While most projections leading into this winter season have been for an overall milder than normal season in Central Texas, one variable in changing that may have us breaking out the fluffy winter coats more often.

The on-average milder than normal winter is a prediction based mostly on the moderate to strong La Niña event in place, where sea surface temperatures in the equatorial Pacific Ocean are much cooler than normal. This throws off atmospheric circulations since the ocean and atmosphere are tied closely together, and leaves Texas drier and warmer.

The shorter-term variable that may throw that for a loop is happening quietly at the North Pole.

The “polar vortex” is a swirling mass of some of the coldest air on the planet at the top of the world. Typically, the jet stream acts as a border of this and keeps it neatly in its place, away from the United States.

But this winter, a “sudden stratospheric warming event” may break chunks of that cold air off, and send them on wild rides much farther south than they belong.

The stratosphere is the layer of atmosphere 18 miles above the ground. This is much higher than where the world’s weather takes place, in the troposphere.

The layer of air in the stratosphere has warmed suddenly by 30 degrees Celsius in the past two weeks. This sudden warming can cause a ripple effect downward into the troposphere, throwing off the thermal wind which controls the jet stream and causing the cold, polar air to spill southward.

This split has been known to bring intense, frigid blizzards to Europe especially, and at times to the U.S.

How stratospheric warming affects weather in the troposphere is still something we are learning about, but it can and has before. Parts of the polar vortex blew into the U.S. in 2003, 2009 and 2013, leading to the cold blast in Chicago known as “Chi-beria”.

It is hard to predict these polar vortex events more than a few days out, and not every stratospheric warming event is the same. How these chunks of frigid air may balance out the warmer pattern brought by La Niña is unknown.

Stay with the First Warning Weather team as we monitor this weekend’s snow threat, and any others that may come this winter.