1. CPC partly defines a specific ENSO phase based on a one-month, persistent SST anomaly of -0.5 degrees C (in the case of La Nina) or lower in the Nino-3.4 region, or a +0.5 degrees C (in the case of El Nino) or greater in the Nino-3.4 region.
2. CPC uses a tool called the Optimal Climate Normals (or OCN) to compute recent trends in temperature and precipitation. The OCN is based on a difference between recent temperature and precipitation and the currently valid 30-year climate normals (1981-2010). It's used to adjust the typical seasonal ENSO composite probabilities of temperature and precipitation for recent trends, like as shown in the bottom two graphs for each season here:
http://www.cpc.ncep.noaa.gov/products/precip/CWlink/ENSO/composites/
3. The three terciles in CPC's extended range forecasts are based on conditional probabilities for above, below and near normal (or median if dealing with precip.). While all three elements are equally weighted (33.3% for above, near, or below), in shaded areas on the forecast charts, the conditional probabilities are higher toward the color code (so red shading would have slightly higher conditional probabilities for above-normal temperatures, etc...).