Thanks for reading. In summary, El Niño is a large and predictable signal and is therefore its is helpful for climate forecasting but, as you also suggest, it is not the only player. The problem is that the other potentially important influences are not necessarily as predictable. I'm not an expert on US climate impacts, so I refer you to the posts by Tom Di Liberto that discuss these issues for the US and California in particular (https://www.climate.gov/news-features/blogs/enso/fun-statistics-el-ni%C3%B1o-and-california-rainfall, https://www.climate.gov/news-features/blogs/enso/no-you-can%E2%80%99t-blame-it-all-el-ni%C3%B1o), and Professor Hartmann's post, which discusses how the tropical Pacific influences the polar vortex (https://www.climate.gov/news-features/blogs/enso/tropics-prime-suspect-behind-warm-cold-split-over-north-america-during).