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A surface low that deepened over the central Plains states sent a warm front surging through the area on Tuesday, April 4. This largely placed central Illinois in a warm, moist airmass where temperatures climbed into the low to mid 80s and dewpoints in the mid to upper 60s. Plenty of CAPE (convective available potential energy) and wind shear were in place, but a few limiting factors prevented storms from firing during the afternoon hours. These were: 1) no source of lift/frontal boundary and 2) a strong CAP or "lid" was in place, which is a layer of warmer air above the surface that prevents storms from developing. 

By late afternoon, the CAP started to erode west of the Illinois River as a subtle upper wave approached from the west. A handful of supercell thunderstorms developed over Missouri and tracked northeast, approaching parts of west-central Illinois by the evening hours. The storms were being fueled by strong instability (3000+ J/kg) and wind shear (50+ kt) as seen on a special 3 pm/20z balloon launch we conducted that afternoon. The first supercell entered Schuyler County just after 5 pm, lifting north along the Illinois River Valley toward Peoria before dissipating as it approached a stronger capped environment. A few supercell thunderstorms that tracked between Burlington, IA and Quincy, IL congealed into one massive supercell, strengthening as it entered Fulton County around 7 pm. The supercell went on to produce baseball sized hail and a few tornadoes, one of which was a long-track EF-3 with peak winds estimated near 160 mph near Lewistown and Bryant in Fulton County. The supercell storm weakened as it moved into eastern Tazewell and Peoria counties, eventually dissipating over Woodford County due to the lack of daytime heating, which strengthened the capping inversion.


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