The Double-Decker River hits Louisiana

August 25, 2016

As Louisiana, East Texas and the Midwest floundered in floods, satellites showed a startling picture. A double-decker river is flowing up the Mississippi! Hovering over the rolling waters of the Mississippi River is a giant atmospheric river!

A giant atmospheric river stretched from South Texas to New England.

Welcome to the schizophrenic nature of La Niña conditions in the Tropical Pacific. Typically, when La Niña conditions dominate the Pacific, we see more extreme weather – more droughts and, strangely enough, more flooding. Rainfall patterns are compressed and concentrated, so that areas that do receive rainfall, frequently receive way too much rainfall. When this happens on a year like this one, with unusually hot Gulf and Atlantic air masses feeding humid marine air into the river, it can create terrible floods. Parts of Louisiana and East Texas have experienced record floods and high river levels.

When pools of cooler water and the cooler air above them, hit warmer water, it causes storms. The cooler La Niña air meeting warm Atlantic air created a record number of July hurricanes in the Pacific. When winds were not right for spinning storms; atmospheric rivers. Thanks to the spin of the Earth, the moisture from these storms stream north and south, away from the equator (and currently over the US).

Atmospheric Rivers flow north and south from tropical storms. Source: NASA

These “rivers in the air” can be huge, thousands of miles long and 250 to 350 miles wide. A strong one carries as much liquid (in water vapor) as 7.5 to 15 times the average flow water at the mouth of the Mississippi River. They bring rain from the tropic to the rest of the world, which is good, but if they are too strong, they cause floods. If one hits the West Coast it is called Pineapple Express, while one that hits Texas is a Mayan Express. So far, poor Texas has been hit by three this year, in March, June and now.