Northern Virginia commuters heading to work Monday morning were brought short by an illogical sight: a tubular-shaped cloud bearing down on them like a steamroller made of smoke:
This perverse cloud stretched from either side of the horizon and bore a resemblance to a massive tornado that was experimenting with spinning on a horizontal axis. Meteorologists at the local bureau of the National Weather Service locked onto the bizarre presence as it approached from the west and quickly loomed right above their heads; one of them snapped this panorama of it arcing over the NWS' forecasting office like a mammoth space slug from Nebulon Blooto:
As Jason Samenow at the Capital Weather Gang has pointed out, today's "rare, menacing" sight is a type of arcus known as a roll cloud. This breed of cloud may look threatening, but it's typically not associated with severe storms. When you want to start running for shelter is when you spot another impressive, danger-emanating arcus, the shelf cloud.
As to how these fantastic floaters coalesce, the folks at NASA explain:
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. A roll cloud is not thought to be able to morph into a tornado. Unlike a similar shelf cloud, a roll cloud is completely detached from their parent cumulonimbus cloud.
Where did Monday's tumultuous roller hail from? According to the NWS, early-morning showers that drove a gust front toward D.C. appear to be responsible for its formation. Other than spurring several weatherheads to dive for their cameras, the cloud passed over town without any incident, although one witness says it was "pretty windy as it passed."