でしょう
(丁寧) and だろう
(普通) can be translated as "probably". So 明日、雨が降るでしょう is translated as "it will probably rain tomorrow". In this sentence the speaker is almost certain that it will rain tomorrow.
For levels of certainty you could refer to http://guidetojapanese.org/learn/grammar/certainty
Edit: about 降っているでしょう
Someone pointed out that 降るでしょう
should be used instead of 降っているでしょう
, but I would like to say that 降っているでしょう
is not quite wrong, however, Japanese people usually don't use. I have asked my Japanese teacher and this is a summary of what I have received:
- 「明日は雨が降るでしょう」:It will rain tomorrow.→シンプル
- 「明日の18:00ごろには雨が降っているでしょう」:It will be raining around 18:00 (Because the weather news said it will rain from 17:00 to 21:00). →18:00には雨が降っている予定です。
Long story short, try to stick to 降るでしょう
and you are safe ;)