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By now, you've probably heard about the colder air coming to Iowa from Thanksgiving into early December. It's all thanks to a weather pattern called the "Siberian Express". As the name implies, the Siberian Express refers to air masses that start in northern Russia and end up in the Lower 48, driving arctic cold into places like Iowa.Like most patterns, this coming Siberian Express will be influenced by numerous factors. We'll cover two of them here. The Arctic OscillationWhenever Iowa gets a cold air outbreak in winter, the Arctic Oscillation is usually involved. This pattern centers on low pressure around the North Pole — the famous polar vortex.Occasionally the polar vortex weakens, allowing the jet stream to move south and letting cold, arctic air escape down into North America, Europe, or Asia.That's exactly what will happen later this week.But how will the cold air end up in Iowa, specifically? That's where the next pattern comes in.Pacific-North American Pattern The Pacific-North American Pattern is driven by variations in the jet stream over the Pacific Ocean.These variations cause pressure changes over the ocean that affect North America's weather.Early next week, this pattern will lead to a large area of high pressure over the West Coast.This high pressure will align the jet stream so that it points straight from northwest Canada to the Midwest. This will provide an easy pathway for arctic air released by the polar vortex to slide straight down the eastern side of the Rockies toward places like Iowa. Just how cold?We're not expecting anything record-breaking, but Thanksgiving through early next week will be quite chilly. High temperatures will only reach the 20s, and a few nights might be close to single digits. Highs around 40 degrees are more normal for the beginning of December.