Similes and metaphors (and other types of description) are best when they come to us spontaneously. If we have to mull them over to find one that works, there is a real possibility that it will ultimately come off as forced to a reader. Sometimes it's best to just skip the fancy description and move on.
However, if you are hell-bent on using a simile, think of an emotion (or emotions) that is/was triggered by the approaching storm or whatever event is dominating the situation. Fear? Anxiety? In your searching, find a simile that also plays to that background emotion. If you are going to insist on a simile, why not make it do double duty? It certainly will help set the tone of the scene, and it might make for some very fluid writing in a situation where a forced simile may stick out like a pube on a bar of soap.
