Learn French: Les prévisions n'ont pas été bonnes.

Using the passé composé (ont été) treats the bad forecasts as a single, completed event with a final result—for example, looking back at a specific weekend that is now over. The imparfait (étaient) would be used to describe an ongoing background state at the time (like saying "The forecasts were bad, so we decided to stay home").

Where does the negation (ne ... pas) go when using the passé composé?

In the passé composé, ne ... pas surrounds the helper verb, which in this case is avoir (ont). It does not go around the past participle (été). Because ont starts with a vowel, ne shortens to n', giving us n'ont pas été.

Why is bonnes spelled with a double "n" and an "es"?

It describes les prévisions (the forecasts), which is a feminine plural noun. To make the masculine adjective bon feminine, you double the n and add an e to get bonne. Then, you add an s to match the plural noun.