ROANOKE, Va. – Most of Southwest and Central Virginia went unscathed Saturday afternoon even though severe weather was widespread all around the region.
Notice the lack of damage markers around our immediate area.
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There is a reason for that. It's called the "cap" or capping inversion. You hear about this weather feature a lot in the Plains during severe weather season, simply put, the cap is a sliver of warm air that hangs out a few thousand feet above the surface.
Thunderstorms locally in Saturday's were developing from the heating of the day. The sun warms the ground and then the ground warms the surrounding air. This process is called convection. When the air gets warm enough, it starts to rise.
Think of this process as you would a hot air balloon rising into the sky. To make the balloon go up, you have to pull the chain that ignites the flame which makes the balloon warmer than the surrounding air.
The balloon becomes buoyant and rises. If at anytime the air around the thunderstorm or balloon in the example becomes warmer, the storm or balloon will start to sink because it cannot pass through. We have a way to keep the balloon warmer, just add more heat, but the thunderstorm does not and therefore it does not develop.
The cap held strong locally yesterday, but broke just east of here. There was a forcing mechanism out west which help to force thunderstorms through this cap. Lucky for us and for outdoor holiday plans that most of us stayed dry, because of the cap.
The cap will be much weaker on Sunday because of overnight clouds. Temperatures typically decrease with height in our atmosphere, but on clear nights, as air escapes back into space, the warmth generated from the heating of the day will keep the air a few thousand feet off the ground warmer than the surface. Sunshine, or another mechanism is needed to get the surface warmer than the air above it to get storms to rise on their own.
With the clouds overnight and into Sunday morning we don't have as strong of a cap, but we also won't be able to generate as much instability as would have otherwise been able to with mostly sunny skies.
Even with the lack of sunshine, storms on Sunday will still have a chance to be strong or severe as a piece of upper-level energy pushes through, aiding storms in their development.
With the lack of a cap in place, storms should be more numerous Sunday afternoon and evening.
A few storms will also be possible on Memorial Day.