Every 5 days, a powerful thunderstorm slams through Stormgate. These storms have been there for all of recorded history, and are localized entirely to the city area. Every dock worker has a different story to explain why.
These storms usually last about an hour, and feature heavy rain, powerful wind, and lightning. The time of day that they arrive fluctuates, but can be mathematically predicted. The Storm Wardens post the times in public places, and run bell towers across the city to give warning. The streets are flooded with rushing water that sweep away anything that isn't chained down. The city is built with a considerable drainage system to accommodate this.
Staying out during a storm can be deadly. Bell towers spread across the city ring thirty minutes before a storm begins, and within one minute of the storm's arrival. There are emergency storm shelters scattered across the nicer districts of the city where people can hunker down.
These storms have been harnessed by Stormgate's citizens to serve important functions. They've created many wind powered devices to accomplish simple tasks like grinding flour or heating water. The rainwater is also used to operate locks in the Stormway, the main canal that cuts through the city. The Arcanum is covered in lightning rods and weather vanes that they use for arcane experimentation.