The skill of many of the model forecasts for Nino 3.4 is available from the providers and generally shows a root mean square error of about 0.5C for forecasts looking out a couple of months, with a correlation of over 0.8.  Here's a link for the skill of the CFS, available from the CPC webpage:

https://www.cpc.ncep.noaa.gov/products/people/mchen/CFSv2HCST/metrics/r…