Also, should a really big area of T-Storms form rapidly, sometimes you can find yourself in the middle of what was a clear path, only now you are surrounded by huge CBs (cumulo nimbus clouds).
My first lightning strike was in a cloud (at night) that the radar was only showing as green (light precip.). After the lightning it got pretty rough. We figured we must have run into a cell that was just going from build-up to mature stage.
Onboard weather radar is a useful tool, but certainly has its limitations.
Q