NOAA Operational Definitions for El Niño and La Niña, El Niño: characterized by a positive ONI greater than or equal to +0.5ºC. La Niña: characterized by a negative ONI less than or equal to -0.5ºC. By historical standards, to be classified as a full-fledged El Niño or La Niña episode, these thresholds must be exceeded for a period of at least 5 consecutive overlapping 3-month seasons. Last four ONI Index are SON 2017 -0.7ºC, OND 2017 -0.9ºC, NDJ 2018 -1.0ºC and DJF 2018 -0.9ºC. The fifth consecutive overlapping 3-month seasons ONI index is surely going to be -0.5ºC or below and hence an official 2017-18 La Nina event will be declared at the end of March 2018. It is surprising that this blog starts with these lines "The great La Niña of 2017–18 is dwindling, and forecasters expect the return of neutral conditions by the March–May season, estimated at about 55% likelihood. Neutral conditions are favored to continue through the summer."