It was suggested to me to use this series for the sums I needed to get (total streamflow for every month over many many years) 

=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=2))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=3))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=4))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=5))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=6))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=7))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=8))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=9))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=10))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=11))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=12))
=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=1))
But the one that represents November in my set (code below) ... 

=SUMPRODUCT($U$2:$U$100*(MOD(ROW($U$2:$U$100),12)=12))
... is the one that will not work. I get a "0" result.

You can see an example of the data setup in the attached screenshot.

Example4.jpg