July 13th Northern Illinois Supercell

Dec 19, 2015


Tracked Classic looking supercell from birth noting dramatic RFD structure but ultimately failed to produce tornado


Tornadoes: 0
Hail: .75″ (Penny)
Wind: None

The Forecast:

Extreme dewpoint values approaching and exceeding 80 leading to extreme CAPE values in northwest flow regime. Outflow boundary from morning MCS would set up across the area and serve as a focus for new storm development. Already in a favorable shear regime, the presence of the outflow boundary and such extreme CAPE values meant supercells and tornadoes were possible with early storm development. Surface convergence was otherwise lacking though, and of course there were questions the morning MCS would move through too late and ruin instability. Still, images for Roanoke (7-13-04) danced in everyone’s mind. If you don’t know the event, look it up. Its quite prolific. SPC went MDT risk, calling for early tornado potential to evolve into derecho.


Detailed Account:

Today looked like a big day. I was originally eyeing a marginal setup later in the week, but had a lot of work to catch up on I had to choose. I rescheduled some things and got the time off to be able to chase this one. Ben Holcomb was in town and after hitting up a local carnival the night before, we were awoken around 7am to a bow echo moving through. It provided a shelf cloud but the winds were diminishing. This bow echo would send south a critical piece of today’s puzzle, an outflow boundary. The forecast appeared to be coming together nicely and we were on the move by late morning with a target generally a couple hours west.

The air was disgusting. Dewpoints in the 80s are not a pleasant feeling. Your shirt gets soaked with sweat just from sitting outside, and to make matters worse the blower motor in my truck went out so we only had some weak A/C coming from the rear at best. We made it to our target area of Princeton and met up with some other chasers at a Culvers for lunch where we went over the day. The sun was blazing, the air was sticky, but there was no wind. This was a bit of a concern, the outflow boundary was beginning to wash out, perhaps the MCS moved through TOO early? I hit a nice jackpot on a gas station casino which would fund the chase. So hooray for that. We moved east a bit and played our waiting game near a Dennys truck stop.

We noticed the first towers of the day going up just to our west.
July 13th 1

Well, at least we targeted right. Cells were a bit disorganized at first and we made a job north to watch the first one, it quickly fell apart, with a new one to the south, so we detoured for that, but soon after a new, stronger looking storm took off and we quickly turned around for it. We hit some traffic snarl going through the touristy areas of LaSalle and Peru. This is the ONE area of Northern Illinois that is awful to chase in, as the older towns that straddle the Illinois river are big tourist attractions, with their beautiful river vistas and state parks. A nightmare for chasers, but we were quickly through after skirting the core and getting some small hail. Much larger hail up to baseball size fell about a mile north from us. Bummer.

Our first look at the storm showed a really classic looking wall cloud and excitement grew. We moved into position in the much better IL terrain/road network. The wall cloud began to weaken as the storm passed overhead. I noted some really dramatic RFD structure. For a day with ridiculous dewpoints everyone thought the storms would be hazy and crazy HP. This was not the case.
July 13th 2

As we kept with the storm the structure improved. The RFD was really carving through the clouds.
LaSalle IL wm

I started thinking to myself, as beautiful as this is…its usually a signal of dry air aloft. That is the only way youre going to get that kind of dramatic carving. Perhaps too much? We kept up with the storm fairly well, which looked great on radar. The NWS had their trigger on the tornado warning button, but using spotter reports from us and others, held off as it appeared tornadogenesis was not imminent.  Near the LaSalle cooling lake (a place I’ve actually gone fishing at) the storm made a serious attempt to produce a tornado, with dramatically lowered wall cloud and even a bit of an inflow tail.
July 13th Illinois

Surface flow was lacking the entire time though, and we could feel the outflow surging ahead. The RFD clear slot was amazing though, and we could stare straight up into the belly of the beast and see amazing mid level rotation. I was pretty awed by the sight. At one point a funnel look-a-like emerged from the RFD gust front.
LaSalle 2wm IL

The storm began a slow demise at this point. The ridiculous CAPE probably kept it from dying a much quicker death, but the structure never re-organized better than this afterwards. The storm became outflow dominant and slowly surged away. It was the only game in town though so we pursued it a little while longer. Some new, junkier looking cells began going up back north towards home so we called the chase and decided to make our way back to Chicago, sampling any cores we could along the way. We stopped for dinner at an amazing Italian place just north of Kankakee, numerous departing storms in the setting sun ended up providing one of the most unreal looking skies I have ever seen. This image is not altered in any way.

It was a major mistake not documenting this with my DSLR – as we were eating and I stepped out for just a quick cell phone picture. I had some really delicious fettuccine alfredo though. Stomachs full, we arrived home around 1030pm.


From a forecast standpoint the day was somewhat of a bust across IL, the large derecho didn’t form until well later, and our storm ended up being the main show for the area. From a chase standpoint, the supercell structure was really quite awesome, but our hopes were a little bit higher. I can’t complain though, it is mid July and any chase, especially so close to home, this time of year is a major bonus. Overall a solid chase, we were in the right spot for initiation and in position on the storm the entire time. Lack of surface flow and dry air prevented the storm from producing. “aint no flow, aint no show.” Hopefully the day I was eyeing later in the week doesn’t produce.


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