Severe weather could bring baseball-size hail to the Midwest

A spattering of severe thunderstorms is expected across portions of the Midwest on Thursday, particularly in Illinois, Indiana and Ohio - a region already frequented by big storms this month. A few storms are likely to be rotating supercells, with the potential for large, destructive hail and a couple tornadoes.

Hail could be scattered to widespread. The biggest hail could be the size of baseballs, particularly in north central Indiana along the Highway 30 corridor between South Bend and Fort Wayne. However, that would depend on storms remaining as supercells and not merging into lines and clusters more quickly.

The National Weather Service Storm Prediction Center is warning of an enhanced risk (Level 3 out of 5) for severe weather. It includes Peoria and Bloomington in Illinois; Indianapolis and Fort Wayne, Indiana; and in Ohio, Columbus, Toledo and Cleveland. That’s where the hail risk will be maximized.

Surrounding that is a broader slight (Level 2 out of 5) risk - Chicago, Detroit and Pittsburgh are included in that zone.

Thunderstorms are expected to blossom as early as 2 p.m. local time, quickly expanding in areal coverage and intensity. A stationary front stretches from near the Iowa/Minnesota border to Madison, Wisconsin to Lansing, Michigan. It will sag slowly south as a cold front during the late morning before crashing southward more quickly in the afternoon.

That’s part of the reason Thursday’s setup may lead to large, damaging hail, but not necessarily tornadoes. That crashing cold front may undercut storms, with cool air sweeping beneath storm bases and limiting the tornado threat. Most of the thunderstorms will end up forming north of the boundary, becoming elevated. That means they’ll ride atop a shallow lip of cool air hugging the ground, rooting themselves instead in milder air several thousand feet above the ground.

There are several additional factors that are conducive to hail-producing storms:

• Steep mid-level lapse rates. The lapse rate describes how quickly temperatures are cooling with height. Over the risk area, lapse rates will exceed 8 degrees Celsius per kilometer. (That is an informal meteorological threshold that usually signifies the likelihood of big hail.) It converts to 23 degrees Fahrenheit per mile altitude. Imagine that - for every mile one ascends, the temperature will drop 23 degrees Fahrenheit. That’s why any thunderstorms that grow tall will loft moisture high enough to quickly freeze, efficiently producing large hail.
• Storms will rotate. That’s because strong westerly winds will be present at the mid-levels. Rotating storms (supercells) are more productive hail-producers. (The rotation will be greatest at the mid-levels of the atmosphere.)
• There is dry air at the mid-levels. That seems counterintuitive, right? That dry air would help hail production. Believe it or not, if the atmosphere is too moist, water droplets coalesce and fall as fat raindrops. But if it’s drier at the mid-levels, there are smaller, supercooled water droplets that have an easier time freezing onto a growing hailstone embryo.
• And lastly, since the storms are elevated, hailstones will fall into slightly cooler surface air. That means a bit less melting on the way down, meaning more of a chunk of ice will survive to the surface.

After about 9 p.m. Central time, storms will grow upscale, eventually merging into one or more broken squall lines or an elongated west-to-east cluster. They’ll continue to move repeatedly along the instigating frontal boundary as it slowly sags southward toward the Ohio River.

Some localized flooding is possible where torrential rains train, or move repeatedly over the same areas, but that would be an isolated threat.

And what about tornado potential? The chance of any tornado activity is low, but not zero. Any tornado risk would ensue if a storm can form in the open warm sector south of the southward-advancing cold front.

There will be less of a trigger there to generate any thunderstorms. But if one can form, it would be able to root itself in a mild, moisture-rich air mass at the surface. Conditions would be supportive of a tornado threat. Most model simulations do not depict surface-based warm-sector storms, but the risk is not zero.