ST. LOUIS, MISSOURI — Isolated severe storms are possible from Missouri to Illinois Wednesday, August 5, with St. Louis, Springfield, Missouri and Peoria falling under a Marginal Risk, Level 1 out of 5, according to an outlook updated at 3:53 AM ET Tuesday, August 4. An upper-level disturbance will trigger afternoon storms across the region, with initial isolated cells capable of bringing localized instances of large hail before organizing into clusters capable of producing damaging winds into the evening.
Large Hail Expected First as Isolated Cells Develop
Forecasters say decent shear and some instability will support initial isolated storm cells developing during the afternoon, with these early cells capable of producing localized large hail before any broader organization occurs. The Marginal Risk corridor stretches in a narrow band from near Cedar Rapids and Davenport, Iowa southeast through Peoria, Quincy, Springfield, Illinois, Columbia, Jefferson City and down through Springfield, Missouri.
This corridor represents where forecasters currently expect the best combination of instability and shear to support organized severe weather development Wednesday afternoon and evening.
Storms Organize Into Damaging Wind Clusters by Evening
As the afternoon progresses, forecasters expect the initial isolated storm cells to organize into clusters capable of producing damaging winds as they track into the evening hours. This evolution from isolated hail-producing cells to wind-driven clusters is a common pattern when an upper-level disturbance interacts with a moderately unstable airmass, and it means the threat profile could shift as the day goes on.
Communities including St. Louis, Springfield, Missouri, Peoria, Quincy and Columbia should be prepared for the potential of both large hail earlier in the event and damaging wind gusts as storms cluster together later in the afternoon and evening.
Timing for Wednesday, August 5
The outlook is valid for Wednesday, August 5, with the upper-level disturbance expected to trigger afternoon storm development across the highlighted corridor. Areas farther east, including Chicago, Indianapolis and Louisville, sit outside the Marginal Risk zone but remain under a general thunderstorm designation…