MLB totally screwed the pooch with this. Could've moved one of the games to this past Monday (mutual off day). Could've had a doubleheader yesterday. Could've moved to a neutral site (Marlins, Rays, Rangers, Brewers all on the road right now if they wanted an indoor venue). But now in all likelihood they'll have to play a doubleheader on Monday, then one of the teams (if not both) will have to travel to start their wild card series the next day (with a cross country flight to either LA or SD a possibility).
Only possible saving grace for the Mets is that it might leave the Braves no choice but to use Sale and Fried against the Royals (who are themselves fighting for their playoff lives) this weekend (and with the weather, that series might end up having a doubleheader, too) while the Mets play the Brewers, who might only have seeding to play for.