Intense weather hit West Coast states last week, feeding a surge of moisture into northern California, Oregon and Washington for three days. A pair of unusual meteorological events called "bomb cyclones" caused at least two deaths, flash floods, rockslides and extensive damage. Trouble began with what's known as an atmospheric river – a large plume of moisture flowing onshore from the Pacific Ocean. It unleashed howling winds that toppled trees and power lines, leaving over 600,000 customers without power in Washington state – the largest outage there since 2006. Some weren’t expected to have electricity until Monday morning.

Heavy rain was held in place by two bomb cyclones, which are swirling storm systems that form over land and oceans when cold and warm air masses clash. (Hurricanes, by contrast, form only over oceans.) The storms also get energy from the jet stream, a band of strong wind in the upper atmosphere.

From San Francisco to Seattle last week, Amtrak cancelled trains and airlines delayed or dropped hundreds of flights. Blizzard conditions shut over 50 miles of an interstate highway in northern California. Seven inches of rain fell in one day on Santa Rosa near San Francisco, which typically gets just over three inches in all of November. An overflowing creek stranded about 150 people at a health clinic and Hampton Inn next door until fire crews rescued them.

Climate change contributes to bomb cyclones and other severe storms, meteorologists say. "Oceans are warming very steadily because of our increased emissions of greenhouse gases into the atmosphere," explains Jennifer Francis, a senior scientist at the Woodwell Climate Research Center in Falmouth, Mass. "That extra warming in the oceans is increasing that contrast between that very cold arctic air coming down and the warm, moist air that’s feeding the air mass." Another factor is that warmer air and oceans increase evaporation, adding moisture to the atmosphere, she added.

Expert says: "We're seeing a major uptick in the frequency of heavy precipitation events, and this is certainly connected to this increase in water vapor that's in the atmosphere as a result of the warming oceans and air." -- Jennifer Francis, Woodwell Climate Research Center

Utility says: "The substantial amount of damage we are encountering from the hurricane-force winds of the storm has made our work slower than expected." – Puget Sound Energy in Bellevue, Wash.

Meteorologist says: "If you picture the atmospheric river as a fire hose pointed at the coast, these frontal waves are essentially the fireman taking his hands off the fire hose and letting it go all wavy." -- Chad Hecht of the Center for Western Weather and Water Extremes at the University of California-San Diego