Thanks Jeff, sometimes storm and thunder phobias can seem to appear from thin air or some traumatic event that may have slipped under the radar. Some theorize that it's the drop in barometric pressure or a static charged field that these dogs are particularly sensitive to. In any case here are a few recommendations.
1. Try high value chewies- stuffed Kongs, marrow bones. I have one colleague that found her Collie did quite well when she ignored him and gave him a pizzle or bully stick to chew on.

2. There is something out there called the "Storm Defender Cape" (no kidding).
And basically you put it on the dog and in many cases it makes an immediate difference (reduces the static charge I believe). Many dogs will try to "ground" themselves in bathrooms- shower stalls or tubs, and it's thought that this is because this is the place in most homes where the electricity is grounded.  

Another thing to consider is an "anxiety wrap" (also a dog product) or a tight fitting garment on him. This in some cases helps release endorphins and is akin to a swaddling a baby in a blanket and may have some calming benefits.

3. You can try desensitizing him to tapes of storms (play on low for a short period of time and feed treats). Another thought is perhaps consulting with your Vet for medication that may be helpful in acute situations (several rainy days back to back in the spring, for example).  

 Hope that helps! And here's to blue skies!