We're receiving reports that servers on the Whirlwind and Emberstorm battlegroups, located in the Seattle datacenter, are currently experiencing downtime. These were the same battlegroups that were scheduled to recieve their local power supply upgrade today, so there may have been some unanticipated issue in the process.
Don't panic! It's likely only temporary and they should be back up soon.
We'll update you guys with more info when we get it. The full list of affected realms is under the cut -- feel free to hit us up on Twitter @wowinsider if yours comes back up and we haven't reported on it yet.
UPDATE 4:18 AM EDT:
Blizzard has posted a thread regarding the issues on the Battlegroup forums.