The following is a technical overview of the meteorological conditions that occurred during the May 3rd severe weather event across central Illinois.

On Monday, May 3rd, thunderstorms developed along a cold front in north central Missouri and progressed into west central Illinois by mid-afternoon. Widespread cloud cover was in place across central Illinois through much of the morning and afternoon, but clouds diminished ahead of the cold front. In areas where the sun came out ahead of the front, temperatures climbed into the upper 70s. Dew point temperatures were in the mid-60s, resulting in a moderately unstable environment with around 1500 J/kg of mixed-layer convective available potential energy (MLCAPE). This instability combined with 35-45 knots of deep layer wind shear was supportive of severe weather. The angle between the shear vectors and the cold front was greater than 45 degrees, allowing for the development of supercells. The first image below shows the surface observations as of 22z/5pm, with the cold front pushing into west central Illinois. The second image shows the MLCAPE as of 22z/5pm. Note the lack of blue shading across central Illinois indicating the atmosphere is unstable and ready for thunderstorm development in the presence of a forcing mechanism (in this case, the forcing mechanism was the cold front). The second image shows the shear vectors at the same time. 

The tornado threat was enhanced by a mesoscale convective vortex (MCV), an area of spin in the atmosphere that formed as a result of thunderstorms the previous day across Kansas/Nebraska, which tracked across north-central Missouri then northeastward across the Illinois River Valley. This MCV contributed to a surface wind shift, from southwesterly to southeasterly, ahead of the cold front. This localized wind shift increased the low-level wind shear, which increased the tornado potential. 

Several severe thunderstorms developed along the cold front and moved through central Illinois. The strongest storm tracked from Cass County east-southeastward towards Coles County. That storm produced at least four tornadoes and up to 2" hail (hen egg sized). (See the "Reports" tab for complete details regarding the location of tornadoes). All four tornadoes occurred primarily in open fields, and a tornado rating could not be assigned due to the lack of damage. The other severe thunderstorms produced large hail, but did not produce tornadoes. Overall, damage reports from the event were fairly limited. Power poles were blown down in Sangamon County, however, it is believed this damage was caused by straight line winds. Ping pong ball sized hail caused damage to vehicles in Virginia, IL. An observer five miles north of Buffalo, IL, reported that enough half-inch sized hail stones fell to cover the ground in hail.  

Satellite Loop

Visible and IR satellite loop showing storm initiation in north-central Missouri during the late morning hours of May 3rd. As it moves eastward, a line of thunderstorms quickly develops and strengthens upon reaching the Illinois border with cold cloud tops (orange colors) indicating persistent updrafts. Additional cells develop southward into the favorable storm environment while the line progresses across the state. After sunset, storms begin to lose energy and weaken as they approach the Indiana border.

Closer Look at Tornadic Storm

The loop below is a base reflectivity animation focusing on the supercell that went on to produce 5 tornados in central Illinois. This radar loop shows the early stages of this storm and spans 16 minutes from 4:41-4:57 PM local time. In this loop, red boxes indicate a severe thunderstorm warning and the purple box indicates a tornado warning.

Photos of the thunderstorms across central Illinois. This section will be updated. If you have photos of the storms that you would like for us to include in the event summary, send us a tweet at @NWSLincolnIL with your picture, location, and permission to use your photo.