I personally had a frozen pitot tube due to failed tube heating in the solid IMC (no thunderstorm though). You just disable autopilot and manually fly by the kn

I personally had a frozen pitot tube due to failed tube heating in the solid IMC (no thunderstorm though). You just disable autopilot and manually fly by the known manifold pressure numbers, vertical speed and attitude. Plus double check with GPS ground speed (estimating the wind). This is not even an emergency (you have to advise ATC about the situation though). Things fail all the time. This is why there is redundancy built into the system. A pilot just need to know how to detect the problem and how to use secondary systems.