I think sjlkh may be incorrect here. If you say ¿Qué es el pronóstico del tiempo?" you're asking for the **definition** of a weather forecast. Thus, an answer would be "A weather forecast is a prediction of what the weather will be like in the future." If you're not asking for a definition but rather for the specific forecast (e.g., tomorrow it will be sunny with highs in the 70s), you'd use "¿Cuál es el pronóstico del tiempo?"
Here are some links to discussions of the difference between Qué and Cuál:
The last one should be all on one line, but the forum's feeble interface broke it up.
I hope this helps.
Cuál es el pronóstico del tiempo