After any major weather event, particularly those that don’t pan out as expected, I like to go back and look at what went right and what went wrong. This time I was wrong, really wrong.
The data retrieved from Monday’s 7 PM weather balloon launches tell the story. The upper air sounding from Lincoln shows a nearly uncapped environment while the sounding from from Davenport shows a capping inversion around 14,000 feet, a layer of warm air that’s about 2,000 feet thick which would act to stop a warm, surface based, air mass from rising.
Peoria is in the middle of both locations, but based on how storms were struggling to become organized, it became clear at 8 pm that we were under the same air mass that was sitting over the Quad Cities. In order for us to overcome that layer of warm air aloft we would have needed our surface temperatures to climb another few degrees.
With the updrafts struggling to gain momentum, the strong shear simply toppled the storms over. This left us with low-topped, low precipitation cells that had some incredible storm structure. The low level wind shear along a convergence boundary located along I-74 allowed the base of the storms to encounter some rotation.
In the picture below from Bob Wilson, you can see the storm clouds are not very tall. Radar estimated that the tops of this storm was somewhere around 15,000 feet. However, you can see the smooth base of the storm indicating that there was some good directional wind shear.
That developing cell would drift east and move over Peoria. You can see the strongly sheared base moving over Peoria below. You can watch a timelapse of this rotating cell moving over Peoria here.
Thanks to the mid-level capping inversion, the severe storms were focused much further south along I-72. The rain and clouds earlier in the day impacted conditions across the region more than I thought they would.
So why did I think the rain would have a smaller impact?
It’s important to understand why the forecast was made in the first place. Throughout the weekend, models were consistent in bringing a strong system into Illinois that would produce widespread thunderstorms. The combination of strong shear and a strong instability would bring in the threat of severe weather. Confidence in the forecast increased significantly Monday morning when the morning model runs all showed storms develop across the entire region.
Below are a number of models that were available to me Monday morning when my forecast was made. All of these models show storms developing near and moving through Central Illinois. It should be noted that not a single model from Monday morning showed storms developing along I-72, which is where they ultimately developed.
Some of the models above suggested that the atmosphere would destabilize as far north as I-80. However, we knew the morning rain and cloud cover would have an impact on the overall coverage of storms, especially that far north. So we shifted the area of focus a little further south which would put most of this development along I-74.
After looking at the new model data as it was rolling in, it appeared we would have strong low level shear and moderate to strong instability across the region by early evening. Satellite imagery backed up this idea by showing clearing skies across eastern Missouri in the wake of the morning showers and storms which would cause instability to increase quickly. Given all the data available at the time, I ended up designating Monday a Severe Weather Alert Day at 10 am.
At 11:30 am, the Storm Prediction Center kept the region in an enhanced risk for severe weather and increased the tornado risk and included the potential for strong (EF-3 to EF-5) tornadoes. They even hinted at the possibility of an upgrade to a moderate risk later in the afternoon. This further backed up my concerns that we would see severe weather.
After 6 pm, one of the Hi-Res models started to shift things a little further south and east. However, these later runs still kept a good amount of severe weather in the viewing area so there as no reason to change the current forecast.
In the end, the models were too far north with the placement of the warm front Monday evening. While a convergence boundary located near I-74 was thought to be the warm front, the true warm front was further south along I-72. Had I taken the time to make a hand drawn surface analysis at 6 pm, I may have been able to identify this sooner.
Temperatures across the Peoria area simply didn’t warm enough to overcome the layer of warm air aloft for severe storms to develop. Unfortunately, that was not clear until the storms began to develop along I-72 and the models started to pick up on their development.