Terry’s Take: The always ominous shelf cloud

Posted on: 10:36 pm, June 25, 2013, by

Terry Swails Weather Blog

Most of you would probably say a tornado is the most frightening cloud formation on earth. No argument from me! However, another feature that brings a great deal of concern and apprehension is the shelf cloud.  Many of you experienced the ominous look of a shelf cloud up close and personal Monday as a powerful line of storms rolled across much of the area.

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Resembling an advancing army, shelf clouds are typically seen at the leading edge of a squall line or feature known as a bow echo. What you’re seeing in a shelf cloud is the boundary between a thunderstorms downdraft and updraft.  As warm moist air is ingested into the thunderstorm updraft, rain-chilled air descends ground-ward out of the downdraft. Upon reaching the earth’s surface it is spread out laterally in front of the advancing storm. Warmer, more moist air is lifted at the leading edge, (or gust front) of this rain-cooled air. When this warm, moist air condenses, you see it as the shelf cloud.

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As the shelf cloud passes, you’ll feel an abrupt wind shift in both direction and speed, which is usually followed within seconds by strong winds, heavy rain, and often hail. The most potent shelf clouds can easily produce winds of 60-80 mph (sometimes stronger).

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Another derivative of this type of cloud is know as a roll cloud. It resembles a giant rolling pin in the sky as it advances with the storm. The big difference between the two is that the shelf cloud is attached to the parent thunderstorms while the roll cloud is not. Here’s some good news, while they both have a frightening look, they are rarely associated with tornadoes!