I'm wondering the same thing.
From team's Facebook:
"Regarding tonight's Nats-Braves postponement: The game will NOT be made up during this homestand. More info at nationals.com as it’s available."
PS: I can't get the team's web page to open.
No official word, but the consensus last night seemed to be toward an August make-up date - which would probably necessitate a twi-nighter like we're doing for the Marlins make-up since the Braves' August visit is a weekight (Monday/Tuesday/Wednesday) series - we have an open date on the following Thursday (23rd) but the Braves start a series at San Francisco that day so that's not an option. Sort of a pity they ruled out this coming Monday though, since that's an open date for both teams.
[I have difficulties with the club site too - like mlb.com-based sites generally it's rather 'busy' with imbedded videos, auto-refreshes and the like - still not as bad as NHL sites]
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Capital Weather Gang today has 'Daytime games can start becoming rather hot by this time of year, but not this one. Soak up the sunshine and pleasant temperatures' - with temperatures in the 70's throughout and 2% chance of rain.