By Chris Spears

DENVER (CBS4) – June is here and Colorado is finally warming up after a long stretch of cool and wet weather that covered much of the west. As we shivered and shoveled snow in Denver a heatwave gripped the southeast United States along with extremely dry conditions.

A late May snow broke tree limbs on East 17th Ave. and Steele St. in Denver (credit: CBS)

In the middle a battle played out between the hot and cold air as a persistent jet stream pattern roared overhead. It resulted in day after day of showers and thunderstorms across the central and southern plains states. There were numerous severe weather days responsible for producing hundreds of tornadoes from Texas to Ohio. Another consequence of the stormy pattern has been historic flooding along rivers such as the Arkansas and Mississippi.

Many have raised the question about why the weather did what it did for so many days in row with little to no change. That’s because we are used to a more progressive storm track here in the middle (30-60°) latitudes of Earth, where it may be unsettled for a day or two and then it’s right back to normal.

The picture above shows what most think about when discussing the polar jet stream that divides the northern hemisphere between hot and cold. The fast-flowing river of air generally moves from west to east with subtle waves called troughs and ridges contained within it. Meteorologists often refer to this as the main storm track. The speed of the jet is driven by temperature contrast. The bigger the difference between cold and warm air on either side the faster the wind flows around the planet and the more progressive the storm track.

But what we’ve been seeing more and more, and many scientists think is what will become more common, is in the picture below. It shows how the jet stream roughly looked during the month of May. It was a much more “wavy” pattern with more of a north to south transport of air within the flow around our planet. This is very important for a few reasons.

As Earth warms, specifically the polar regions, the temperature contrast that drives the jet stream will lessen, and thus the speed of the jet stream will slow down. Add in a more north to south flow of air, called meridonal, and you have yet another factor that plays a role in a slower storm track. As Meteorologist Jeff Haby points out, a highly meridional flow can cause atmospheric blocking, and that results in stalled or stagnant weather patterns. That means one area can stay much below normal for several days while another is way too warm. In between a region can get battered with day after day of severe weather, like we saw in Texas, Oklahoma, Kansas and Missouri.

Meridional flow along the jet stream also generates more vorticity, or spin, within the atmosphere. This is important to note because vorticity aids in the development of stormy weather.

A wavier jet stream is something we aren’t used to in the United States and it poses a lot of problems, ranging from weather patterns that seem to get stuck, to helping someone who might deny global warming to understand why it’s colder-than-normal where they live, should they be caught in a cold part of a wavier jet stream.

A recent study that talks more about the link between the warming arctic and a wavier jet stream causing extreme weather in other parts of the world can be found here.

Chris Spears

  1. Robert Chase says:

    “Persistent [sic] Weather”?!? Gibberish!

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