Powerful storms to batter East and West coasts as California faces risk of flash flooding and snow.

wo strong storm systems are expected to batter the East and West coasts simultaneously as California faces a fresh threat of flash flooding and heavy mountain snow.

The arrival of the "separate but potent" low-pressure systems has left 22 million people under winter weather alerts and will see both sides of the country slammed with severe weather. Widespread precipitation linked to a system across the Pacific Northwest is expected to move southward into California, the National Weather Service said.

The system, fueled by an atmospheric river, is forecast to slam into the already storm-battered state Monday across Northern California, and eventually bring heavy rain to central and Southern California on Tuesday. Around 17 million people in California were under flood watches Monday, and southern parts of the state will likely be added to the flood watch list.

Heavy snow is also forecast for higher elevations of the Sierra Nevada, the agency said.

The UC Berkeley Central Sierra Snow Lab, a research institute based in Soda Springs, a census-designated place about 89 miles northeast of Sacramento, tweeted Monday morning that the area had already seen 13.2 inches of snow in the last 24 hours and was expecting up to 30 more through Tuesday.

Rainfall amounts could reach 6 to 8 inches in parts of the Sierra Nevada and Southern California in the coming days. Snowfall amounts will be up to 4 feet above 7,000 feet and 8 feet above 9,000 feet.