Shelf Clouds: Not Tornadoes, But Still Scary and Ominous

Shelf clouds are typically seen at the leading edge of a thunderstorm or squall line of thunderstorms. While menacing in appearance, shelf clouds are not tornadoes or wall clouds.

What you're seeing in a shelf cloud is the boundary between a downdraft and updraft of a thunderstorm or line of thunderstorms.

Rain-chilled air descends in a thunderstorm's downdraft, then spreads laterally when reaching Earth's surface. 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 the shelf cloud.

As the shelf cloud passes, you feel an abrupt shift in wind direction and increased wind speed, followed within minutes by heavy rain or hail. Wind gusts once the shelf cloud has passed may be quite strong, causing downed trees, tree limbs and power outages.

http://www.weather.com/science/news/shelf-cloud-photos-20130412