Its a bit more complicated than that, you have a period during the spring called the spring persistence barrier that makes skillful prediction of a developing el nino difficult. What is impressive is that multi month model predictions before this spring barrier had a historic el nino developing which turned out to be correct (an out of sample prediction).
It's a massive oscillating system (millions of square miles), so we have plenty of lead time. It's easy to measure and watch evolve.
Its a bit more complicated than that, you have a period during the spring called the spring persistence barrier that makes skillful prediction of a developing el nino difficult. What is impressive is that multi month model predictions before this spring barrier had a historic el nino developing which turned out to be correct (an out of sample prediction).