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What causes the warm before the storm in Colorado?

It often gets warm and sunny in Colorado right before a winter storm rolls in. Here's why that happens.

DENVER — Warm sunshine spread across the Front Range on Tuesday, and high temperatures could reach the upper 60s by Wednesday afternoon – all while a winter storm bears down for Thanksgiving night.

This happens a lot in Colorado. We call it the "warm before the storm." It’s caused by a wave pattern in the jet stream, which is a river of fast-moving air just above our heads.

Sometimes the jet stream takes on a very wavy pattern, and when it does, there’s usually abrupt changes in the weather underneath it.

When the wave bends upward, you get sunny weather – the warm before the storm. As the wave progresses from west to east, the downward bend moves over Colorado – that’s our Thanksgiving storm.

As the next upward bend in the wave moves overhead, the skies quickly clear up again, and you get something often known in the Colorado ski country as "bluebird" – sunny skies over the fresh fallen snow.

This wave pattern is more typical in Colorado winters, which is why last year’s blocking jet stream pattern was so shocking. We had three months of nearly constant cloud cover because we were stuck in a downward jet stream bend with no bluebird and no snowmelt.

Credit: KUSA

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