Deke Arndt, Chief, Climate Monitoring Branch, National Climatic Data Center
We spent much of this winter and spring in La Niña conditions, and that tends to push storminess a little further out into the Western Pacific. That releases a lot of energy, and that energy can disturb the Jet Stream. The Jet Stream is just a big river of air, and, like any river, if you disturb that upstream, it's going to have consequences downstream. With La Niña, for the United States anyway, what that often means is the Jet Stream is displaced a little farther north than the average position. What that meant this May was that the organized storm systems occurred in the Northern Plains much more often than they typically do. This brought a colossal amount of rain over a huge footprint of area.
All of this rain has to go somewhere, and it made its way into the rivers and streams in the Missouri River system. What that lead to was historic rainfall totals. Add that on top of record snowmelt in the Northern Rockies, and you're going to end up with exactly what we saw: flooding in Omaha; flooding in the Dakotas; flooding along the entire tributary system of the Missouri River.
It may have got that initial nudge from La Niña, far away in the Pacific Ocean. We call those "teleconnections", and they influence our weather and seasons more than we might think. When you're dealing with a river of air like the Jet Stream, influences upstream can have consequences downstream. And that may be what we saw this spring. We may have seen La Niña's influence on the outcomes of the flooding in the Northern Plains.
For ClimateCast, I'm Deke Arndt.
ENSO Forecast Discussion (NOAA Climate Prediction Center)
ENSO Quicklook (International Research Institute for Climate & Society