I find that photos sometime help. When I can I leave the pop door open and let them decide if they want to come out or not. A few years back we had an ice storm and I literally could not open the pop door without doing serious damage so they were stuck inside that day and the next.
I took this photo when it was 4* F. I know your question is snow and not cold, but cold is not really an issue for them. If a cold wind were blowing they would not be outside but on a calm day cold does not keep them inside.
My chickens do not like change. When mine wake up to a white world they are not going outside immediately. If I leave the pop door open it may take the bold ones two or three days to venture outside, some never do. I think it helps if they have a reason to go outside, like to forage.
This snow did not start to fall until after they were already outside. The change was gradual enough that they never bothered going inside. This is only an inch, not what you will be seeing, but you can see how cold their feet are and how much the snow itself scares them. Not at all. The wind was not blowing that day, it was a beautiful day.
The most snow I've seen with chickens has been 9". That fell overnight. It took them three days before any wandered outside. A couple waded through 9" of snow to check out the compost pile, they were used to finding food there. They were out of luck that day. A few made treks to other favorite spots, it was easy to follow their trails. Some even went out to wander in the snow seemingly at random but I think they were pecking at grass that was sticking up through the snow. Even the bold adventurous ones spent most of their time in the coop.
What should I do if it's too snowy for them to be outside?
Personally I give them the option to do what they want. They may be OK in your coop or they may not. There is a lot more involved with that than square feet per chicken. They may be OK outside. If it gets violent inside due to crowding some might decide to go outside.
Before it snows can you shelter the run to keep snow out? That's not just above, snow will blow in from the sides. Putting something up to block wind may be a big benefit. Mine really do not like a cold wind. From what I've heard this storm has a lot of wind with it so any wind block needs to be strong.
Perhaps you can shovel the snow out afterwards or spread bedding on top of it so they have a place to walk. Putting pallets or lumber on top of the snow to give them a place to go might be good.
I wish you luck.
