National Weather Service, Lincoln IL -- SWOP event summary

April 15, 2011

Severe thunderstorms impacted parts of central and southeast Illinois on Friday, April 15th. A deepening area of low pressure across northern Missouri helped pull a warm front northward into the region. To the north of the front, windy and cool weather prevailed for the entire day. Meanwhile, much warmer and more humid conditions were observed south of the front across Missouri into southern Illinois. Central Illinois remained between the two contrasting airmasses, with the frontal boundary reaching a Jacksonville...to Effingham...to Robinson line by late afternoon.

Instability parameters remained rather modest, with the evening upper air sounding at NWS Lincoln showing CAPE values of less than 1000J/kg. However, large amounts of low-level wind shear were present in the vicinity of the warm front. Surface winds were generally from the E/SE at around 10kt, but quickly veered to S and increased to 40kt at around 3000ft aloft. This strengthening and turning of winds with height aided in the formation of rotating updrafts within many of the developing thunderstorm cells.

As a result, numerous supercell thunderstorms formed near the warm front between 4 PM and 7 PM...mainly southwest of a Canton...to Lincoln...to Decatur line. A survey conducted by NWS Lincoln staff members on Saturday, April 16th uncovered evidence of three tornado touchdowns across parts of Menard and Mason counties.

In addition to the tornadoes, numerous reports of funnel clouds and large hail were received. Golf-ball sized hail (1.75" diameter) was observed at the Springfield airport, while hen-egg sized hail (2" diameter) occurred just west of Waverly in Morgan County.

As the evening progressed and the weak instability waned, the storms eventually congealed into a linear structure and began accelerating east-northeast. The southern portion of the line produced gusty winds from south of Taylorville southeastward to Lawrenceville. A gust to 53 mph was measured at the Flora airport, while a SWOP observer just east of Birds in Lawrence County recorded a 56 mph gust.