"It will be very windy tomorrow."
in the previous lesson I just saw 现在的风很大 for "it's very windy now", but here, it requires 刮 to mark the solution as correct (I wrote 明天会大风 and it was marked wrong). Why is that?
Pleco seems to suggest that: 风很大：wind is big (ie windy) 刮大风：wind is intense (ie gale force wind)
Im not sure though.
I think it is that you have no verb in the sentence (you need 刮 as the verb). You have 会 to indicate that something will happen but have not followed it with the verb that describes what will happen. In the first sentence you gave, the verb is implied, as you are just describing the noun 风 by following it with qualified adjective 很大.