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July 10, 2020

50-50% Chance of La Niña in the Fall

On Thursday, July 9, the National Oceanic and Atmospheric Administration (NOAA) announced a La Niña Watch. NOAA issues a watch when conditions are favorable for the development of either a El Niño or La Niña within the next six months. Currently, there is a 50-55% chance of La Niña developing in the fall and lasting through winter.

As many may remember, El Niño or La Niña events can change atmospheric circulation patterns across the globe influencing our seasonal average temperature and rainfall patterns. Knowing there is an El Niño, or La Niña, also helps make seasonal patterns more predictable.

To see what La Niña typically looks like for the contiguous U.S., we have pulled a couple of charts from NOAA that shows seasonal patters (Sep-Nov; Dec-Feb; Mar-May; Jun-Aug) for both temperature and precipitation for seven La Niña events (1973-74; 1988-89; 1917-18; 1970-71; 1916-17; 1955-56; 1975-76).

Seasonal temperatures and precipitation: Difference from average (i.e. anomaly)

What does this mean for Texas and the Southern Plains? As you can see from the temperature maps for the seven past La Niña events, about five out of seven La Niña years results in above normal temperatures for the Sep-Nov season, while six out of seven years results in above normal temperatures in the winter (i.e. Dec-Feb). La Niña is less influential in the spring and summer, so the seasonal patterns for Mar-May and Jun-Aug are less clear.

For precipitation, six out of seven La Niña years results in below normal precipitation in both the Sep-Nov and Dec-Feb seasons.

For both temperature and precipitation, though, there is some variation within Texas and the Southern Plains. So this is just a rough approximation of broad trends for the region. Also, the boundaries you see within each state are climate divisions designated by NOAA. In some cases, these climate divisions can be quite large, and you can have even more variation in each one of these divisions that gets averaged out.

Before we get too excited or worry about La Niña forming in the fall, there is still a 40-45% chance we could remain in ENSO-Neutral conditions through the fall and winter, and there is even a small chance (i.e. 5-10%) of conditions moving into El Niño territory. It should also be stated that while historical trends from previous La Niña years helps us plan and assess risk, every La Niña event is unique and can often result in surprises. We are coming out of the time when models have a hard time predicting El Niño or La Niña events so there will be more updates coming as the models start zeroing in on what we can expect in the fall.