SSRS Matrix row sort

I have looked at multiple posts but haven't been able to get a working solution.

I have the following matrix:

Matrix:

Risk                         9/2013          10/2013       11/2013     Total

Critical

High

Informational

Low

Medium

Total

I need this to sort Critical, High, Medium, Low, Informational.  Can somebody tell me what the Row Group sorting expression for this would be?

I tried different variations of iif and switch statements but I can not get anything to work for the life of me.

Thanks