"It will be very windy tomorrow."
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.
很大 (adverb+adjective) is a viable predicate in Chinese.
大风 (adjective+noun) is not a viable predicate in Chinese. Since you have a noun in your predicate, you need a verb (that noun needs to be the object of a verb). 会 is only a modal verb and does not count as the "real verb" of the sentence. I does not take an object. That's why the second sentence needs 刮 "to blow". 风 is the object of 刮.