National Weather Service, Lincoln IL -- SWOP event summary

May 20, 2013

A large outflow boundary generated from dissipating convection west of I-55 during the early morning of May 20th pushed eastward toward the Indiana border by midday.  The boundary generally stretched from near Paris southwestward to Effingham.  Behind the outflow, lower humidity values kept the airmass relatively stable across much of central Illinois throughout the afternoon.  Further upstream, a highly unstable airmass developed across Oklahoma into parts of Missouri, aiding in the development of severe thunderstorms ahead of an approaching cold front during the late afternoon and evening.  These storms eventually spread northeastward into the Illinois River Valley after 7 PM. 

The evening upper air balloon launched from NWS Lincoln showed a modestly unstable airmass by that point, with CAPE values of around 2000J/kg.  In addition, low-level wind shear had increased to between 45 and 55kt.  This environment was able to support and sustain the developing convection to our southwest.  As a result, numerous strong to severe thunderstorms tracked from southwest to northeast across the area, producing strong winds, vivid lightning, and locally heavy rain.  Wind speeds within the stronger storms were estimated at 50 to 65 mph, with both the Peoria and Springfield airports measuring a peak gust of 54 mph.  While most of the resulting wind damage was rather spotty, three concentrated areas of damage were evident.  One area was from northern Fulton County northward into Peoria, where numerous trees and tree branches were blown down.  Another area was further north across Stark County, where a likely downburst produced widespread tree damage and minor structural damage from Toulon northward through Wyoming and Bradford.  A third location that experienced widespread wind damage was across Moultrie, far northern Shelby, and western Coles counties.  Numerous trees were knocked down across this area, while a roof was torn off a house near Sullivan. The storms weakened as they crossed I-57 into east-central Illinois after midnight.

Rainfall was highly variable...with the highest totals in excess of 2 inches across southern Sangamon County and the lightest amounts of only around one quarter of an inch across eastern Champaign County into Vermilion County.