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The answer is a Complex sentence

Explanation:

Complex sentence is the type of sentence whereby there is atleast an independent clause and one or more dependent clause. since the independent clause can stand alone on its own, it makes it easer to separate from a dependent clause.

From the statement given above, The weather forecast has predicted severe thunderstoms is an independent clause which stood on its own. The dependent clause, the dark, heavy thunderclouds never materialzed is being linked together with an independent clause by a linking work but.