During the afternoon hours of June 18, 2010, between 2 pm and 5 pm, a line of strong to severe storms moved east-northeast through extreme southern Wisconsin, basically south of a Madison to Milwaukee line. The storm produced wind gusts of 40 to 50 mph over a good part of the southern two rows of counties, small hail, brief heavy rains, and some localized urban flooding.
Within the line of storms, there were embedded severe storms with damaging winds. There were at least two swaths of storm damage in which wind gusts were in the 60 to 76 mph range. One swath extended from south of Monroe in Green County northeast to the Janesville and Milton area. Another swath extended from northeast Walworth County through southeastern Waukesha County to Milwaukee Mitchell Field. A severe weather spotter measured a wind gust of 76 mph in Mukwonago, an estimated gust of 75 mph in Muskego, Waukesha County, and a gust of 66 mph was measured at Mitchell Field. There were other scattered reports of powerful wind gusts south of these two swaths.
Below on the left is an image showing suspected thunderstorm wind damage swaths. There were other scattered reports of damage but these are not plotted on this map. Refer to a link later in this story that will lead you to a plot of all damage or wind reports. On the right is a radar image showing the position of the line of strong to severe storms at about 415 pm.
In the radar image you can see that the line of storms has a bow shape. We call these "bow echoes." Bow echoes generally produce some of the strongest non-tornadic, straight-line winds. Additionally, bow echoes usually have a well-developed accessory cloud feature on their front side called a "Shelf Cloud." Shelf clouds sometimes look like a snow plow, but they may also have very low hanging fragments of cloud material (scud) that are not rotating and resemble funnel clouds. Consequently, these cloud fragments attached to shelf clouds scare people and they may end up calling the County 911 Dispatchers with false funnel cloud reports. The NWS office near Sullivan did get a citizen report of a funnel cloud in the Wales area of Waukesha County. Our Doppler Radar system didn't show any rotation in the thunderstorm line and the atmosphere wind structure didn't support the tornado idea on June 18th.
Below are some pictures from severe weather spotters in southern Wisconsin. You can clearly see a horizontal shelf cloud in some of the pictures.
Bill Stolte, Waukesha County Emergency Management Director, took a picture of a scary-looking cloud associated with the shelf cloud as it moved through Waukesha County. His picture is below on the left. Note the funnel-shaped cloud feature. Bill said it wasn't rotating, and only lasted a couple minutes. It sure looks like a funnel cloud, doesn't it? Everyone, ranging from private citizens to trained severe weather spotters, have to keep in mind that a true funnel cloud rotates and it shows signs of persistent rotation on a vertical axis. If the cloud feature you are looking at isn't rotating, it can't be a true funnel cloud, no matter how much it looks like a funnel cloud and no matter how scary looking it is. Penny Zabel, a meteorologist here at the Milwaukee/Sullivan NWS Office, took a picture of a similar scary-looking cloud fragemnt in Waukesha County - her picture is to the right.
The following link will lead you to a plot of the local storm reports we received on June 18th. http://www.crh.noaa.gov/mkx/lsr/lsr.php?&elm=tstorm&dates=20100618
The following link will lead you to another article on this web site about Scary Looking Clouds. http://www.crh.noaa.gov/mkx/?n=scary-clouds