From NPR.

A major storm is sweeping across the northwest U.S., battering the region with strong winds and rain, and causing power outages and downed trees that killed at least two people.

The worst part of the powerful storm system, known as a "bomb cyclone," will occur Wednesday through Friday as parts of the northern California coast are at high risk of excessive rainfall, with areas receiving 10-15 inches in a 48-hour period.

A "bombogenesis" or what is commonly known as a "bomb cyclone," is a midlatitude (between the tropics and polar regions) cyclone that rapidly intensifies over a 24-hour period and when atmospheric pressure drops significantly, by at least 24 millibars, according to the National Ocean Service. This occurs when cold and warm air masses collide.