Although California (especially southern) has a greater chance for good rains this El Nino winter than during an average winter, it is not guaranteed. To see how California fared during past El Nino winter, spanning from the stongest El Nino to the weaker ones, see the blog piece by Mike Halpert from last year: https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3…

Note that although most strong events do bring good rains to California, not all of them did. The chances are pretty good that at least southern California, and possibly central and northern, could get good rainfall in the coming 5 to 6 months. The Government still does not want any more water unnecessarily used (or wasted) than necessary.

Besides increased chances for heavy rainfall in California, the same is true for most of the remainder of the southern US, all the way from Arizona through Texas, through Florida. What happens in Oklahoma and Texas does not necessarily also happen in California, although both places are likely to have wet winters, or at least not dry ones. The season of most likely effect in the south-central US is Dec through March, while in California it has tended to be more from Jan thorugh mid-April, just a bit later. Some of the storms that bring rain to California may continue eastward and affect Texas and then maybe even Florida in the following days. But some storms may affect one place without also affecting the other.

Yes, El Nino involves a weakening of the trade winds along the equator in the Pacific Ocean. This has been happening quite a bit during the last couple of months. All symptoms of El Nino that are expected in the tropica Pacific, both ocean and atmosphere, are being observed. This is not a "partial" event.

Bottom line: Your prospects for a good rainy season are quite favorable this year, especially if you are in the southern part of the state, but also if you're in the central part. Just study those maps in Mike Halpert's blog (again, https://www.climate.gov/news-features/blogs/enso/united-states-el-ni%C3…).