Some cats are very fearful of the noise made by thunder and lightening storms. Make sure your cat has access to her/his refuge when it is storming outside. Play a radio or TV to help cover the noise of the storm. Provide hiding areas that will allow the cat to escape the sound. For example, leave a closet ajar so the cat can hide inside. If you are home during the storm, sit with the cat. Try to engage the cat in play to distract her/his. Reward her/his with treats and praise to help create a more pleasant association with storms.
Cats that are extremely agitated or wild during storms may need to be confined to a small space such as a cat carrier or bathroom. If your cat fits into this group ask your veterinarian about medication that can help.