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Storms to make West Coast Christmas travel tricky

Temperatures are expected to rise to record-breaking levels east of the Mississippi this week, but snow will keep falling along the West Coast, making for tricky travel for some on Christmas Eve.

Winter storms dumped snow in Washington, while heavy winds downed trees, knocked out power and smashed cars in Oregon, and rain brought further relief to a drought-stricken California.

Both snow and rain have been falling heavily in the Pacific Northwest and in Northern California, due to a surge of moisture and warm air coming in from the Pacific. Later this week, the pattern is likely to turn a bit colder, meaning more snow will fall through Christmas Eve, Andrea Bair a meteorologist with the National Weather Service's Western Region Operations Center, said.

"That will make travel a bit tricky on Christmas Eve, especially at mountain passes," Bair said. "It will impact a lot of people."

Heavy rains in the San Francisco Bay area have already delayed more than 100 flights at San Francisco International Airport, and caused traffic jams and rock slides, according to NBC Bay Area.

Windstorms are not uncommon in winter in the Pacific region, but the storms that hit Oregon yesterday were especially strong, said Bob Oravec, lead forecaster for the National Weather Service in College Park, Maryland.

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While the snow is falling on the West Coast, the Eastern United States continues to have some of its warmest December weather on record, and snow is unlikely to fall until at least after Christmas.

"It is a pretty big dichotomy across the country right now," he said.

Snow has been sparse in much of the United States, especially in the eastern half of the country.

El Nino is partly to blame, but there are other factors that are also explaining why the two regions are diverging.

"El Nino does have its footprints in what is going on the West right now," said Bair.

As often happens during strong El Nino cycles, the Pacific jet stream has extended into North America, and has lately carried a surge of moisture with it from the Pacific Ocean.

The jet stream is running through the country at a higher latitude than it normally does during a strong El Nino, which explains why the Pacific Northwest is getting so much rain and snow.

National Weather Service monitors at Portland International Airport measured 13.52 inches of rain since the beginning of December. That makes this December Portland's wettest month on record.

Seattle has had its third-wettest December on record, and it's the ninth-wettest month overall, Bair said.

El Nino is partly responsible for the warmer weather in the East, but it's largely due to a climate pattern known as the Arctic Oscillation, which is particularly positive right now. Very low pressure at the North Pole is trapping cold air in the Arctic, preventing it from drifting down through Canada and into the Eastern United States.

This lack of cold air in much of the country has restricted snowfall to mountainous regions, especially the Rocky Mountains, with some falling the Sierra Nevada range and Cascade Mountains, farther west.

Snow will likely continue to fall along all three ranges over the next three days, Oravec told CNBC.

That snow is much needed in the Sierra Nevada. Snowpack is an important source of water for California, and NOAA research released in September determined the Sierra Nevada snowpack levels are at their lowest in 500 years.