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A stronger southern jet stream can bring repeated rain and flooding into fall and winter.
SAN ANTONIO — El Niño begins thousands of miles away in the tropical Pacific, but it can reshape weather patterns across the United States. As unusually warm water spreads across the central and eastern Pacific, thunderstorms and risking air shift eastward as well.
This change alters the flow of air high above the ground, typically strengthening the subtropical jet stream and extending it farther south and east across the southern United States.
The jet stream acts like a highway for storm systems. During an El Niño fall and winter season, that storm track often stretches from California and the Southwest through Texas, the Gulf Coast, and the Southeast. Storm systems moving along the jet can pull in moisture from the Pacific and Gulf, producing more clouds and frequent rounds of rain. Starting usually in the fall season, but it usually becomes stronger and more consistent during winter.
The flooding threat can grow when several storms follow a similar path. Even if one storm does not produce extreme rainfall, repeated rain can saturate the ground, fill drainage systems and raise water levels in rivers and streams. Once the soil becomes saturated, additional rainfall runs off more quickly, increasing the risk of flash flooding and problems in urban areas.
During a recent strong El Niño in 2015-2016, several cities in the South recorded a surplus in rainfall. Some leading to significant flood events such as the South Central Texas floods in 2015, Southern Louisiana flooding in August 2016, and catastrophic floods in South Carolina in October 2015. From slight increases to heavy amounts of annual rainfall for several areas across the South.
While El Nino does not guarantee that every storm will be strong or that every southern state will remain wet and cool all winter, it simply shifts the odds towards a more active southern storm track, making rainfall and flooding more likely when the right ingredients come together.
