your on paper right? i failed a UA (not for thc) back when i was first on probation....my PO gave me a choice of 30 days in jail or a 90 outpatient rehab/group 3 times a week....took the 90 day program, i wasnt ready to get sober yet, blew it off, and my sobriety lasted one and half days...ive heard some PO's just give you a warning, some go with the outpatient rehab, or some just say fuck it, and throw your ass in jail...
best of luck....let us know how it went!