This afternoon and evening we are tracking strong to severe storm chances which is why we are under a Weather Alert Day. The main driver of this storm development is a cold front that's passing through Missouri in the afternoon and evening hours.
Whenever we are experiencing hot and humid conditions like mid-Missouri is currently, a cold front is the perfect driver of storm systems. For starters, moisture is a key to experiencing more well structured storms, and we have that criteria met. Instability is also needed, which Missouri will also have by the time the cold front moves through. This is in the form of the air near the surface being warmed or "excited" by daytime heating. The final factor we need is called lift, which is the mechanism that forces this more excited and moist air into the upper atmosphere. This is where the cold front comes into play.
The cold, dry air behind the front is much more dense than the warm, moist air ahead of the front. So, as this cold front moves into the warmer atmosphere, it "lifts" this warmer, more excited air into the atmosphere. As this air is forced up, it reaches a point where it's able to move freely. This is where we see large Cumulonimbus growth, and more importantly, thunderstorm development.