunt of it will start Thursday, with heavy rain at lower elevations. Some communities can expect to see 3-4 inches of rain by the weekend.
Agencies like Caltrans and the CHP were mobilizing Tuesday for expected road closures and preparing to help drivers stuck on mountain roads.
"It's gonna be a safety concern," said Officer Monique Mischeaux with the CHP. "If it's not safe for the motoring public to use those highways, we will shut them down. That's why we want people to prepare in advance."