When you release your dog from the stay, offer lots of calm praise and perhaps even have your visitor casually hand or toss a treat to your dog. (tulsapetsmagazine.com)
If your dog does calm down and stops reacting to the storm, respond with calm praise and rewards. (thesprucepets.com)
With any of these options, make sure you offer calm praise when he is allowing you to do it, so he understands this is what you want. (dogtrainingbasics.com)