It is possible that the La Nina advisory will not pan out in the coming months -- that is, that the La Nina conditions that have developed recently will not last long enough for the event to be considered a La Nina in retrospect. The advisory is issued when the recent (past one month) conditions satisfy the La Nina criteria, which includes both oceanic and atmospheric conditions, and when the prediction for the coming months suggest a continuation of La Nina conditions. Sometimes the prediction turns out not to happen, and then in a sense we have a false alarm. Even if that happens, the current conditions are triggering some climate impacts through the teleconnections, and some of these will likely continue for a while even if the NINO3.4 warms to less than half a degree below average. The 0.5 degree threshold is a man-made cutoff, in contrast to nature, which works on a continuum. The climate prediction for winter (DJF) for the U.S. shows some familiar La Nina features, such as the tendency toward below normal precipitation over the southern tier of states, based on the expectation of weak La Nina. If the La Nina conditions dissipate sooner than expected, such as during December, these climate impacts will be less likely to happen, but may still occur weakly or in very rough fashion even if the SST is weaker than -0.5 but still, say, -0.3 or so. The ENSO condition historically shows a strong tendency toward persistence between October and December, and this is reflected in many of the dynamical model forecasts for this weak episode. By January, persistence begins decreasing. So, even though the most recent week shows a weakening of the La Nina conditions, we are not ready to say that the forecast is a bust. Intraseasonal fluctuations affect the SST and the SOI noticeably on a week-to-week timescale. But getting back to your question, yes, it is possible that we will fail to achieve the duration required to call this a La Nina, in retrospect. It may only last for 3 or 4 running 3-month periods instead of the required 5. But the collection of predictions from the models suggests it will last long enough to make the 5 required periods.