The heart of this system will pass well north of St. Louis. The greatest potential for severe storms (wind, hail and tornado threat) from far northern Missouri up into Iowa.
Most of the viewing area will see little more than an uptick in clouds during the afternoon on what will be an otherwise warm, windy and dry day. However, as the tail end of that disturbance brushes our region during the afternoon a very narrow ribbon of isolated storms may try and develop in a north / south band.
If the storms develop they will zip rapidly across the area sometime between 3pm and 7pm. Because of the set-up in the atmosphere any isolated storm could produce gusty winds, hail and even a very low potential for a tornado (but not zero).
Again, I have to emphasize that at this point it looks like most of the action will remain just outside our viewing area to the north with only a small chance of an isolated strong/severe storm developing southward into our area during the afternoon or early evening.
Thursday also looks mainly dry but chances for more widespread rain/storms will ramp up quickly Thursday night into Friday.