By Pedro Di Nezio | June 17, 2025
When spring arrives in the Northern Hemisphere, many climatologists turn their attention to the tropical Pacific Ocean. Around this time, the Pacific starts to show early signs of warming or cooling patterns known as El Niño or La Niña. These ocean temperature patterns typically emerge in summer and last through the winter, allowing us to anticipate their effects well in advance. El Niño often brings extreme flooding and coastal erosion to the U.S. West Coast and drought in Indonesia. La Niña is linked to droughts and wildfires across the southern U.S. and an increased frequency of Atlantic hurricanes.
Growing up on a farm in Argentina, I witnessed firsthand the rhythm of rainfall and drought often attributed to la corriente del Niño–the common name given to these patterns in South America. That early exposure sparked my lifelong interest in understanding how the ocean and atmosphere interact and how we might better predict their changes. Today, as a scientist at the University of Colorado, my research focuses on developing models to improve forecasts of extreme El Niño and multi-year droughts caused by La Niña.
Accurate forecasts of El Niño and La Niña help governments and businesses prepare for droughts, floods, or heat waves, and reduce risks of economic disruption. Early warnings also support disaster preparedness, minimizing damage to infrastructure and loss of life. Overall, timely predictions enhance resilience to climate variability. Unfortunately, significant discrepancies often exist among models, making expert interpretation crucial for understanding how El Niño or La Niña may develop and impact global weather.
Building on years of research in El Niño forecasting and my work advising public and private sector stakeholders, I’ve put together five practical tips to help you get the most benefit from El Niño and La Niña predictions:
El Niño events usually begin in late summer and reach their peak in winter. Forecasts made earlier—particularly in spring—are often unreliable, as short-term, chaotic atmospheric patterns can give the false impression of an emerging El Niño.
We saw this situation very clearly back in 2014 when atmospheric patterns in February prompted warnings of a potentially extreme El Niño. By June, those signs had fizzled, and the event never materialized. That’s why I recommend waiting until May to place confidence in forecasts showing that El Niño is on the way.
La Niña events are driven by slow-moving waves beneath the ocean surface, which are more predictable and less vulnerable to disruption from random atmospheric fluctuations. For this reason, it’s generally safer to trust La Niña forecasts—even those issued in spring—especially following an El Niño year.
For instance, in February of 2024, at the height of an El Niño, official forecasts predicted neutral conditions for the following year—meaning neither El Niño nor La Niña. However, experts like me anticipated a La Niña since this sequence of events has happened consistently before. As expected, La Niña did eventually develop and contributed to dry conditions in the Southwestern U.S. and California in the winter of 2024-25 and the late activation of the Atlantic hurricane season in 2024.
Not all El Niño events are the same. In some years, ocean temperatures in the tropical Pacific can rise explosively–increasing more than 2°C above normal within just a few months. These variations are measured using the Niño index, a standard metric for tracking El Niño events.
Therefore, even if forecasts show an El Niño is forming, it’s critical to monitor how strong it could become. Intensity is difficult to predict but greatly influences the scale of El Niño impacts–particularly extreme rainfall and the risk of flooding. So don’t stop paying attention once May has passed. The most impactful events often build quickly around August and September.
Over recent decades, we’ve seen that La Niña can persist for two or more winters, especially when it follows a strong El Niño. This behavior creates an opportunity for longer-term impact forecasting, sometimes beyond the usual six-to-eight-month limit.
In 2016, after a powerful El Niño, we predicted La Niña conditions by the end of the year and again for the following winter. That prediction held true.
Furthermore, it’s rare to see El Niño develop the very next year after La Niña. So, if a forecast does predict such a transition, be at least a bit skeptical–it might not happen that way.
Three-year La Niña conditions are very rare, they happen once or twice in a century, therefore it is difficult to study their causes and predictability. However, they pose a significant risk due to their protracted impacts. The last three-year event occurred in 2021-2023 producing prolonged drought across the Americas–particularly South America. Montevideo, Uruguay’s capital city, at the epicenter of this drought nearly ran out of water during the summer of 2023.
Regions in the tropics are directly affected by Pacific Ocean conditions, so rainfall and drought patterns in that part of the world tend to be more consistent from one event to the next. Outside the tropics, however, impacts become less predictable due to other atmospheric influences.
Take California, for example. While strong El Niño events in 1982 and 1997 brought heavy rains, that pattern didn’t repeat in more recent El Niño events–highlighting gaps in our understanding. In contrast, tropical regions continue to show more reliable responses to both El Niño and La Niña. For instance, Panama experiences extremely heavy precipitation during La Niña and low precipitation during El Niño. This effect, combined with increasing demand, led to record low water levels in Panama Canal during the El Niño of 2023–as reported by a recent rapid attribution study.
El Niño and La Niña are the dominant drivers of year-to-year variability in the ocean and atmosphere, shaping global weather patterns. Forecasting has improved thanks to more advanced models and longer climate records, but expert judgment is still essential.
The tips shared here are grounded in patterns observed over the past 50 years, but I do worry that, as the climate continues to change, they may become less reliable.
January to April During this time even the best forecast systems can’t predict whether El Niño or La Niña will arrive by the end of the year. If an El Niño is just ending, based on historical patterns, I would expect La Niña to follow. If La Niña is present, I would not expect an El Niño to follow. Instead, I’d split the odds 50:50 for La Niña to persist for a second winter or for the tropical Pacific to shift to neutral (normal) conditions.
May to July Now I start to pay closer attention to the forecasts. But they are really only able to predict the direction that the tropical Pacific will take (El Niño, La Niña or neutral), but not the strength of the pattern. If you need to know if a developing El Niño or La Niña will be moderate or strong, you need to keep waiting.
August to October Now the forecasts are good at predicting the intensity of the developing event and are particularly accurate when an extremely strong El Niño is expected. If an El Niño or La Niña has already begun, those patterns can nudge tropical cyclones to become less or more common (respectively).
November to December By this point, if an event is going to happen, either El Niño or La Niña will be in full bloom. Random weather patterns can cause their downstream effects to be stronger or weaker, so their impacts outside the tropical Pacific might not be known until February or March the next year.
WTW hopes you found the general information provided here informative and helpful. The information contained herein is not intended to constitute legal or other professional advice and should not be relied upon in lieu of consultation with your own legal advisors. In the event you would like more information regarding your insurance coverage, please do not hesitate to reach out to us. In North America, WTW offers insurance products through licensed entities, including Willis Towers Watson Northeast, Inc. (in the United States) and Willis Canada Inc. (in Canada).
