We really don't know when the alt season has started yet or not. Ethereum though, the prime mover in Altcoin are close to being reaching it's precious highs. But if you have to look at the dominance, bitcoin still takes over 70% so again, will there be any alt season when all the money is still on BTC.

And as far as past trends, as bitcoin narratives has shifted, this kind of correlations might not be true at all, just saying. Altcoin investor has mature as well, no longer the irrational buyers that we have witnessed in 2017.