Acepromazine does work very well, but it's a pretty heavy duty sedative so probably not something you'd want to give prophylacticly on a regular basis.

My 13 y.o. has developed thunderstorm phobia in her later years. We've been managing it with a combination of tools:

A body wrap. My husband couldn't believe it the first time I took out the ace bandages... he thought I'd finally fallen completely over the edge! But now he knows how to do it and is a firm believer. The wrap helps! I have two 4" ace bandages tied together and use the TTouch method to 'tie' front and back end together. You can find details of the method online or you can buy a wrap.

Melatonin. Has a high degree of safety, commonly used by airline employees to facilitate sleep in different time zones. My old girl gets 300 mg which is the usual dose for large dogs.

Benadryl. This is 'the kicker' that seems to boost the effectiveness of the others. Benadryl also has a high degree of safety, it's usually well tolerated by dogs and has minimal side affects (the primary one being drowsiness which in this case, is a "good thing"). Because of the safety of the drug and lack of side affects, it doesn't bother me to give Benadryl prophylactically when there's just a possibility of thunderstorms. Usual dosage is 1mg/lb body weight (up to 100mg) and it can be repeated up to 3X a day if necessary.

Management. Figure out what makes your dog most comfortable by carefully observing his behavior. Kalie heads for the far end of the powder room, a room that's long and fairly narrow and is an interior room (ie: no windows). I 'support' her choice of that as her "safe room" by putting down throw rugs (more comfortable for her than the usual bare tile) and kicking Granger out.

Your behavior. I find that I can help her get through the worst storms by being a calm, solid, presence. I curl up around her on the floor and give her long, slow, head to tail, strokes. Again, watch your dog for clues to what makes him feel more secure.