If it's probable, then odds are it's going to come true. If rain is probable this weekend, you shouldn't plan a picnic.
Things that are probable are probably going to happen; they're likely. Meteorologists are always trying to figure out what weather is probable, though they're often wrong. Sportscasters and gamblers try to guess the probable outcome of big games. You can also call a candidate for a job or political office who's likely to get the position a probable — they've probably got the job in the bag.