"La Nada" prevails for now - La Nina likely by fall
Posted July 25, 2016
Not so long ago now, we had a very strong El Nino pattern in place, with well above normal sea surface temperatures near the equator in the central and eastern Pacific. However, those temperatures ramped down steadily in March through May and we have since been in a "neutral" pattern that falls in the between El Nino and La Nina (hence the unofficial "La Nada" in the headline).
It isn't always the case that the Pacific temperature patten simply oscillates between one of those extremes and the other. Occasionally, the neutral pattern will persist for an extended time, or will return to the most recent extreme. It is more common, though, for a gradual transition to occur, and for now most computer models and experts on the Pacific pattern are leaning toward the formation of a weak to possibly moderate La Nina pattern, most like developing somewhere in the August to October time frame. The graphic here shows the probability of El Nino (red bars), La Nina (blue bars) and a Neutral pattern (green bars), and you can see that while confidence about La Nina remains modest, it is considered to be a little over 60% likely by December or so, while the likelihood of a neutral pattern by then is only about 35%.
While all of this can exert an influence on large scale weather patterns that can affect what happens here, a lot depends on how strong the potential La Nina would become and how fast, and how it interacts with other patterns and smaller scale features. One influence La Nina is known for is allowing for an increased number and intensity of Atlantic tropical cyclones (while they tend to be suppressed somewhat during El Nino), and the potential for La Nina is one reason many of the tropical forecasts for this season show normal to above normal numbers of storms. How strong that effect is this season will depend on the rate at which this La Nina takes shape.
Farther into the cool season, later fall through winter, the effect of La Nina on North Carolina weather averages out over the long term to tilting the odds a little toward drier than normal, and warmer than normal, winters. That influence is reflected in current long-term outlooks for winter from the Climate Prediction Center that show our state a little more likely to be drier than normal and warmer than normal this winter, compared to lower probabilities of being near or above normal on precipitation or near or below normal on temperature.