Maybe if you make it a happy special thing on rainy days it would work. I used to save a special treat for Crystal when I left since I knew it was hard for her. It got to the point where she just couldn't wait to see me leave
Hard on my ego but it worked. hahaha.
Cheese a tiny piece of cheese works for almost anything.
and I wouldn't give it to her until the end of the walk. She can see it and her mind will be on that until you are back.
Don't make it too long a walk.
My friends have a dog I take care of now and then and she was so frightened of thunder and lightening. I played the radio and sang, ignored it and did the thing with cheese and then later no cheese until now, she still doesn't like it and jumps on my lap but no shivering or crying and I tell her what a big girl she is.