A few gustnadoes developed near the Big Sioux River Valley on the evening of Wednesday, May 11th. Gustnadoes formed along an outflow boundary (as noted in the radar images below) from a line of storms to the south. Gustnadoes are different from tornadoes in that they are circulations that form near the ground and are not attached to the base of a cumulonimbus cloud. This is different from a tornado in which the circulation extends from the surface to the base of the parent thunderstorm which is also rotating.
The first gustnado formed as the outflow boundary interacted with developing storms in the area. An outflow boundary results from a thunderstorm as cold air moves away from it. Following the radar images are several photos of lifecycle of one of these gustnadoes, taken north of Hawarden, Iowa, shortly after 6 pm CDT. Photos are courtesy of Sue VanBochove of Hawarden, Iowa. You can also view a short video of this gustnado (provided by Eric Anderson of Hawarden) on our Facebook page.