The Office of Personnel Management (OPM) made the announcement at 7:17 PM tonight. All local schools and many offices had already announced closures for tomorrow, and some of them for Wednesday, too.
OPM is being, perhaps, excessively cautious. According to the WaPo, it's the first back-to-back closure since 2003. Nevertheless, speaking as someone with kids in school and a schoolteacher wife, I welcome the unexpected family vacation.