Thunderhead goes by many names. Most of them are not repeatable in polite company.
Thunderhead is a massive perpetual storm that travels the breadth of the Empire, with one exception. It has not visited Throneseat in a thousand years. None have been able to accurately predict Thunderhead’s path otherwise, but it is often slow moving enough to get several days warning of its approach. It has been known to harass the Maze That Walks more than would be expected.
The coming of this storm is usually a brutal blizzard with several feet of snow, interspersed with sleet and hail. A community that is unprepared for Thunderhead is usually doomed, and even those prepared have been known to be lost to the snow.
Legends say that Thunderhead is actually the home of a kingdom of giants.