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.
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.