SQL Server 2012 :: Assigning A (Group ID) Based On Consecutive Values?

I have a data set that looks something like like this:

Row# Data
1 A
2 B
3 B
4 A
5 B
6 B
7 A
8 A
9 A

I need wanting to assign a group ID to the data based on consecutive values. Here's what I need my data to look like:

Row# Data GroupID
1 A 1
2 B 2
3 B 2
4 A 3
5 B 4
6 B 4
7 A 5
8 A 5
9 A 5

You'll notice that there are only two values in DATA but whenever there is a flip between them, the GroupID increments.