Supercell

According to a National Weather Service report, a supercell thunderstorm spawned a tornado near the town of Tyler in Clearfield County on Sunday afternoon (8/9/2009). I was blessed to get a good glimpse of the storm without being near enough for trouble. Here's my picture:

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I admit, this is a rather poor image. Light conditions were not very good, although it was only partly cloudy in Curwensville when I took it. The wedge shape in the sky on the left is pale blue sky. But the image is good enough to explain a few things about the storm.

To the right is the body of the storm, called a supercell thunderstorm. Any thunderstorm is caused by warm, moist air rising high into the atmosphere because of heating. As it rises, the water vapor in the air condenses and becomes clouds and rain. Sometimes the wind and rising forces are very strong, causing a strong swirling vortex to form in the middle of the storm. These storms are called supercells.

In this photo, you can see the main channel of air on the right half of the sky. These clouds are made of water that is condensing around the vortex. The friction and forces in here made a great deal of lightning. At the top of the picture, you can see what is called the "anvil" of the storm. This is where the uplifted air gets so high that it enters a cold thin part of the atmosphere where strong winds blow it out in all directions. This is somewhere between 35,000 and 50,000 feet.

Now here is the radar image from the National Center for Atmospheric Research:

This picture shows conditions about the time of the tornado. The tornado occurred in the red part directly above the letters FIG, which is Clearfield-Lawrence Airport. The storm front came through Pennsylvania from the northwest. As it approached Clearfield County, the storms were aligned along a single front, almost a straight line. You can see part of the line in the upper right-hand corner of the photo.

Notice how the portion of the storm that spawned the tornado has fallen behind the main line. If you are familiar with the geography, you will notice that it has stalled over Bennett's Valley between Weedville and Penfield along State Route 255. This is a very deep valley with Boone Mountain on the northwest and Penfield Mountain to the southeast. Apparently when the storm came directly across the valley, the face of Penfield Mountain caused the storm to stall and the winds to start spinning up to a great elevation, which eventually led to the tornado.

This is not the first time that Penfield Mountain has caused Parker Dam State Park to suffer tornadoes. The same thing happened on May 31, 1985, and you can still see the damage to this day.