Answer:
The tropical storm flooded half of the city and left thousands of people homeless, which the meteorologists had predicted.
Explanation:
An independent clause has a subject and a predicate. It is capable of expressing a complete thought on its own. On the other hand, a subordinate clause is formed by a subject and a verb and starts with a subordinate conjunction or a relative pronoun - in this case, "which". A subordinate clause does not express a full idea on its own, it needs the information expressed by the main clause in order to make sense.
The subordinate clause "which the meteorologists had predicted" lacks information to make sense since it is not attached to an independent clause. By adding "The tropical storm flooded half of the city and left thousands of people homeless" and connecting the subordinate clause to it, we have a complete piece of information. In this case, since the subordinate clause has the function of explaining something said in the independent one, we should separate them with a comma. The result is:
"The tropical storm flooded half of the city and left thousands of people homeless, which the meteorologists had predicted."
