When forecasters issue a severe thunderstorm watch, it’s a signal that the atmosphere over a broad region has the right ingredients for dangerous storms — damaging wind, large hail, and even isolated tornadoes. It is not a guarantee that a storm will hit your exact address, but it is a call to stay alert. Across Pennsylvania, where the terrain ranges from the Allegheny Plateau to the Piedmont and the storm-prone Susquehanna Valley, these watches…