I have a 2year old black Lab called Roo, and she is terrified of going on her walk (of which she has ALWAYS loved) in the rain. My younger brother took her out last week and it chucked it down with rain and there was thunder and lightning on the cliff top... Roo has always hated thunder and lightning... however she has never before experienced it on a walk. As my brother was meerly wearing a jumper and Roo was pulling like crazy trying to run home, he started to run home... though as Roo was pulling so crazily he slipped over. I think this was probably an extra fright for her. I then took her out when the rain had stopped as she hadn't really been on her walk. Once we got passed the bit she heard the thunder and lightning she loosened up a little, and started chasing her ball again. However the minute we turned around to head back she freaked out again, walking by my side off lead. She wouldn't run for the ball anymore, and kept checking the sky in fear.
As we hardly ever get thunder and lightning I thought she would be okay. However my mother took her out this morning and it was chucking it down with rain. She was completely terrified and wouldn't run for her ball. Mum decided maybe it was because she was associating it with that particular spot, so she walked her up the opposite direction, however she was still pulling like crazy and clearly just wanted to go home. So now every time it rains I think she thinks there is going to be thunder.
Please can somebody help?
Thanks x

My suggestion would be to try playing some games in your backyard with a hose or sprinkler... just to get her comfortable with the sensation of falling water and try to turn it into a fun game.
Once she is happy playing under a sprinkler and having fun try introducing some loud noises to replicate thunder... all while keeping her happy and distracted.

I have never had to do that, it just seems like the logical process to me, but if it's raining, me and my dogs stay rugged up inside :D

The dog is already afraid of loud noises, such as thunder. Her going on her walk, and what she fears all of a sudden came after her while she is on her walk, probably caused her to be more afraid of thunder and thunderstorms.

Dogs can sense a thunderstorm long before a human can.

What you can do... and its similar to what shaz suggested... Play thunderstorm videos in the home. While doing this.. Play with her. Interact with her posetively. Whatever you do.. Don't "feel sorry" for her. Its human reaction to want to cuddle and make something feel good while frightened. In the dogs world... That type of reaction is enforcing the fearful behavior "its ok to be afraid".

Take her on walks. Act as if nothing strange is going to happen. And if it does? Oh well. Still act the same way. She is going to really watch your reaction. If your facial expressions are worried or your tone of voice raises, she's going to go into panick mode again. So try your hardest to maintain your cool when a storm comes through.