One of the difficulties of forecasting seasonal temperature out West is the prevalence and dominance of the "trend," which favors warmth (some combination of decadal changes and climate change).  As you can see in these historical ENSO composites of temperature ( https://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/composites/ , click El Nino temperature and toggle over AMJ, MJJ), you can see the trend clearly favors above-average temperatures, but, historically, El Nino runs counter to that, helping to drive below-average temperatures over the Southwest and north-central US.  In the last month (May), we've definitely seen some of those characteristics, so it's possible that El Nino is helping to drive some of the wetter, below-average temperatures in those regions.  There are possibly other weather and climate factors too, so hard to say definitively without more focused research.