That's why I got my mare a blanket with zero fill (to add to her already massive blanket collection). :) That way, she can be dry and out when it's raining.
I worried about her shivering off weight because anytime it rains (which is pretty much all the time in NW Oregon) and she gets wet, it's inevitable that she'll be shivering. Usually, when she has access to shelter, she'll go stand in it until it stops raining. However, in Oregon we get days of rain as opposed to showers and since her main source of food currently is grass, I don't want her just standing in her shelter and not eating since that's what she would do. When she has her blanket on she's totally fine going around in the rain.
So why don't you get Beau a lightweight, no fill, blanket to wear when there's a chance of rain? That way, you can just stick it on him when it might rain and not worry about if it's starting to rain or whatever since he's already protected.
Fabio - 13 year old Arabian/Lipizzan gelding
Rest peacefully, Lacey.