Why do forecasters track solar-wind speed and density?

Answer: They affect energy input

They affect energy input ✓ — Correct! Solar-wind speed tells forecasters how fast the stream is arriving, and density tells them how much material is packed into it. Faster and denser solar wind can push harder on Earth's magnetic field and carry more energy into near-Earth space. That does not guarantee a spectacular aurora by itself, but it does raise the chance that geomagnetic activity will intensify.

They predict cloud cover — Wrong. Cloud cover belongs to ordinary weather forecasting, not space-weather forecasting. You need both forecasts for a successful aurora night, but solar-wind speed and density do not tell you whether your town will be cloudy.

They set the moon phase — Wrong. Moon phase depends on the positions of the Earth, Moon, and Sun. Solar-wind conditions do not control whether the Moon is full, half, or new.