National Weather Service, Lincoln IL -- SWOP event summary

August 16, 2012

A strong cold front pushed through central Illinois on August 16th, triggering widespread showers and thunderstorms.  Some of the storms became severe during the afternoon, producing damaging winds of 60 to 80 mph and large hail up to the size of quarters.  Total rainfall from this system was highly variable...with some locations picking up beneficial amounts of 2 to 3 inches and others receiving less than one quarter of an inch. 

By the early morning of August 16th, the cold front was positioned over eastern Iowa.  Southerly winds ahead of the front were bringing warm and increasingly moist conditions into central Illinois, with surface dewpoints well into the 60s.  Meanwhile, the airmass behind the front across the Plains was much cooler and drier, with dewpoints only in the 30s and 40s.  This sharp contrast between airmasses helped spark the showers and thunderstorms across the area.  One round of storms developed during the pre-dawn hours across Iowa and far northern Missouri, then tracked eastward into north-central Illinois during the morning.  While these storms were generally sub-severe, frequent lightning and locally heavy downpours occurred along and north of I-72.

Additional thunderstorms developed along the advancing cold front further south and east during the afternoon.  Thanks to cooling aloft associated with an approaching upper-level wave, the airmass became moderately unstable...with Convective Available Potential Energy (CAPE) values ranging from 1500 to 2500J/Kg.  In addition, a strengthening low-level jet brought increasing wind energy to the environment.  The upper air balloon launched from NWS Lincoln during the evening of August 15th showed winds 2000ft aloft of around 15 mph.  These had increased to nearly 50 mph by the morning of August 16th.  The combination of instability and wind shear led to the development of long-lived thunderstorms along the front during the afternoon and early evening.

The strongest storms formed along a Pana...to Windsor...to Mattoon line, where numerous trees and power lines were downed.  The storms then dropped southeastward, producing sporadic wind damage and heavy rainfall throughout southeast Illinois into early evening.

Here is a sample of some of the severe weather reports received on August 16th: