Thank you for your thoughtful question. A narrative essay is actually one of the easiest kinds of essay to write, once you have your idea. That should encourage you about what happens after you decide on the topic of your narrative.
A narrative just tells a story. It can be something true or something you make up, and I do not know what restrictions you have about that for this assignment. I would think any narrative, real or fiction, would be acceptable, but that may not be so.
Your prompt, "the coming storm," is a good one, and your idea to use it as a metaphor is also a good one. Anything which sets up a scenario in which things are going along without any problems and then a problem arises will work for this idea.
On a humorous note, perhaps the person with the terminal illness is contemplating the mayhem that will ensue when his or her family reads the will and discover something surprising, like a family secret. Or perhaps the money (and of course there will be a lot of it) will be given to the most unexpected family member and the others will create an uproar as the one who gets the money quietly goes on to do something unexpectedly wonderful with the money.
On a more serious note, perhaps a poor farmer's crops are finally about to yield a harvest when a natural disaster (flood, locusts) strikes and the farmer has nothing left. (Think Dust Bowl--this really happened to thousands of people.)
Realistically, the possibilities are endless. As outlined in the Purdue Owl site I attached below, all you have to do is the follow these guidelines:
"The essay should have a purpose.
Make a point! Think of this as the thesis of your story. If there is no point to what you are narrating, why narrate it at all?
The essay should be written from a clear point of view.
It is quite common for narrative essays to be written from the standpoint of the author; however, this is not the sole perspective to be considered. Creativity in narrative essays often times manifests itself in the form of authorial perspective.
Use clear and concise language throughout the essay.
Much like the descriptive essay, narrative essays are effective when the language is carefully, particularly, and artfully chosen. Use specific language to evoke specific emotions and senses in the reader."
Once you decide on something that interests you (for you can set this story at any time and in any place, including a medieval castle, an alien planet, or a classroom), just decide where you want the story to go and start writing. Be consistent in tense (past, present, or future) and tell a good story. Good luck!