Instability.

We talk about it a lot. We talk about as if you know what it is. But you may not know, we may be assuming, and you may be lost.

So let’s explain. First…a definition from the officially official experts:

The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather – the greater the instability, the greater the potential for severe thunderstorms.
-NWS Glossary

The basics of instability is that warm air in the atmosphere tends to rise, so when especially cold air is above warm and humid air — expect instability.

Here’s a video to explain it deeper.