which type of sentence is this? the weather forecast had predicted severe thunderstorms, but the dark, heavy thunderclouds never materialized.

This sentence is a complex-compound sentence (also called a compound- complex sentence). It has two independent clauses joined by the coordinating conjunction "but":

• "The weather forecast had predicted severe thunderstorms,"
• "the dark, heavy thunderclouds never materialized."

And the first clause contains a past perfect verb phrase ("had predicted"). The sentence shows contrast between the forecast and the actual weather.