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All About Weather

What to expect when jet streams overlap

storm from two jet streams overlaping

Early spring in Illinois can be a battle between the last of cold winter weather and the mild air of spring. On relatively rare occasions in March, we can experience warm, muggy air that normally doesn’t reach us until April or May. When this occurs early in spring, it’s best to be on guard for severe weather. 

Jet streams

Jet streams are very fast, high altitude winds that blow from west to east. Illinois is generally influenced by what’s called the polar jet. The polar jet is located about 5 to 7 miles above ground. Not only is it a fast wind, it is also a boundary between colder air to the north and more mild air in the middle latitudes. The low-pressure systems, or L’s that you see on a weather map, are the weather makers and ride along the jet stream from west to east. This jet stream can be near or over Illinois generally from mid-fall to mid-spring, but moves north during the warmer part of the year.

Farther south, there is another jet called the Subtropical jet stream. It is about 8 to 10 miles up, and is a boundary between the mild air of the mid-latitudes and the warm, muggy air of the subtropics (think Miami). Summer is usually the time of year when this jet can move far enough north to bring us that uncomfortable, sticky weather.

One jet over the other

Once in a while, the atmosphere will have a situation where a section of the subtropical jet will be above the polar jet. This may happen in early- to mid-spring. When this occurs, there is colder air right next to warm, muggy air near the earth’s surface. If this occurs, the chances for severe weather is greatly enhanced and may lead to thunderstorm and tornado outbreaks. Bomb cyclones, when low-pressure systems get stronger very quickly, can also occur. There may be several days prior to storms where the air temperature is 20 or more degrees above normal along with high humidity. If the cold air starts pushing into the warm air though, that’s when the storms may start.

So if you watch the weather this spring and hear a weathercaster talk about a combined jet, pay close attention to weather changes.


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Duane Friend is an energy and environmental stewardship educator with University of Illinois Extension, serving the organization in many roles since 1993. Duane provides information and educational programs to adult and youth audiences in the areas of soil quality, weather and climate, energy conservation, and disaster preparedness. These programs provide practical solutions for families, farms, and communities.  He assists families in creating a household emergency plan, farmers with the implementation of soil management and conservation practices, and local government officials and business owners with energy conservation techniques.

All About Weather is a blog that explores the environment, climate, and weather topics for Illinois. Get in-depth information about things your weather app doesn't cover from summer droughts to shifting weather patterns.