National Weather Service, Lincoln IL -- SWOP event summary

March 2, 2012

A powerful storm system triggered a widespread outbreak of severe weather across the Ohio and Tennessee River Valleys on Friday, March 2. Numerous reports of tornadoes, large hail, and damaging winds were reported across this area. Further northwest, a variety of less severe weather occurred across central Illinois…including hail, strong winds, and even some sleet and snow.

Low pressure tracking from southern Missouri during the morning of March 2 to eastern Michigan by early evening was the main culprit of all the wild weather across the region. As the low moved northward, warm and moist air riding up and over an advancing warm front triggered scattered strong to severe thunderstorms across south-central Illinois during the morning hours. Many of these cells produced hail…with the largest stones being observed in Cumberland County where our Neoga SWOP reported ping-pong-sized hail and an observer 2 miles northeast of Neoga picked up golf ball-sized hail. Other storms further north into the cool sector of the system produced large quantities of very small hail. Numerous observers across Coles, Champaign, Edgar, and Vermilion counties reported pea-sized hail nearly covering the ground after the storms passed.

As the low tracked to near Terre Haute, Indiana by early afternoon, a portion of east-central and southeast Illinois was able to punch into the warm sector of the storm system. The airmass in this part of the state became increasingly unstable as temperatures climbed into the upper 60s and lower 70s and dewpoints reached the 60 degree mark. In addition, a tremendous amount of low-level wind shear existed in the atmosphere due to southerly winds at the surface increasing and turning to westerly aloft. Supercell thunderstorms quickly began to initiate in this environment along the trailing cold front. Some of these cells produced small hail and gusty 45 to 55 mph winds as they tracked along and south of I-70. Further south and east, the storms eventually became severe…with several long-lived supercells producing strong tornadoes across southern Indiana and Kentucky.

Meanwhile, further northwest into the cold air, temperatures hovered in the 30s along and west of the Illinois River throughout the day. As colder air arrived behind the departing system, light rain mixed with sleet and snow during the afternoon, with northwest Knox County picking up a slushy accumulation of around one inch. The snow was even heavier and more persistent across central and eastern Iowa, where a few locations measured as much as 2 to 4 inches.

Once the low lifted into northern Indiana and the cold front had completely cleared the state of Illinois, strong westerly winds ushered colder air into the region during the mid to late afternoon. Winds gusted over 40 mph at times as temperatures dropped into the 30s and 40s.