Understanding the science behind the recent severe thunderstorms in Edgar County is easier when you know the science behind a general thunderstorm.
Thunderstorms require three ingredients to form – moisture, lifting, and instability. Highs on April 26 topped out near 80 degrees as southerly winds brought in warm moist air from the Gulf of Mexico. These conditions fulfilled the first ingredient; the second and third arrived along an advancing cold front. A cold front is the leading edge of an air mass that is cooler than the air already at the surface. Since cold air is denser than warm air, the front lifts the warm air up into the atmosphere and it is replaced by the cooler air. This interaction between the air masses destabilizes the atmosphere and with that instability, forms a thunderstorm. Similar conditions were in place Sunday as the sunshine heated the atmosphere and it became unstable.
Once the ingredients are in place, the lifting produces cumulous clouds as the rising air condenses and establishes an updraft. The updraft supports the thunderstorm by ushering warm, moist air into the storm. Eventually, the moisture condenses into rain drops and they fall when they are too heavy for the updraft to keep them in the cloud.
The storm that would drop hail on Paris developed around 3 p.m. April 26 across northern Clark County. With ample instability and moisture, the cell thrived, and continued to develop over Paris. The winds in the storm’s updraft began to lift raindrops high into the atmosphere where they froze and collided with other droplets. As these droplets started falling back to the surface, the updraft caught them and once again lifted them into the sub-freezing portions of the atmosphere where they grew into hailstones. This cycle continues, forming larger and larger hail, until the updraft can’t support the hailstones and they fall to the surface.
Around the same time, the storm stalled along the cold front, barely moving to the east. The nearly stationary storm drenched the city in heavy rains and dropped up to golf ball size hail across eastern portions of the city and points east along the Clinton Rd. and U.S. Highway 150.
As one section of the storm weakened, another rapidly developed and took its place, resulting in periods where the hail would briefly stop before another round began. This phenomenon is known as “training” where storms will continually form over the same area and barely move. Sunday’s storm; however, was a little different.
Developing over Moultrie County in central Illinois, this cell grew in strength and intensity by feeding on moisture and daytime heating as it pushed east-northeast over Coles County. Once it crossed Interstate 57, its updrafts rapidly intensified, blowing rain and hailstones high into the atmosphere. The updraft was strong enough to support two inch size hail as it passed over Oakland, but continued to intensify and produce three inch size hail south of Brocton.
As the storm moved into western Edgar County, it began to rotate. Rotation in a thunderstorm is caused by wind shear and a strong updraft. Wind shear is found in the upper atmosphere where horizontal winds shift suddenly in direction and speed. If a thunderstorm grows tall enough, its updraft – the winds supporting the thunderstorm – can force the horizontal winds to spin vertically, causing the storm to rotate. If these spinning winds in the cloud connect with winds at the surface, a tornado will form. This storm didn’t produce a tornado, but a funnel cloud was spotted near Redmon.
Sunday’s storm was considered a “supercell” because of its strong, rotating updraft that generated very large hail and funnel clouds.