After California Fires Ease, More Homes Likely Will Be Destroyed In Mudslides

Devastation after a debris flow killed 23 people in 
Mendicito, California in January, 2018. A large wildfire
a month earlier set the stage for this disaster.
The same or worse could happen again in the '
aftermath of this month's wildfires if the
weather turns very wet in February and March. 

 In what might at first glance seem like a strange development, a Pacific Palisades house miraculously spared by this month's wildfires was destroyed by a mudslide on Wednesday.  

No, unfortunately,  it did not rain in southern California yesterday.

 Instead, the home above where the mudslide started was destroyed in the fire.  Firefighting efforts and water leaks caused by the blaze soaked the steep hill, causing the mudslide.

This brings about a bigger worry:  If the rains finally returns, southern California could face devastating mudslides. 

This is always a risk on and below hillsides in the few years in the aftermath of a wildfire.  Vegetation that hold soil and rocks in place has been destroyed.  

Also, the heat of the fire makes soils "hydrophobic."  Yes, kind of afraid of water. 

The wildfires create a layer on top of the soil that water can't penetrate. Burning vegetation forms a sort of wax that oozes into the soil, then hardens when it cools. That creates that "hydrophobic" layer that rainwater can't get through. 

Then, when it finally rains on steep slopes, the rain starts running downhill, gathering fire debris, rocks, stones and whatever else it can grab.  These turn into devastating debris flows that wipe out everything in their path.  

The problem can last months or even years after a fire, but the sooner it rains heavily after the wildfire, the more critical the risk. 

Which leads us to the California wildfires. 

It's supposed to be the rainy season right now in and around Los Angeles. That it hasn't really rained yet this winter is why this wildfire devastation was so surprising. Had it rained like it normally would have, there still would have been an awful windstorm with lots of trees and power lines toppled. But there would have been either no fires, or blazes that wouldn't spread much and be easy to suppress.

The rainy season in southern California goes into March.  It could suddenly turn very wet.  Especially in this age of climate change, there's a greater propensity for "weather whiplash," in which conditions go abruptly from hot to cold and back again, or very wet to super dry to soggy again. 

I'll have more on this intensified "weather whiplash" in an upcoming post.

There's no real sign of rain in southern California anytime soon. Los Angeles might go the entire month of January without seeing any rain at all. On average, L.A. usually gets a little under three inches of rain in January.

There are subtle signs the weather pattern could change toward something wetter in February, though nobody knows for sure. 

POST-FIRE DEBRIS FLOWS

A worse case scenario is if the things change so abruptly that atmospheric rivers start plowing into the southern California from the Pacific. Atmospheric rivers are narrow bands of deep moisture that can create torrential rains and flooding.  

Atmospheric rivers hit the Los Angeles area last February, dousing the city with ten inches of rain that month - with seven inches coming in just two days.  To nobody's surprise, that caused some horrible flooding and mudslides. and causing some serious flooding. 

If the same thing were to  happen this February, it would be catastrophic. Freshly burned hillsides would slip away or send intense debris flows flying down hillsides. Also, the wreckage of destroyed homes would not have been removed yet, so that would add to the scary debris flows.

These things hit fast, and there's little time to get out of their way.  Usually, there's less time to evacuate ahead of a debris flow  than people had when they fled this month's wildfires.

There's a history of devastating debris flows after wildfires, both in the L.A. area and elsewhere. 

On December 4, 2017, the Thomas Fire broke out and consumed 282,000 acres in Ventura and Santa Barbara counties and consumed 1,000 structures. At the time, it was the largest wildfire in California history. 

Later that winter, the usual winter rains arrived in southern California. On January 9, 2018,  torrential rain fell on the Thomas Fire burn scar and sent a huge, devastating debris flow through Montecito, California, killing 23 people, injuring 150 and leaving hundred of homes and businesses destroyed or damaged.   

Last June, wildfires raced through the hills above Ruidoso, New Mexico, a community of about 8,000 people. Then the normal monsoon thunderstorms hit later in the summer. At least eight times, debris flows from the fire area crashed through the city during and after those summer downpours. 

As of today, the fires still rage in southern California. More strong, dry winds are possible next week, which could make the situation worse. 

Everybody wants it to rain in southern California. But we had better hope and pray the wet weather comes in the form of weeks with long periods of drizzle, light rain and fog. Such gentle wet weather would minimize debris flows.

The worst case scenario would be thunderstorms and downpours, which would a southern California catastrophe so much worse.