Rare "roll cloud" appears in the Memphis sky

So I was driving into work this morning and saw this...

A roll cloud! It extended north/south as far as I could see and was moving east across the city. I kept an eye on it before it disappeared over the eastern horizon. These pics have gotten a fair bit of attention on our Facebook, Twitter, and Instagram feeds and several people asked for more details, specifically how does it form? So here's the scoop on the rare "roll cloud" from the Glossary of the American Meteorological Society:

"A low-level, horizontal, tube-shaped arcus cloud associated with a gust front of a convective storm (or occasionally a cold front).  Roll clouds are relatively rare; they are completely detached from the convective storm's cloud base, thus differentiating them from the more familiar shelf clouds. Roll clouds appear to be 'rolling' about a horizontal axis [similar to a tornado on its side] because of the shearing effects and horizontal vorticity [rotation about a horizontal axis] provided by the differing air masses."

Roll clouds and shelf clouds (which are much more common and we have seen regularly lately as storms move into the area, pushing an outflow of cold air from the parent storm out ahead of the storm) are both of the arcus variety. The difference, as noted above, is that shelf clouds are attached to the parent storm while roll clouds are completely detached from other clouds.

In this case, the roll cloud likely originated as a shelf cloud from the squall line of storms that moved into central AR last night before falling apart. The outflow from the storms continued, and with no "parent storms" to attach to, the shelf cloud likely became a roll cloud. The fact that it made it this far east is fairly impressive!

A comparison of the two types of arcus clouds, from Ecography.com.

Roll clouds are most common in the U.S. along sea breezes and coastal locations, but are most likely (and more easily predicted) in Queensland, Australia, where they are referred to as the Morning Glory. (Check out this link for more on the Morning Glory and impressive pics!) We happened to luck out with one overhead this morning! Don't ever miss an opportunity to look up - you never know what you'll see!

Erik Proseus
MWN Meteorologist

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