Jail time is certainly a possibility. You need to get an attorney immediately. Unless your attorney advises you otherwise, I suggest that you immediately get into alcohol/substance abuse counseling. Knowing that you are on probation, and being tested, and still not being able to stop using is a pretty good indication of some problems with substances, which you need to address for your own future well-being.
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What will happen depends on the judge. I would guess jail time is likely. You've clearly been pushing your luck and the judge is running out of other options. You clearly lack the discipline to stay sober. As my colleagues have already pointed out, you need to contact your attorney and make him/her aware of the situation. You should also get yourself into treatment.
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