A Pineapple Express storm system is thumping California with 10 inches of rain and 70mph wind gusts, leading to 22million people being placed under flood advisories.

All lanes of the 710 Freeway at Pacific Coast Highway in Long Beach have been closed, and numerous streets and highways across the state have become inundated due to the pouring rains on Thursday morning.

Both Los Angeles and San Diego face flooding from the historic storm that pounded northern California a day earlier.  

The National Weather Service office based in Los Angeles warned: 'Start preparing now. Adjust plans. Grab sandbags. Mountain communities, grab extra supplies in case you are stranded.'  

Two atmospheric systems, known as a Pineapple Express, are targeting California this week. The storm comes as the state has already dealt with massive rain and washouts just last week. 

Atmospheric rivers are defined as 'narrow regions in the atmosphere that transport much of the moisture from the tropics to northern latitudes' by the National Oceanic and Atmospheric Administration. 

The weather phenomenon is known as 'Pineapple Express' because it brings warm subtropical moisture across the Pacific from near Hawaii. 

Showers began in Northern California on Wednesday, drenching the area with as much as eight inches of rain, Fox Weather reported.