It is sort of a regular tradition for Blizzard to take their servers down early each Tuesday morning. As in today's Tuesday morning.
The downtimes come in two flavors. Typically, it is down for the whole morning and if you are on the east coast that drags on to almost the mid afternoon. Sometimes, you are lucky and Blizzard serves you just 15 minutes of downtime because all Blizzard wants to do is reboot each realm.
Unfortunately, today was not one of those days. Blizzard wanted to ready their servers for a new patch. So they took their servers down for a number of hours.
