Explain this strange weather pattern

I’ve seen this exact same occurance twice in as many weeks on the weather RADAR. Basically, a large rain system passes through the area from W to E. Then, a very long, thin line moves from the system heading S at a rapid speed. IIRC, I’ve seen this once or twice years ago, as well.

I have a screen shot of the RADAR:

The main system of rain to the NE of Chicago is heading E. But if you look at the long, thin line that runs E/W, just N of Geneva, that line was heading SSW at a rapid clip.

What’s the deal?

That is an outflow boundary. After a thunderstorm, the air is cooled and what you are seeing is the line that separates the cooled air from the warmer air away from the storm. It is essentially a miniature cold front. They are fairly common.

Google “outflow boundary” for more info.

THat appears to be what our meteorologist calls a “gust front”. Basically a big wind system that moves out away from the storm as the supercells collapse. I’ve experienced this many times. We won’t get any rain (and damnit, we need some!), but there will be a very sudden cool blast of air. It lasts 5 to 10 minutes and I have seen it snap limbs off of trees or peel back roofs of buildings.

Google “gust front”. Here’s a link from University of Illinois.

I’ve seen the gust front on radar and have always wondered about it. Thanks for the question and answers!

Once I saw this as a static line between two separated storm areas.

Here’s Wikipedia’s page, including a link to a cool mpeg of satellite pictures. Outflow boundary - Wikipedia

I found I still have an image of a static line from 2003. It’s an animated gif.


The line shows when the radar apparently switched to clear air mode (as mentioned in the articles), judging from the increased ground clutter.

But perhaps this example isn’t quite an outflow boundary, because it isn’t parallel to the other storms’ motions. I remember seeing another one that was parallel.

Cool stuff - thanks all. Looks like I’m seeing outflow boundaries. A bit of ignorance fought.

That mpeg video is pretty sweet - thanks.