The NOAA home page has a prominent announcement
“Parts of the U.S. Government are closed. This site is being updated, however, only with furlough-related information to support NOAA employees; NOAA websites and social media channels necessary to protect lives and property are also being maintained.”
Apparently the need to protect lives from the threat of icing was not considered, when they decided to shut off the distribution of upper air data.
I’m completely sympathetic with unpaid employees, but I have to wonder about the choice of not distributing potentially life-saving data that the models keep churning out.