Kate Chopin's use of the storm in the story is meant to echo the violent, sudden outpouring of passion between Calixta and Alcee. Storms are a force of nature, and the mutual attraction that these two people share is similar in that it is wild, natural, and uncontrollable. Storms can move in quickly with little warning, and that is what happens with both the storm and the eruption of sexual energy in the two characters. They have a history of sharing pent up passion that did not experience a release, but in this meeting, there is an eruption of what had been suppressed.
Once the storm outside is over, the air is cleared of its extra energy with no apparent harm to the environment. Likewise, Chopin leaves no suggestion that the act between Calixta and Alcee will be repeated or that it will negatively affect either of their lives.