Choose a loose fitting outer jacket that covers to about crotch level. Wear a couple of under layers for warmth, and tuck them into your pants. Wear thin gloves with good texture for grip that will allow you to easily slide your trigger finger inside the trigger guard and manipulate controls like a magazine release. Police supply stores carry those kinds of gloves. With a good belt/holster combo in the appendix or crossdraw positions, you should be able to quickly and easily raise the jacket hem with your non-dominant hand to access your firearm with your dominant hand. It takes the right gear and practice. Big outer pockets are ideal for a holster designed to keep the firearm upright for an easy draw.

Here's some options I ran across that might be of interest: Shooting Illustrated | Winter Carry