CM – The jet stream took a sharp turn and the US experienced unprecedented tornado weather in December


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December 16, 2021

by William Gallus, The Conversation

Extremely strong winds swept over a large part of the United States on December 15, 2021 and hit several states with gusts of hurricane strength. Record temperatures helped create tornadoes in Iowa, winds spread grassfires and clouds of dust in Kansas, and wind damage was reported from northern New Mexico and Colorado in the Midwest. The National Weather Service described it as a « historic weather day » with an « unprecedented storm prospect ».

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We asked atmospheric researcher William Gallus, whose office at Iowa State University was in the center of the storm, to explain what caused the extreme weather and why it was so unusual.

We saw very strong winds because of a very strong disturbance in the jet stream. This disruption helped create a very intense low pressure system that creates high winds and storms. But the low pressure wasn’t what made this event unusual.

It was unprecedented because an incredible amount of warm air was drawn in from the south before the storm.

Here in Iowa, December temperatures were mid-December In the 1970s, December 15th was hotter than ever, and there was a very unusual amount of moisture at these temperatures. Because of this, we saw tornado warnings – and reports of tornado damage – across the region.

Tornadoes are extremely rare in December in Iowa. Minnesota, which had never seen a tornado in December, also had tornado warnings and a possible sighting.

The wind speeds with this particular system were about as strong as we saw. But it was all the other weather parameters that came together in December that put this storm system out of scale.

In the warm, humid air, we also had thunderstorms, and thunderstorms tended to increase the winds even more. If you were to climb 300 meters into the sky, you would find that it is much more windy up there. During thunderstorms, the rain helps create a downward wind flow that we call downdraft. When you have that downdraft, it tends to carry the really strong winds to the ground. Thunderstorms in the conditions we saw could bring about winds that could easily reach over 80 miles per hour.

The winds must rise above the Rocky Mountains. If you get a temperature inversion where the temperature actually goes up, rather than going down, when you get higher in the atmosphere just above the peaks of the mountains, it can act like a lid catching the momentum of the wind blowing over the mountains. The wind can’t really spread, but rushes down the east side of the mountains.

Wind gusts increased dramatically in southeast Colorado, with Lamar Municipal Airport gusting 107 mph in the last hour. This is due to a cold frontal passage that, with the help of sloping winds from the Rocky Mountains, mixes intense mean winds to the surface. #COwx

When everything goes down, gravity accelerates it, just like when you drop a ball from the top of a skyscraper. The same thing happens with these winches. As they flow down the east face of the Rockies, they accelerate.

If you are on the leeward side of a mountain range like Denver and Boulder, the winds in those areas can get very strong as you descend.

If we get a low pressure system, it’s because of wobbling in the jet steam appear. We call them troughs in meteorology.

If you look at a map that shows the jet stream, the jet stream looks like a roller coaster – it swings up and down from north to south. Whenever you are in front of one of these troughs, where the jet stream bends south and then north again, the air in front of it has to rise, and a low pressure system is created. The winds that blow around him can get very strong.

In this case there was a particularly sharp depression in the jet stream, almost in a « V » shape, which increased the effect.

Is there any connection between this storm and the deadly tornadoes that struck Kentucky and other states December 10-11?

It’s hard to say if there was a trigger anywhere on the planet who managed to generate these two different waves in the jet stream.

It is interesting that La Niña takes place in the Pacific Ocean. When we have La Niña conditions we often find that the far north of the United States is getting colder than normal and the south is getting warmer.

This article was republished by The Conversation under a Creative Commons license. Read the original article.

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