shelf cloud

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A flat, wedged-shaped, sometimes multilayered cloud, descending from the front edge of a thunderstorm and accompanied by strong winds.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun meteorology A low, horizontal cloud typically forming at the leading edge of thunderstorm outflow. An arcus type cloud often found along squall lines.