Meghan Danahey: November 2009 Archives

I got a good question from Sheryl Arldt last week. I thought I would share my answer with her here in my blog. It might offer a bit more understanding on the topic of Jet Streams in the Northern Hemisphere and how they affect our weather.

"It seems that we usually don't get cold fronts or bad weather coming from the east or northeast and I was wondering why?"

The polar (or mid latitude) jet stream winds blow across the northern hemisphere from west to east all the way around the globe. It is the polar jet stream that drives our storm systems (cold fronts, low pressure systems, etc.).

Imagine you a little piece of wood floating down stream in a small creek. That wood will move downstream along with the current. Sometimes the current picks up, so the piece of wood moves faster. Sometimes the current is very slow, so the wood moves very slowly. In stagnant water, the wood would just float until more of a current came along to pick it up and move it back downstream.

Low pressure systems bring unsettled weather. Low pressure systems are like the wood moving in that creek. The twists and turns in the jet stream, along with the jet stream position, determine the track of those low pressure systems.

In the summer, the polar jet stream migrates up toward the Canadian border. That's why we don't see a whole lot of cold fronts in the summer! Most of the active weather in the USA is along the northernmost states in July and August. In the Spring and Fall, the jet stream migrates back to the south. We start to see cold fronts and periods of unsettled weather with this transition.

In some instances we do see weather come at us from the east or northeast. In those rare cases, it is usually a tropical feature, easterly wave, or other low pressure system sitting just off to our east. The circulation around those features can bring in showers or storms from the opposite direction.

For more information on Jet Streams, cold fronts and pressure systems check out this article courtesy of the National Weather Service Office in Louisville, KY.

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