Depends on what timescale you're forecasting.  If you're interested in seasonal (3-month) averages then CPC outlooks already leans toward wetter for the upcoming winter/spring (so is already 'forecastable' thanks to predictors like ENSO and trends).   For other climate averages (1 week, 2 weeks), it may be predictable as much as a month in advance (see this interesting paper).   With day-to-day weather, you may be able to predict it as far as 7-10 days out.  We talk a bit about these timescales and different drivers in this past blog article.