Snow Warnings in United States Come From Tropical Zones

Changes to the polar vortex winds hovering within the atmosphere dictate winter weather patterns in North America. However, the scientists found that the tropics more likely influence the main significant cold snaps.

The first-ever study conducted by a team from the University of Reading identified how the four main winter weather patterns in North America behave counting on the strength of the stratospheric polar vortex. 

This is a ribbon of wind and low that circles the Arctic at heights of 10-50km, trapping cold air inside.

It is already well established that the vortex wind strength influences weather in Europe and Asia, and therefore the study revealed it also features a substantial effect on three out of the four main winter weather patterns in North America, giving forecasters a further tool to know potentially high-impact weather during winter.

The most extreme cold snaps affecting the whole of North America are not most likely to occur after a weak vortex unlike in Europe, according to the study published in Geophysical Research Letters. Instead, the shape of the whirlwind and conditions in the tropics were identified as stronger influences of these conditions.

According to lead author Simon Lee, atmospheric scientist at the University of Reading, vortex strength should not be considered as a cause despite the 'polar vortex outbreaks' experienced in North America.

A weak polar vortex, according to Lee, allows cold air from Arctic over Asia and Europe. However, Lee said it is surprisingly not the case on the other side of Atlantic.

Lee recommended checking the equator's conditions for the cause of those widespread freezing conditions in North America.

The study did unveil that the power of polar vortex gives useful information on the likelihood of most weather patterns over the US and Canada beforehand, including some potentially disruptive temperature changes or heavy rain. 

A more precise weather forecast could make towns prepare better in times of calamities, said Lee.

One of the most explicit suggested effects of an active vortex was a 10-15% likelihood of icy conditions in western parts of North America, including Alaska, but milder conditions in central and eastern parts of the US.

Vortex wind speeds - in an average - brings temperatures 5°C above normal and wetter weather in the eastern US, another weather pattern found by scientists.

Researchers noted that the weather pattern with the highest chance of the most widespread extreme cold in North America, where average temperatures in the central US are more than 5°C below average, strongly depends on a weaker vortex, as it does in Europe.

Widespread extreme cold is more common when an area of high pressure extends up to Alaska, according to the researchers, and the polar vortex stretches down towards North America - driving cold Arctic air southward in the lower atmosphere.

The influence of the stratosphere on weather patterns, according to scientists, also interacts with long-term weather patterns within the tropics like El Niño, according to the scientists. The pattern, according to the study, should be studied further and incorporated into forecasts to enhance their accuracy.