July 13, 2015

July 13 was forecast to be an extreme CAPE, low directional shear severe weather setup across central Illinois. Dewpoints exceeding 80 F aided by evapotranspiration from the corn were forecast to contribute to CAPE values of 7000 J/Kg or more across the area. With decent northwest flow aloft but rather marginal directional shear at the surface, this setup had many strong parallels to past significant Illinois tornado events. The two most infamous being the August 28, 1990 Plainfield F5 and the July 13, 2004 Roanoke F4. Both events were defined by extreme instability, moderate to strong northwest flow aloft, and marginal to weak low level directional shear. It was remarkable that this event was setting up 11 years to the date of the Roanoke tornado too. Despite the lack of directional shear, a discrete storm with an incredibly powerful updraft can wind up taking control of the local environment and basically generates its own directional shear through a combination of enhanced storm relative inflow and deviant right turning motion. A violent tornado is no guarantee on such setups, however. Many other extreme CAPE, northwest flow summer setups have resulted in a derecho, severe MCS, flooding event, or a cap bust with nothing happening at all. It's what I like to call an "all bets are off" type of setup. However, the potential was there for a violent tornado in the "backyard" so I was ready to chase.

SPC was in tune to the significant tornado possibility this time. A 5% was issued for the Roanoke event and only a severe thunderstorm watch was issued during the 1990 Plainfield event in the days before data was available to recognize the characteristics and potential of these setups.

My initial target was just east of Peoria. I setup to wait for early afternoon storm initiation in Morton, IL. Due to their extreme volatility, these setups have a tendancy to go earlier than later. With temperatures around 90 F and dewpoints around 80 F the heat and humidity were absolutely oppressive. It's the kind of air that makes your A/C vents start billowing white condensation, your glasses fog when you step outside, and you're instantly coated in a layer of condensation on your skin that feels slimy. You don't want to be outside for long.

A storm fired northwest of Ottawa and quickly took on a nice pendant shape with a developing hook echo. I went up I-39 for the intercept, stopping short of the Illnois River as a gorgeous stacked plates supercell started to come into view (see video). As the storm approached, a broad and ragged gust front also came into view.

Another shot of the gust front, looking like it was pushing out as a shelf cloud for a moment.

The base lowered a bit and what looked like a wall cloud was starting to form instead.

Then the rear flanking downdraft kicked in with a nice surge, carving the updraft base into that characteristic horseshoe shape I'm always after when anticipating tornadoes. I let the storm pass to my east as it tracked southeast, pushed along by northwest flow aloft.

A tiny nub of a tail cloud started to form off the tip of the wall cloud/RFD surge. Things were looking promising for tornadogenesis.

A wide angle, processed shot from the DSLR shows the whole base, exhibiting rather dramatic structure as it wrapped up. I was hopeful that we were moments away from a tornado at this point as the storm tracked over empty farmland with remarkable color and contrast.

It wasn't to be, however. While the RFD winds were surging well, it appeared that the storm did not have the right balance of matching inflow winds. Indeed, inflow into the storm seemed quite stagnant. The cycle petered out with nothing materialzing at the surface.

I stair stepped down the road grid with the storm for quite awhile. The storm tried again a few more times with some lackluster cycles and RFD clear slots visible. Nothing had the robust appearance of that initial attempt, however. With pretty sunset colors striking the deteriorating storm (see video), I decided to call the chase and make for home.

The tornado threat did not materialize over the target area on this event. Other reports came in from elsewhere, or with the overnight activity.