Shelf Clouds

Shelf clouds, also called arc clouds, are typically seen at the leading edge of a squall line of thunderstorms. Although they might look similar, shelf clouds are nothing like tornadoes; what you see in a shelf cloud is the boundary between the downdraft and the updraft of a thunderstorm. Rain-cooled air descends in the downdraft then spreads when it reaches the earth's surface, then warmer, moist air is lifted at the gust front of the rain-cooled air. When the warmer air condenses, a shelf cloud forms.

Shelf clouds, after they have passed, can cause strong wind gusts and heavy rain or hail. These wind gusts can be so strong that they are able to knock over trees and power lines.


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