Spider storm

Spider storm is a term for a torrential downpour that was originally coined in the Varisian city of Korvosa, where the sewers and water pipes are infested with arachnids known as drain spiders. Unusually heavy rainfall causes the water level in the sewers to rise, forcing the drain spiders out of their hiding places, and causing them to erupt in a flood from gutters, outhouses, and sewer openings. Varisian traders brought the term east, where it is now most frequently used in the lands around Lake Encarthan.