Relentless Approach

The shelf cloud in the top two images was moving swiftly and relentlessly toward my camera's position at Noelridge Christian Church on C Avenue NE near Boyson Road in Cedar Rapids, Iowa around 5:33 PM, Friday, June 29, 2012. The ragged-looking shelf cloud was the leading edge of a very windy gust front that was spreading out in a bow shape. The radar "bow echo" can be seen in the bottom image as a thin dark red color, indicated by the arrow. Gust front winds in certain areas of Eastern Iowa reached speeds of from 50-80 mph. Also known as "arcus" clouds, shelf clouds are products of cool, sinking air from a storm cloud's downdraft that spread out across the land's surface on the leading edge of the gust front. This cool outflow cuts under the warm air being drawn into the storm's updraft, lifting the warm air and making it condense. A sharp, strong gust front will cause the lowest part of the leading edge of the shelf cloud to become ragged and lined with rising fractus (scud) clouds (see top image). Striations in the shelf cloud (as seen in the middle image) are caused by cool and warm air acting upon each other creating convergence (lifting). The top two images look southwest.