Ongoing La Niña affecting our weather

Published 1:40 pm Saturday, April 16, 2022

By Skip Rigney

Wednesday night’s rain helped erase part of the precipitation deficit that Pearl River County has accumulated during the past six months. Historically, the average rainfall from October 1st through April 15th is between 29 and 33 inches depending on the exact location in the county. Since October 1, 2021, radar and rain gauge data indicate that most of the county has received only 20-25 inches. A strip of land in northwest Pearl County and also an area between I-59 and the Stone County line have received greater than 25 inches, but those locations have still been slightly drier-than-average for the period.

Abnormally dry conditions are typical in the southern United States during months when sea surface temperatures over a broad expanse of the tropical eastern and central Pacific Ocean cool to about one degree or more below average. When those cool anomalies persist in that region of the Pacific for a month or longer,  scientists know that La Niña is occurring.

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The huge pool of cooler-than-average water results in changes in the atmosphere, not only above the tropical Pacific, but across much of the globe. One such change is in the path of the high speed winds in the upper atmosphere known as the jet stream. The resulting pattern of high altitude winds tends to suppress rainfall across the southern United States.

The La Niña now underway began in September last year. The previous La Niña had ended only five months earlier in May 2021. You may recall that during the period between the two La Niñas we had one of our wettest summers ever.

It’s not that unusual for one La Niña to follow within one year of another. In fact, it is much more common than it is to have consecutive years with recurring El Niños. El Niño is the nearly opposite, warmer sibling of La Niña.

Not only do the high altitude global wind patterns associated with La Niña and El Niño affect how much rain we get in the southern U.S. during the fall, winter, and spring, those wind patterns can also enhance (La Niña) or suppress (El Niño) the number of hurricanes that develop in the Atlantic, Caribbean, and Gulf of Mexico.

Long-range forecasters at NOAA’s Climate Prediction Center are leaning towards La Niña continuing through the summer. That’s why the odds are stacked in favor of yet another busier-than-average hurricane season according to the outlook issued last week by Colorado State University’s Tropical Weather Research Group. Hurricane season begins each year in June and runs through November.

However, it’s worth noting that forecasts issued in the spring for La Niña, El Niño, or the neutral state in between, are less accurate than predictions made at other times of the year. Forecasters acknowledge that there is about a 40 percent chance the near-surface waters of the tropical Pacific could trend back to more average, so-called “neutral,” conditions by late summer or fall. If that happens, we could have a more “average” hurricane season.

Let’s hope so. Even a return to the average number of named storms in the Atlantic, Caribbean, and Gulf, which for 1991-2020 was 14, would be a welcome change compared to our last six busier-than-average seasons.