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Fox News Weather Center

Cold Air to Plunge Into Western Half of US During Third Week of December

Cold weather lovers in the West will be getting another present from Mother Nature during the third week of December.

The jet stream will take a big southward dip over the western half of the nation during much of the third week of the month.

The jet stream is a river of high-speed air high above the ground that guides weather systems along. It often marks the boundary between cold air to its north and warm air to its south.

According to AccuWeather Chief Long Range Meteorologist Paul Pastelok, "The air that settles into the western United States next week will originate in part from Alaska and part from the Pacific Ocean."

"The end result of the air mass blend will deliver temperatures of 10-20 degrees Fahrenheit below average," Pastelok said. "For much of the interior West and in portions of the Rockies and Great Basin, this will be the coldest weather of the season so far."

An area that may escape the brunt of the cold air will be west of the Cascades in the Northwest, where more ocean air will dominate.

An extensive area of highs in the teens, 20s and 30s will develop over the interior West next week. Nighttime temperatures will dip into the teens in Salt Lake City and Boise, Idaho, and the single digits in Denver.

A substantial amount of cold air will also nose onto the Plains and part of the Midwest, before fading farther to the east next week. Minneapolis will have multiple days with highs below freezing, following an temperature departures of 6-10 degrees above normal during much of the autumn.

The cold will dip into central and Southern California.

The pattern will set up multiple nights with a significant frost or freeze over the Sacramento and San Joaquin valleys, as well as the Inland Empire. Temperatures are likely to dip several degrees lower than the chill that struck during late November into the start of December.

As the cold air settles in and is funneled southward and westward through the canyons and gaps in the mountains of Southern California, moderate Santa Ana events can occur.

"From Tuesday through the end of the week, there will be some chilly and gusty winds every day in the traditional Santa Ana areas of Southern California," AccuWeather Meteorologist Dave Samuhel said. "Wednesday could bring the strongest winds."

The winds may not become strong enough to down tree limbs and cause sporadic power outages. However, the main effect will likely be to cause low AccuWeather RealFeel® Temperatures. So even though some areas may not eclipse the lowest temperatures of the season so far in the coastal areas, it will feel like it. RealFeel Temperatures could dip into the 20s from time to time, factoring in the wind, actual temperature, humidity and other conditions.

The leading edge of the colder air will spread southeastward with areas of snow over the mountains and interior of the West during Monday into Tuesday.

A reinforcing batch of cold air with the possibility of more areas of snow will spread southeastward Wednesday into Friday.

The cold air may have staying power over the interior West.

"Once cold air settles into the Great Basin and the Intermountain West this time of the year, it is difficult to route out," Pastelok said. "And, this certainly looks like a situation where the cold will linger with areas of fresh snowcover offering assistance."

In some cases, the cold may actually intensify over the interior West as the month progresses.

The cold air with and without snow in the West should have the ski industry in prime condition for the upcoming holiday season.