News•August 5, 2014

Putting a Climate Context on SoCal’s Flash Floods

A confluence of extreme atmospheric conditions over Southern California, possibly with an assist from the intense drought in the state, set the stage for the torrential rains and flash flooding that washed away cars and homes in some areas like Mt. Baldy on Sunday.

Volunteers and rescue workers clear mud and debris at the #Mt.Baldy flood. pic.twitter.com/dOdzHs7Dgl

— SB County Fire (@SBCOUNTYFIRE) August 5, 2014

And while the U.S. Southwest is expected to increasingly dry out in a warming world, when it does rain it is more likely to come as a heavy downpour. Heavy rainfalls are expected to increase around the world as the warmer atmosphere packs more moisture to fuel downpours.

The rains that fell across parts of Southern California into western Arizona and parts of Nevada on Sunday were the result of a summer monsoon over the Southwest. A monsoon is a seasonal reversal in the winds over a region that can bring with it changes in rainfall patterns. The winds switch directions because the beating of the summer sun on the ground in the Southwest contrasts strongly with the relatively cool Pacific waters, with air rising over the land and the wind blowing in from the ocean to replace it. The ocean winds bring with them plenty of moisture to fuel seasonal summer rains that fall in bursts and account for more than half of the annual precipitation in some parts of the Southwest.RELATEDWhy California’s Drought Isn’t Going Anywhere
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With this seasonal setup, it is not unusual to see monsoon thunderstorms and subsequent flash floods at this time of year. But “this is one of the more significant events that has occurred in recent years,” said Steve Harrison, a forecaster with the National Weather Service office in San Diego.

One reason was the unusual amount of moisture that was available to budding storms, even for a monsoon. There were 2 inches of what is called precipitable water in the atmosphere — essentially it is what all the water vapor in a column of air would measure if it condensed.

That amount “is high for pretty much anywhere in the world, but especially for our area,” Harrison told Climate Central.

Another factor was a low pressure system that moved in over California. It was stronger than is usual for this time of year and moved slowly, Harrison said, both of which are factors conducive to flash flooding events.

“So that just made things even worse,” he said.

Rains fell so hard that they reached rates of 3 to 4 inches per hour on Sunday, with some of the largest total rainfall amounts hitting 1.5 to 4.7 inches. The highest amount fell on Mt. Baldy in the San Gabriel Mountains to the east of Los Angeles. The locale saw a stunning 3.98 inches fall in one hour — a 1-in-500 year event for them, according to the NWS.

Near 500-Year Rain Event @ Mt. Baldy last Sunday @EvelynTaft @crystalNBCLA @henrydicarlo @RICKatFOX @KEYTNC3Meredith pic.twitter.com/F71M0rUjQC

— NWS Los Angeles (@NWSLosAngeles) August 5, 2014

Such intense downpours — other examples were seen earlier this year in Pensacola, Fla., and last year to devastating effect in Calgary — are a feature of a warming world and are expected to become more common, even in dry places like the Desert Southwest. As the average temperature of the planet warms — and it has already warmed by 1.6°F since the beginning of the 20th century, according to the Intergovernmental Panel on Climate Change — the water vapor content of the air goes up. Essentially there is more “juice” for storms to ring out of the atmosphere, which weights the dice to more extreme precipitation events.

Such increases have already been observed in every part of the U.S. except Hawaii, according to the National Climate Assessment released in May. While the regions with the largest increases in extreme downpours are the Northeast and Midwest, the Southwest has seen at 12 percent increase since 1958.

Strong t-storms over #SanBernardino city bringing heavy downpours. Flash flooding likely! #cawx pic.twitter.com/ahq9go4eN8

— NWS San Diego (@NWSSanDiego) August 4, 2014

Intense rains can lead to flash floods and landslides anywhere they occur, but the record-setting drought conditions in place in California — something else that could become more the norm in a warming world — could be making them worse. The ground already has trouble absorbing rain that falls at such fast rates, leading to large runoffs that can displace rocks, trees and even buildings. Parched soils are baked by the hot sun and become even more impervious to falling rain.

“Most of [the rain] doesn’t get captured in the places it needs to be,” said Stuart Seto, a weather specialist with the NWS Los Angeles office. “It just washes that stuff over the roads.”

That leads to scenes like those in Mt. Baldy of residents digging their cars out from packed muds after the waters are gone, as well as the tragedy of homes and lives washed away.