La Nina has started in the Pacific Ocean and will likely persist through this upcoming winter and into 2021. The phenomenon is a natural occurring oceanic-atmospheric connection marked by cooler than normal sea surface temperatures over the central and eastern Pacific Ocean near the Equator. The opposite is, of course, El Nino which features warmer than normal sea surface temperatures in the same region. As La Nina started to take shape this past summer, it likely played a role in the tropical season of 2020.

Last September saw a very active Atlantic basin filled with up to 5 named storms at once. While September is usually active in the tropics given the time of year (very warm waters), last year was extremely busy. La Nina years can lead to an increase in Atlantic tropical cyclone activity because wind shear (changing of wind speed with height) is weaker over the Caribbean and Atlantic Basin. If wind shear is high, it rips storms apart and doesn’t allow them to reach their full potential. With a lack of shear, storms can blossom over warm waters and continue to grow into major hurricanes. The developing La Nina could have certainly played a role in the very high count of tropical systems last year.

This image depicts a typical La Nina pattern and its effects on the U.S. weather. The southern part of the country is usually dry while the north is cold and wetter toward the Northeast. Depending on where the polar jet stream sets up shop, the winter will either be quite active for us locally, or warm air from the south will win out and prevent a lot of snow if the cold air stays bottled up in the northern Plains.

Meteorologists at the national level believe La Nina will be in place through at least February 2021. What does this mean for the U.S. weather and our local forecast? Typically, La Nina brings above-average precipitation and colder than normal temperatures for the northern part of the country with drier weather and warmer conditions for the southern U.S. The last time La Nina occurred was during the winter of 2017-2018. That winter locally featured nothing impressive. Looking back, the mean temperature was 1° above normal and our snowfall was below normal by almost 7”. That certainly bucked the typical La Nina signals for northern parts of the country.

But given the hot summer last season and the active tropics from last fall, perhaps this La Nina is going to be stronger than the previous one. We also had almost no snow locally last winter, so perhaps we are due to stay busy this year. Time will tell!

2021 ABC27 Weather Almanac