Monday June 15th saw a handful of tornado warnings issued across the Chicagoland area. The forecast was not calling for much severe weather, mostly a heavy rain and flooding event, similar to what the area has been the past several days. Indeed a serious flooding threat developed, but some storms were able to rotate and produce funnel clouds and tornadoes.
The first of which formed over Western Cook County around 4:45pm. This prompted the first tornado warning. Ahead of that storm, another storm formed and displayed a classic hook echo shape on radar, right over the heart of the Chicago metro area.
This storm produced a funnel cloud that was observed near Midway Airport. No confirmations of any touchdowns were received though. Through social media a few images emerged, the best being this one as the storm moved over Lake Michigan just south of Downtown Chicago. Original photographer is unknown.
These types of radar signatures are what can prompt tornado warnings. When you see or read that a “tornado has been detected by radar” this is what they are referring to. Indeed at this time a distress call was received by the coast guard from a ship in the area due to an ongoing waterspout/tornado. This would have been east of 87th street in Chicago.
No injuries or serious damage has been reported from this storm. As storms began to congeal, a second cell developed rotation just east of Wilmington. This storm DID produce a stovepipe tornado somewhere between Symerton and Peotone. We received many photos, and ISC chaser Danny Neal managed to catch a glimpse of the tornado himself. These images were shared with us and come from Sara Wischover.
Radar at the time showed an embedded cell within the training line of thunderstorms. There have also been no damage or injury reports associated with this tornado either.
So, what happened? There were no watches out. The SPC risk had no tornado threat highlighted in Illinois. Even us here at ISC were not thinking tornado threat today. Everything about the forecast pointed towards the heavy rain threat we saw.
Well, for one, the air was very moist, dewpoints were in the 72-74 range across the area, with surface temperatures not much higher. Air this moist is incredibly buoyant. In addition, the closer your dewpoint is to the actual air temperature, the lower the cloud bases will be. In the case of today, with dewpoints in the 72-74 degree range and surface temps not being much higher, cloud based were incredibly low.
Now, we had earlier storms in the day. When these storms collapse they left behind what is called an outflow boundary. These boundaries can enhance the spin in the air, and are one of the things we as chasers look for when deciding where to target on a chase day. We also had a stationary front draped across the area, the same front that has been stuck here giving us multiple days of storms. All these ingredients seemed to come together last minute to promote storms that briefly rotate. Wind shear, another key ingredient was largely lacking, and for this reason a bigger severe threat was not forecast. In low shear environments storms can basically rain themselves out, and you end up with heavy rain being a primary threat. That is mostly what we saw today.
Taking a look at wind shear values across the area just before the tornadoes shows things were a bit enhanced in those areas. For reference, 30kts is considered minimum criteria to get rotating storms for severe weather. Forecast values today were in the 20-30kt range. Barely there. Thanks to some help from frontal and outflow boundaries, shear was able to increase into the 40-45kt range. This is certainly enough to promote better organized storms.
This is why we did not experience a more widespread, prolonged event. I think these locally enhance shear values at the right time are the primary reason we saw some storms briefly rotate today.
A couple things to point out.
1 – This illustrates once again that Lake Michigan DOES NOT protect Chicago from the development of tornadoes and
2 – WHENEVER storms are forecast, it is advised to pay attention to the weather. The atmosphere can change at the last minute and give us things we aren’t expecting.
Stay tuned to ISC as we will post any new information that gets released about the tornadoes.