San Francisco, Nov 19 (IANS): A powerful "bomb cyclone" fueled by a Category 5 atmospheric river is expected to explode across the West Coast of the US this week, threatening to unleash hurricane-force winds, catastrophic flooding, and massive snowfall in the mountains, Weather Nation TV reported.

The intense mid-latitude storm is expected to hit Tuesday through Thursday, potentially affecting millions of Americans from Washington State down to Oregon and Northern California, Xinhua news agency reported.

Some areas along the California coast could see level 4 impacts, which is in the "extreme" category, according to Weather Nation TV.

"This intense storm system is expected to bring major impacts to areas of the Northwest and northern California with heavy rain, strong winds, and big swells," it noted.

Atmospheric rivers are narrow channels in the atmosphere that can carry a large amount of moisture. They form when cold air from the Arctic meets warm, moist air from the tropics, cooling it to form heavy precipitation.

A "bomb cyclone" forms when a cold mass of air collides with a warm mass, intensifying a cyclone, bringing more violent winds and a greater likelihood of coastal flooding.