A bomb cyclone occurs when a storm's pressure drops quickly, which intensifies the storm and ramps up wind gusts. Atmospheric rivers are a "long, narrow region in the atmosphere—like rivers in the sky—that transport most of the water vapor outside of the tropics," according to the National Oceanic and Atmospheric Administration.

A high-end atmospheric river is continuing to pummel northern California on Thursday as it slowly meanders through the region on the tail end of a historic bomb cyclone that developed off the Pacific Northwest.