According to the newest
However, similar or colder average temperatures have to exist in the equatorial Pacific for five consecutive overlapping three-month periods for the La Nina-like conditions to be declared a La Nina episode.
Although models are split, I think it is more likely than not that the conditions will actually become a La Nina episode. Note that the last complete 3-month period (MAM 2010) was still safely in the El Nino range, +0.8 °C!
The expected cooling may help to keep the average 2010 temperatures below those of 1998.