Shelf clouds develop along the leading edge of the gust front associated with the downdraft area of a thunderstorm.

It marks the leading edge of the rain-cooled air and is usually accompanied by an abrupt change in wind speed and direction. These winds can be extremely strong and produce tornado-like damage. Shelf clouds are common along squall lines.

A shelf cloud features a relatively long linear shape that slopes down away from the precipitation area. It forms when warmer air is lifted up and over cooler air. As this air is lifted, it condenses and forms the shelf cloud. The presence of a shelf clouds almost always indicates a severe thunderstorm!

Sometimes the shelf cloud will accelerate and move out ahead of the actual thunderstorm. When this occurs, it is possible for damaging winds to occur before thunderstorm rains arrive. Often low hanging clouds on the gust front or shelf cloud are mistaken for funnel clouds. These are actually called fractocumulus clouds.

Shelf clouds are often mistaken for wall clouds.

Keep in mind the shelf cloud is usually much longer than the wall cloud. Wall clouds often point toward the downdraft rain area while shelf clouds slope down away from the precipitation and downdraft. The shelf cloud often marks an area of shear.

Often times, low level inflow clouds can be seen moving toward the shelf cloud while the motion of the shelf cloud is partially opposite. These opposing cloud motions are often times mistaken for rotation. Take a good look at this incredible shelf cloud observed over Lake Erie just off Northeast, Pa.

Photo courtesy: Erie County EMA.