"It will be raining tomorrow."
Translation:Estará lloviendo mañana.
What do you mean? Do you want to know why "Va a llover mañana" is incorrect? As jindr noted, it's the wrong tense. That's the phrasal future, whereas the English sentence uses the future progressive tense (I believe I have the right label). To form that in Spanish you use the future tense of estar with the gerundio.
Also, DL never explains.
Because that's a different verb tense. Please note that the point is not to teach how to say weather forecasts. It's to show different sentence structures, part of which involves using verb tenses that may seem unusual in a given context.
Suppose you needed to say, "She will be running tomorrow" (i.e., replace "it" with "she" and replace "raining" with "running"). That's an entirely different sentence using the exact same construction. It wouldn't help to know only standard weather forecasting statements in that situation.