Yes they can be.
Thunderstorms (Cumulonimbus clouds) contain lightning, precipitation, hail, extreme turbulence, supercooled water droplets, ice, microbursts and violent winds, all of which can be hazardous to commercial passenger jets. They have caused passenger jets to crash before, and could possibly be a contributory part in a crash in the future.
Lightning
Commercial aircraft usually get struck by lightning a few times a year. The damage the aircraft receives from a lightning strike varies. The structure of the aircraft is designed to dissipate the electric charge overboard, but the entry and exit points can cause damage to the aircraft’s skin. It can also potentially interfere with the aircraft’s electrical systems but it rarely causes any significant problems.
Icing
Severe icing is dangerous for commercial aircraft. Ice forming on the aircraft structure increase the weight of the aircraft, which in turn increases the stalling speed. It also disrupts the airflow over the wings, reducing the total lift produced by the wings. Lift created by the wings counteracts the weight of the aircraft which is what gets, and keeps the aircraft airborne. Most commercial aircraft are certified to fly through light to moderate icing but not severe icing. Pilots will therefore always look to avoid areas of severe icing.
Hail
Large hail stones found within large thunderstorms cells can cause structural damage to the aircraft and its engines. They have been known to cause damage to the leading edge of the wings and nose cone as well as crack the windshield and damage the engines.
Updrafts / Downdrafts (Wind Shear)
These can disrupt the aircrafts flight path and airspeed, potentially causing the aircraft to overspeed (go to fast) or stall (go to slowly). It can also push the aircraft towards the ground .
Microbursts
Microburst is a phenomenon found underneath a Cumulonimbus cloud, where a strong downdraft causes a large change in wind direction over a small area. An aircraft flying through a microburst will likely see a large increase in airspeed followed by a dramatic reduction. If the aircraft is close to its landing speed at this point, it’s flying close to its stalling speed. A sudden and dramatic reduction in airspeed is very dangerous to any aircraft.
Microbursts also have the potential to cause significant downdrafts which ‘push’ the aircraft towards the ground. In extreme circumstances, the force at which the aircraft is pushed downwards is more than the thrust force produced by the engines and the rate of descent can’t be arrested.
