On June 22nd, 2016 a trio of supercell thunderstorms tracked across Northern and Central Illinois. These storms accounted for over two dozen tornado reports and persisted well after dark. A map of the preliminary SPC storm reports is posted above.
The latest total from NWS Chicago lists 8 tornados as of this writing and of the most notable, an EF-2 tornado that moved through the south side of Pontiac, IL just after 10pm CST. The storms were well forecasted by the Storm Prediction Center (SPC) with a 10% hatched tornado risk across much of Northern IL by Wednesday afternoon along with a moderate risk for severe weather for much of the area which is depicted below.
The focus of severe development was a warm front that was forecasted to move north-ward through the state during Wednesday afternoon. Not only can a warm front provide a source of convective initiation but it is also a valuable source of low-level wind shear which is an important factor in tornadogenesis. Just north of the front, on the cold side, one would expect backed winds at the surface which then veer rapidly with height. This low level turning of the winds with height provides a relatively narrow corridor of amplified low-level helicity. Nearly all of the tornado reports from this day would fall within 20-40km of the warm front.
The most persistent supercell, and parent of the Pontiac tornado, seemed to move southeastward along the front as the evening progressed. The map below shows a closer inspection of a 21z surface analysis issued by the WPC before initiation along with the tornado reports from the SPC for 6/22/16.
However, forecasting the timing and the location of this warm-front proved to be a challenge as a morning mesoscale convective system (MCS) moved through the state. An MCS is characterized by a large line or complex of thunderstorms and stratiform precipitation. While this MCS was forecasted well in advance of Wednesday morning, the exact influence it would have on the north-ward extent of the warm front remained a large question. By Wednesday afternoon, the front had managed to make its way to I-80 resulting in storms firing just to its north and west along the IL/IA border.
The winds aloft were classic of a late June set-up in the Ohio Valley with 50-60kt winds out of the WNW at 500mb. 00z soundings from KDVN and KILX also depicted a stout low-level jet at 850mb with winds at 30kt out of the SW. These soundings also showed ample instability with CAPE values exceeding 3500 j/kg across much of Illinois. The 00z sounding from KDVN is posted below.
Despite a multitude of favorable ingredients for severe weather, one concern highlighted by the upper-air data was a cap just above 700mb with an attendant 50kt westerly flow. This nose of warm air just above 700mb was actually the lowest extent of remnant elevated mixed layer (EML) which had been advected into the area from the west. This cap would indeed hold until just around 6pm when storms began to initiate.
While the orientation of the wind-vectors wasn’t that of a classic plains set-up — often with a southerly LLJ — it isn’t rare to see SW flow at 850mb with midwest severe outbreaks, especially with this upper-flow regime.
By 6:30pm storms had initiated along the IL/IA border and were moving SE at around 30kt. Three distinct supercells had formed around this time and all showed early signs of rotation as a result of the well-sheared environment. The storms would soon after produce numerous funnel-cloud and brief tornado reports as they tracked toward I-80. The storms were also producing particularly dangerous amounts of cloud to ground lightning during this time. Pictures of the centermost cell are posted below.
Originally it was believed that the environment was at least somewhat supportive of stronger tornados (evident via a hatched region per SPC). However, upon materialization the storms were battling strong RFD/outflow, HP modes, as well as upper level flow blowing near parallel with the front. The later caused the storms to rain into one another. The last super-cell to produce tornado reports past sun-down may have avoided this as it flanked the western edge of the line.
Regardless, this event was reminiscent of other warm-front set-ups that have produced damaging tornadoes in the state in recent years. On the same date last year a similar system moved through N. Illinois causing extensive damage to Morris and Coal City, IL. Earlier in the year, on April 9th, 2015 a warm front set-up was responsible for the violent EF4 Rochelle/Fairdale tornado which killed 2 individuals and injured over 20 as it tore a 30 mile track through N. IL.
The footage from link below was recored by a police officer in Pontiac, IL as a tornado passes just by his cruiser (Via WGN Chicago). Police dash cam catches dramatic tornado video