[GWN7]

Call an electrician. As stated the middle feed is arcing. Look at the metal filings on top of the breakers from the short. The same is happing to the metal bar in the back the screw mounts to. It might be welded to the screw or will break when you try to undo it. If you try to do it hot you might end up with no power at all if things at the back of the box are screwed up.

When you get it repaired build a wood box to cover the breaker box from the weather. This will stop any direct snow/water/debris from coming in to contact with it.