"Downpour" is a noun. The verb is "to pour." Oddly, you can't use "downpour" as a verb, and you can't use "pour" as a noun. Right: "It is pouring." "It has started to pour." "It is a pouring rain." "It is a downpour." "It is pouring down." Wrong: "It is downpouring," or "this is a pour." The second sentence is wrong, and the easiest way to fix it is to add a subject for the verb "need," such as "I need" or "We need" or "You need," The tenses in the two sentences don't match. The easiest way to fix this problem to put everything in the present tense. "It's pouring! We need to close all the windows to prevent wind and rain from blowing in!" This is also correct, but has a slightly different meaning: "It has started to pour! We need to close all the windows to prevent wind and rain from blowing in!" That carries the idea that they left the windows open while the rain was light. Now it has started to pour and they need to close them. If we don't want to say who should close the windows, we can use the passive voice. "All the windows need to be closed to prevent wind and rain from blowing in."

It started raining, a heavy downpour. We need to close all the windows to prevent the wind and rain from coming in.