While conditions on the ground caused the flood, there were directors behind the scenes giving the actors their cues. According to an analysis led by Dr. Robert Webb of the NOAA’s Earth System Research Laboratory about the climate causes of the floods, La Niña was most likely one of the directors. La Niña is a periodic cooling of waters in the eastern tropical Pacific. The cool waters are accompanied by stronger-than-usual easterly trade winds.
These changes affect seasonal climate in places far “downstream” of the Pacific. During La Niña events, the polar jet stream tends to shift northward in a region stretching from the Pacific Ocean to the western United States, and then dips southward into the Missouri River Basin, bringing colder air into the region.
Historically, the result of these disturbances is that La Niña tends to tilt the odds towards colder-than-normal winter and spring temperatures across the upper Missouri River Basin, and snow cover and depth tend to be more extensive. This past winter and early spring, the climate followed that pattern.
The heavy rains in May probably aren’t part of the La Niña story, however, according to Dr. Martin Hoerling of NOAA’s Earth System Research Laboratory. For one thing, La Niña was petering out that month. In addition, La Niña’s biggest influence on climate occurs in winter; its influence is weaker in the spring. “From historical data [of past La Niña episodes], you cannot reconcile the spring 2011 wetness with a La Niña impact,” said Hoerling.
In a description of the record rainfall in Montana, the Billings National Weather Service Office website notes, “Certainly, one or two of these especially wet systems are not uncommon during the month of May.” In May 2011, however, a persistent “block” of high pressure sat over western Canada. Like a boulder in a river, this mass of air guided the two storms from the Pacific straight over the Missouri River Basin, where they tapped into moisture coming up from the Gulf of Mexico.
The downpours were extreme, but extreme weather doesn’t always require a special explanation. As Hoerling puts it: “Weather happens, and extremes and records are meant to be broken from time to time.”
The Next Act
According to Dr. Wayne Higgins, Director of NOAA’s Climate Prediction Center, “This is a ‘double dip’ La Niña, following on the heels of the 2010-2011 La Niña event.” While not common, the phenomenon of a double dip La Niña is not unheard of. Predicting the strength of the second dip can be challenging, though.
“At this time, we cannot predict the ultimate intensity of the La Niña event,” said Higgins, “but we are confident that La Niña increases the potential for major flooding in the north-central U.S. next spring.”