Popping on here to comment that this seems like a pile-on. (Having been the target of one years ago, I easily recognize it.)
OP - your BM was overreacting - not sure whether there was some underlying reason for her scorched earth policy?, or whether you just caught her on a bad day?
Based on what you explained: been there a year, had a good basic working relationship with the BM, paid board on time, cleaned up after yourself, helped BM with various barn/farm tasks, never asked that your horse be held for vet/farrier, etc. - this response by her is extreme.
One thing you could have done differently (which has been covered by other posters) is to have couched your text in a question: “It’s storming heavily right now so I’m concerned for my safety and Dobbin’s - I brought him in early, hope this is okay? But wanted to give you a heads up that he’s in” Later, you could have explained that when you are there and it’s storming badly, you would prefer to bring him in; this really just saves them the labor (and you can pick his stall) - I can’t imagine that your BM would have a problem with this…
I’m happy to hear that you found a better barn so quickly, and that you are out of there. It sounds like your BM is a bit thin skinned and reactive, but maybe she was having other stressors and you just happened to be in the line of fire.
Caveat: YES!, horses are out in storms all the time without shelter, and survive! Mine are and have been since during the almost 12 years I’ve been at my boarding barn. All I can do is not think about it and hope for the best. That said, if I wanted my horse (the one on stall board, not the one on field board) to stay in on occasion (she’s out at night and in during the day 365) my BM wouldn’t have a problem with it. And she’s a blunt, straightforward type whose bad side you do NOT want to be on.
Best of luck to you, OP!