I agree with those saying that just showing up, regardless of how good the intentions, can be a big problem and maybe even make a bad situation worse. I'm guessing that there are a lot of restricted roads and even finding a motel room anywhere close, could be an issue (possibly taking one away from someone who needs it).
We trust these people and after a catastrophe, they usually have a page that show the best charity organizations for that particular catastrophe. Here's the one for the recent disasters.
"Highly-rated nonprofits providing relief and recovery to communities impacted by Tropical Storm Marco and Hurricane Laura."
Tropical Storm Marco and Hurricane Laura : Charity Navigator