Blizzard has announced their newest limited-time event Storm Rising starts April 16 and continues through May 6. A tweet on Overwatch's main page first detailed the event with a new trailer.
The trailer contained decrypted audio for an archive mission briefing. The audio ends with the voice identifying themselves as Sojourn. Whether or not, Sojourn is an upcoming playable hero is unsure but it seems she'll play a substantial role in the upcoming limited-time event.
Update: Blizzard has now confirmed further details about Storm Rising. It will indeed take the form of a special co-op PvE mode for four players, who will choose between Tracer, Genji, Winston, and Mercy. They'll take on Talon operative Maximilien in Cuba. There will be a ton of new cosmetics available to earn, presumably including new skins you'll be able to collect through loot boxes. If you're not taken with the new mode or simply miss those from the past, both the Uprising and Retribution modes will also be playable during the event.