Currently I am trying to update the Forecast Category that is standard in SFDC when the % changes on my Oppty.  I cannot seem to reach the field with standard workflow so a field update will not work.  Does anyone have any triggers that may be able to help with this issue?  I would hope it would run something like

If Probability is changed Forecast Category updates to x

I have an idea of how I want this to work but any additional help would be awesome!  All standard fields