Good morning! The past several days have been quite active in the meteorological community.We began last week on the tail end of a pattern featuring stagnant warmth, with ridging in the Eastern United States and cold air bottled up in Western Canada and the Pacific Northwest. A well advertised change has occurred since that time, however, with cold air surging southeastward into the Great Lakes and Northeast States.
Good afternoon! We continue to monitor developments with La Nina this weekend, as it will one important features will be considering in our monthly and winter forecasts. Sea-surface temperatures this past week, have cooled over more of ENSO regions in the Central and Eastern Pacific, as anticipated in our last major update. The thermocline (sea-surface temperature gradient) has been rising this week, with an easterly trade wind bust causing more upwelling cooler sub-surface waters into the Central Tropical Pacific. The latest 30-day moving SOI is now at +13.03. So La Nina conditions have been strengthening this week.
Much fuss has been made over the past several months in regards to the development of La Nina this year, coming on the heels of one of the strongest El Nino’s on record. There is a propensity for these things to occur, after all, and a significant cooling of Nino-region pacific temperatures led many to believe that a La Nina was not only on its way — but could be moderate or strong by the time cold season arrived in the Northern Hemisphere. Those ideas will not come to fruition.
The lack of La Nina development has, instead, been notable — with a significant lack of depth to any cooler sea surface temperature anomalies. Trade winds aren’t cooperating (we’ll get into that more later) and tropical forcing seems to favor this pattern continuing through the next few months. It appears likely, now, that La Nina will never truly get off the ground. Forecast models have responded, with monthly and seasonal data now pointing to a Neutral (La Nada) pattern through at least the first half of North America’s calendar Winter.
While we had been tracking heat and severe thunderstorms over the first few months of the summer, the atmosphere was still undergoing a major transition away from El Nino and towards a La Nina. This transition has already resulted in sensible weather changes across much of the country, with a huge heat ridge building in the Central US last month — typical of a La Nina. However, to the despair of some long range forecasters, certain aspects of the transition have not gone as smoothly, and more recent forecast models have trended weaker with the eventual strength of the La Nina over the next few months.
During last year’s El Nino conditions, the trade winds in the Equatorial Pacific were strongly weakened, allowing warm water near Australia to pool eastward. Those trade winds have strengthened over the last few months, which pushed that warm water back to the west, and allowed cooler water to upwell towards the surface. However, climatology favors trade winds remaining a persistent feature — in other words, we are supposed to have east-to-west trade winds blowing warm water towards Australia keeping cooler water near Peru. We can only have a La Nina when those trade winds are consistently stronger than average, and that has yet to be the case this year. For more background on what causes El Nino, La Nina, and the demise of an El Nino, refer to our article from late April.
