Washington, Mar 14 (IANS): A sudden drop in barometric pressure unleashed a rare "bomb cyclone" over the US state of Colorado, paralysing the region with heavy snow and high winds.

According to the National Weather Service (NWS), as many as 74 million Americans could be affected by the storm, which hit southern Colorado with Category 2 hurricane conditions on Wednesday and 100 mile per hour winds, Xinhua news agency reported.

More than 55 million people are "threatened" by high winds, 10 million by winter storms and over 17 million by floods, with "severe storms capable of producing damaging winds, hail and tornadoes, the NWS said.

Thousands of cars were abandoned along the Colorado roadways as "the bomb cyclone wreaked havoc, triggering innumerable crashes and leaving people stranded in cars", the Denver Post reported.

"This is a very epic cyclone," said Greg Carbin, chief of forecast operations for the National Oceanic and Atmospheric Administration's Weather Prediction Centre. "We're looking at something that will go down in the history books," he remarked.

It could develop into the worst storm of its type in 35-40 years, he added.

Colorado's capital Denver was the first major metropolitan area hit by the storm, where Governor Jared Polis declared a state of emergency and called in the National Guard for help in providing emergency relief.

"A bomb cyclone happens when there's a rapid pressure drop, with a decrease of at least 24 millibars (which measures atmospheric pressure) over 24 hours known as 'bombogenesis'," the Weather Channel said.

"This storm has dropped 33 millibars since Tuesday morning and continues to strengthen," it added.