"It will probably snow tomorrow."
会 can be used as a modal verb for the future tense with some degree of certainty, i think '会' (or some other modal verbs) is mandatory after '大概'
You can translate 会 to will in this case and many others. 我会去的/I will go. 那会起作用的/That will work.
It's often used when talking about the future (possibilities). Kind of like "may" or "might" in English I believe.
Where I am from, we only use 大概 for 'approximate'. We use 可能 or something along those lines for possibilities.
Saying, "明天下雪的可能性很大" is literally: The possibility of it snowing tomorrow is very great = it is probable that it will snow tomorrow.
Don't let the strict beta test answers get you down! Keep at it (:
Came here to say more or less the same thing. In this module, they are using 大概 for the English "probably" when most Mainlanders would use 可能. I have only heard 大概 used in expressions such as 我明白他大概的意思