SUM Values based off Lookup Table

I'd like to SUM all values in DATA TABLE col_b where DATA TABLE col_a has a value of MED in the LOOKUP TABLE.

For example using the tables below..
I want to sum col_b in the data table for all Cat# that have a lookup of HIGH. This would be Cat1, Cat2, and Cat5. Thus, equalling: 5+3+9+8 = 25

Any ideas on how to get this to work?

LOOKUP TABLE
    col_a    col_b
    Cat2     HIGH
    Cat5     HIGH
    Cat3     MED
    Cat4     LOW
    Cat1     HIGH

DATA TABLE
    col_a   col_b
    Cat1     5
    Cat1     3
    Cat3     8
    Cat1     9
    Cat5     1
    Cat4     3
    Cat2     8 
    Cat3     3

Thanks,
JC