What Was Up with That Rain?

 This post first published in the Morning Report. Subscribe to the daily newsletter here.

Our MacKenzie Elmer contacted the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography to ask, what’s up with all that rain yesterday? 

While the weather models are still modeling to determine the exact cause, scientists are pretty sure a unique kind of storm called an atmospheric river is to blame. 

Atmospheric rivers are bands of flowing water vapor in the sky. They live lower in our atmosphere, often beneath the clouds, which means it’s difficult for satellites to pick them up. It’s why the U.S. military flies aircraft equipped with sensors through them to understand their dynamics, climate scientist Julie Kalansky said.  

They ride on currents of air known as jet streams that are formed by the turning of the Earth, and why these sky rivers travel in a westerly direction (west to east), picking up moisture from the oceans, often in the tropics, and dumping it along the world’s western coasts. 

Monday’s deluge dropped over four inches in a matter of hours in some parts of San Diego. But this atmospheric river was a weaker, offshore storm, unlike the series of severe rivers that pummeled northern California last January. Topography can have an impact on where atmospheric rivers deliver their rain. Mountains quickly drive air up causing all that moisture to condense and rain out. 

This storm hit unstable air masses upon its arrival to San Diego, causing the atmospheric river to cry out its contents in short order. 

As humans continue to burn fossil fuels causing the climate to warm quickly, atmospheric rivers are expected to get more intense. 

“Projections show that as temperatures increase, the atmosphere is able to hold more water,” Kalansky said. “That means more precipitation in atmospheric rivers.”