1. "мне нужно будет уехать" or "мне понадобится уехать". 2 is incorrect. Maybe you meant "мне нужно уехать"? But it is the same without "will". 3. is correct but it has additional shades. Imperfective уезжать can either mean - repetetive action ("всякий раз мне нужно будет уезжать", "мне нужно будет уезжать каждую пятницу") or - progressive action, similar to English "I must BE GOING now". When it si progressive, we say "мне нужно идти" in present to specify two things: 1. it is already the time to do it 2. it is continuous process including preparations. we say "мне нужно уезжать" in present for the same purpose,. though for "уезжать" it is not necessary "this minute" it can be "today" or "this week", for example. "Уезжать" is a more serious process, it can be moving to another country in some contexts:) we say "мне нужно будет идти/уезжать" in future, when speaking about particular moment or situation: "если [похолодает/к власти придёт Трамп] мне придётся уехать" Here I inform you of the fact itself: if [it gets colder/Trump wins] then I will have to go. "если [....], мне придётся уезжать". Here emphasis not on the result (I'm in another country) but on the process: I learn that Trump won elections and more or less the same day/week/month I'm packing my bags and going. That is, I'm emphasizing either urgency or the process (which can be, say, tiresome). It is properties of continuous/progressive aspect: it is attached to some moment and it is developing in time.

Большое спасибо за объяснения!

We sometimes prefer such forms because "I" is not really the source of necessity. Logially, I'm not the "actor" with respect to need/necessity, but rather a patient:) For exmaple, "я хочу" and "мне хочется" mean almost the same (I want) but the second is a bit less deliberate, like you _feel_ an urge to do something.

Rebecca, the future tense of уехать is, depending on person (я) уеду, (ты) уедешь, (он, она, оно) уедет, (мы) уедем, (вы) уедете (они) уедут. But here you don't need future tense of уехать. You need the future tense of "нужно". "Нужно" literally means "it-is-needed". "Мне нужно..." means "I need", but very LITERALLY it is "to-me it-is-necessary...". Even though we don't say "it" and "is", but they are implied. Comapare: "сегодня солнечно" - it is sunny today. So the future tense is "будет нужно" (it-will-be-needed). Уехать doesn't change.

Спасибо за ответ! Is the future tense of уехать "будет уехать" or "уе́детs"? I see the conjugation table in this link here: http://masterrussian.com/verbs/uezzhat_uehat.htm