Metro has just announced that they will discontinue above-ground rail service at 1pm today, until the snow has stopped. That will close 39 stations until the snowfall has been managed.
In addition: Metrobus and Metro Access will stop running at 1:30pm.
The reason for the closures at 8″ of snow is that if the third-rail is covered, it can no longer transmit the necessary running power for the trains, meaning you could get stranded by the snow. Once they’ve cleared up the snow, they’ll return to normal service, but I’d expect that to be tomorrow at the very earliest. You can read more about Metro’s snow policies on their site.
Steven Taubenkibel of Metro has just said that road conditions are far too treacherous to continue service. For the remainder of the day, they’ll run empty deicer trains on the above-ground tracks, so you may still see trains on the tracks, but there will be no service there. They are emphasizing safety, asking people to stay off the roads so they can be cleared for service.