Force storm was a Force power that presumably combined Force Whirlwind and Force lightning to create something akin to a dry tropical storm. It was most likely intended to be used in concentration in a small area.
Also known as "Summon Storm," this power telekinetically manipulated the nearby atmospheric conditions to whip the weather around the user into a frenzy. A skilled user could create storm conditions in areas up to several kilometers in diameter, lasting for hours at a time.
Sheets of rain reduced visibility and thoroughly soaked anything in the area, while driving winds whipped foliage and hurled loose objects about. When combined with the random lightning strikes, the storm could be hazardous enough to ground flying vessels and dramatically interfere with any combat or other activities in the area.
Obviously, this ability could only be used outdoors, and was constrained by the local climate. Though a user could not conjure a rainstorm in a desert, a dangerous sandstorm was certainly not out of the question.
While not inherently a dark side ability, it was most commonly used by darksiders; such as Sith and the Nightsisters of Dathomir; due to the power's wide-ranging and indiscriminately destructive nature.
Some versions of this power were much less damaging than Force lightning.