The first storm's impact will be felt with heavy rain starting tonight, especially from the Pacific Northwest coast to the northern part of the state, the NWS office in San Francisco said.

The second will be a “powerful low pressure system” that will deliver another round of rain across the state Sunday night into Monday morning, he said.

“The West Coast will once again be the center of inclement weather for the next few days as low pressure systems from the Pacific make landfall,” the NWS explained.

According to the forecast, “heavy rain” warnings are in place for much of Northern California and the state's central coast this Sunday and Monday. There will also be a risk of moderate rain in Santa Barbara and Santa Monica during those days.

“The heaviest snow will hit the Sierra Nevada on Monday, where a few feet of new snow can be expected,” the NWS said.

More than 27 million people are under flood warnings in the northern part of the state and parts of Los Angeles due to the severe weather forecast, according to CNN.

“Soils around the Bay Area are close to saturation, so rain can flow quickly and cause numerous landslides,” the NWS added.

They occur worldwide, but are particularly strong on the West Coast of the United States, where they make up 30% to 50% of annual precipitation and are essential to water supplies. However, according to NOAA, they can also cause storms that lead to flooding and landslides. Formed by winds associated with hurricanes, 'atmospheric rivers' are typically 250 to 375 miles wide and move under the influence of other weather conditions.

Many atmospheric river phenomena are weak. But the most powerful ones can carry extraordinary amounts of moisture. According to the US Geological Survey, studies show they can carry seven to 15 times the average amount of water discharged daily by the Mississippi River.

According to the Scripps Institution of Oceanography's Meteorological Center, at least 46 'atmospheric rivers' have been reported along the US West Coast in the 2023 hydrological year. Nine are classified as strong, two as severe and one as exceptional. California experienced major flooding and massive snowfall.

According to NOAA, when moisture-rich air moves over mountain ranges like the Sierra Nevada along the California-Nevada line, the vapor rises and cools, turning into heavy rain that falls as rain or snow.

While traditional cold winter storms from the North Pacific produce snow in the Sierra, the atmospheric rivers are warm. Snow still falls at higher elevations, but rain usually falls on top of the snow at lower elevations. This can cause rapid melting, runoff and flooding, and reduce the amount of snowfall needed for California's water supply.