The brewing storm is actually a place of interest called Dwarf’s Bane. This ritual site is positioned just south from the Nchuthnkarst public dungeon, east of the southern part of the Blackreach cave system. As you get closer, you’ll spot a small stone pathway with some posts. Follow it and it will take you to the larger plateau where a storm is brewing during the ritual.
Once you climb the plateau at the place of interest and take a few steps inside, you’ll be able to see the skill point shard after rounding the large boulder blocking the view to the east. It is on a small rock, next to some glowing crystals, mushrooms and other cave flotsam and jetsam.