According to the latest forecast, La Niña is expected to develop later in the fall, increasing the likelihood of further weather events such as Tropical Storm Debby in South Carolina.
The National Weather Service’s Climate Prediction Center released its updated La Niña forecast Thursday morning, sticking with its July prediction that the weather pattern would likely form in the fall. La Niña typically leads to a stronger Atlantic hurricane season and drier winters in the Southeast.
Tropical Storm Debby began as a Category 1 hurricane when it made landfall in Florida on Monday. As it moved toward South Carolina, Debby was soon downgraded to a tropical storm. The slow-moving storm brought heavy rains and caused significant flooding in parts of the state throughout the week.
La Niña forecast
According to the latest forecast, the probability of La Niña occurring between September and November is 66%. The probability of La Niña lasting from November to January is 74%.
The Atlantic hurricane season began on June 1 and lasts until November.
What is La Niña?
La Niña is the opposite of El Niño. During La Niña events, the trade winds are even stronger than usual and push warm water towards Asia. Off the west coast of the USA, the upwelling increases and brings cold, nutrient-rich water to the surface. This cold water in the Pacific pushes the jet stream north.
What does La Niña mean for hurricane season?
According to NOAA, La Niña has been shown to increase hurricane activity in the Atlantic basin.
NOAA is currently forecasting an above-average hurricane season with 17 to 25 named storms. What’s particularly notable for South Carolina as Debby moves through is that the Atlantic hurricane season is typically most active between mid-August and mid-October – the period when La Niña is currently expected to occur.
Due to La Niña’s effects on wind shear and atmospheric conditions over the Atlantic, hurricane activity tends to increase under La Niña. In the tropics, wind shear decreases during La Niña, which can favor hurricane formation. Wind shear is a change in wind speed and direction with height in the atmosphere.
La Niña also promotes increased hurricane activity because it reduces atmospheric stability and reduces sinking motion in the atmosphere.
Impacts of La Niña winter weather
According to the National Oceanic and Atmospheric Administration, the effects of La Niña tend to cause droughts in the Southeast.
“Dryness over the Southeast is often associated with the Pacific-North American (PNA) climate pattern,” NOAA explains. “In the negative phase of this pattern, the general flow of the atmosphere becomes ‘stuck’ in a wave chain that repeatedly brings higher than normal pressure to the North Pacific, lower than normal pressure to Western Canada, and further downstream, higher than normal pressure to the Southeast. High pressure generally leads to dry skies.”
Below is a map showing the average impact of La Niña on winter precipitation in the United States over the past 60 years.
Despite the rainfall from Debby, much of South Carolina is still experiencing drought, according to the US Drought Monitor.