Re: Allergic reaction to... Snow?
Doubtful if there will be spiders in the snow.
The reaction to most insects are "normally" just a red spot with maybe a bit of swelling and redness.
The reaction to just on the face and head sounds like (to me) as some sort of contact irritation but I'm not a vet.
Does she stay close when she is out during this time?
Does she have access to stored chemicals or anything that could be an irritant?
I'm not much of a help because I know you've probably gone through all these questions yourself.
The words of a fool offend only another fool!