Image of the week : a roll cloud

Pictured above, a roll cloud extends far into the distance in 2009 January
above Las Olas Beach located in "Punta del Este" in Maldonado, Uruguay.
Credit & Copyright: Daniela Mirner Eberl

From TheTelegraph

Explanation: a roll cloud

What kind of cloud is this?
These rare long clouds may form near advancing cold fronts.
In particular, a downdraft from an advancing storm front can cause moist warm air to rise, cool below its dew point, and so form a cloud.
When this happens uniformly along an extended front, a roll cloud may form.
Roll clouds may actually have air circulating along the long horizontal axis of the cloud.

Morning Glory Clouds Over Australia
Credit & Licence: Mick Petroff

A roll cloud is not thought to be able to morph into a tornado.

Unlike a similar shelf cloud, a roll cloud, a type of Arcus cloud, is completely detached from their parent cumulonimbus cloud.