Atmospheric rivers are driven by various weather and climate phenomena and are not always related to ENSO.  El Nino can help increase the occurrence of them, but that is, by no means, guaranteed.  Our ENSO outlooks are probabilistic and each month we offer updated chances for an El Nino event, which we also discuss here on the ENSO blog.  https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/ensodisc.shtml