Brakes squeal

normal .just pump your brake while backing out to clear the rust on your rotor

 

Is it high pitched, and only for the first few minutes, backing out when just starting up? We've had that, noticeable but not "horrible", comes and goes.

 

when your car sits, the brake rotors rust. when you back up, the pads scrape the rust off the rotors.
the reason it doesn't happen when driving is that the motor generator does a lot more of the braking than the brake pads down to 6 mph when the pads take full control.
my car has been sitting in my moist garage for 5 weeks. the sound is brutal and the pads were stuck to the rotors when we got home. i had to gun it in reverse to break them free.
the easiest thing to do is throw it in neutral once in a while in a deserted area. this disconnects the regen system, and allows the pads to work. then gently apply the brakes to clean the rust off the rotors.
be careful, when you're traveling 30-40 mph, and you put in neutral, the car really jumps forward into a true glide as the regen is disconnected.

 

You did write squeal.
Yes cold weather and rust.
Plus some vehicles do squeal when reversing, but no squeal in forward.

I'd think normal.

You can try this to re-bed the pads.
Drive 75 mph and grab the brakes hard utilizing ABS to 55 mph.
Let the brakes cool for a mile or two.
Repeat.
Do this four/ five or so times.

If race car...remove your calipers flatten the pads on a machinist tables, chamfer the pad edges, polish the retainer pins and deglaze (or re-ture) the rotors. Refer to Toyota re-grease specified components and for anti rattle chemicals and where to place.

If not a squeal, since you mention winter and snow, maybe you have a rock or debris trapped between the pad and rotor.
Got to remove the caliper to inspect.
That would make a horrible noise...not a typical squeal.

Good hunting.