It's not about forecasting the weather: good gas peakers can respond in under 10 minutes. I meant predictable in the sense of day/night/season.
http://www.hellogrid.com.au/energy-exchange/1117/
As solar utility scale sites become more common (and they are), a cloud growth can wipe 100s of MW off the grid very quickly and the problem is it is very hard to tell how much will get wiped off and when (measured in minutes).
Gas will be in the mix for longer than other fossil fuels due to its reaction time but as storage becomes cheaper and gas more expensive, it will be used less and less IMO.
To illustrate, storage is already really cheap in practice thanks to Pumped-storage, we just have really sort supply of locations for such plants.