Nearly the entire population of California is under weather alerts on Presidents' Day as a multi-day atmospheric river storm dumps heavy rain across lower altitudes and snow in the mountains. State officials have warned residents to stay off the roads as they face the risk of flooding, landslides, and even tornados. 

About 37 million people are under severe weather alerts as California experiences heavy rainfall, with Southern California being the primary area set to see the heaviest downpours. Heavy rains will cause flooding in Santa Barbara and Ventura counties through Monday morning. And the greater Los Angeles metro is expected to receive a round of heavy rain in the afternoon. 

According to the National Weather Service, flash flood warnings were issued Monday morning for Santa Barbara and Ventura counties, where upwards of 2 to 5 inches of rain had already fallen. The foothills are expected to receive the heaviest amount of rain, increasing risks of landslides later today. 

"Unfortunately, all of that rain is along an atmospheric river that is literally like a big, giant fire hose in the Central and parts of Northern California today, tomorrow, and Wednesday," said FOX Weather Meteorologist Michael Estime, adding, "It's one of the reasons why we have a flood threat."

The tornado risk runs high from Chico to Sacramento to Stockton and Modesto. 

Winds will be high across the state. 

So far, power outages are limited. 

In the higher altitudes, snow will burry Sierra Nevada. 

"I mean, we're talking about 1 to as much as maybe 3 feet of snow, especially in the higher elevations," said FOX Weather Meteorologist Kendall Smith.