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.
If your dog does calm down and stops reacting to the storm, respond
with calm praise and rewards.
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.
Your use
of calm praise and even a small treat (not necessary but ok to do) is good.
Reward your dog with
calm praise or treats for maintaining a calm and quiet demeanour.