mike-s,
All you have to do is read the label on the windshield washer fluid as to the freeze point. Never seen a decent brand yet that didn't give the freeze degrees point sold in the USA, including when in Florida. Using logics, you sure wouldn't buy washer fluid in Florida for use in Minnesota in the winter as it would freeze solid just being in the vehicle washer reservoir also. There's a little very important thing in life called "commonsense".
Here in Michigan, the freeze point of washer fluid in gallon plastic bottles sold does NOT change as to freeze point on the labels during the year at all. Walmart, Meijer, Target, auto stores, etc have the very same washer fluid on their shelves year around! Been buying and using the stuff for over 50 years now and never seen real washer fluid with below -20F on the label. The alcohol in it actually aids in cleaning the bugs etc off the glass much faster and aids in faster liquid evaporation leaving the windshield looking much cleaner/clearer.