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In a vicious cycle California knows too well, parts of the state scarred by recent wildfires now face an elevated risk of flash floods and debris flow.
The National Weather Service warned on Sunday that Santa Barbara, Ventura and Los Angeles counties ― where some of the worst wildfires in state history started last month ― should be prepared for “dangerous land and water flow” in the coming days as 3-to-5 inches of rain pound the region.
“If you live in the recent burn areas including the Thomas, Creek, and LaTuna burn areas, there is a high probability that debris flows will occur Monday night into Tuesday, when highest rainfall rates are expected,” the NWS said.
Flash Flood Watch in effect Monday Night-Tuesday for recent burn areas of SBA, Ventura, LA counties. Significant threat for flash flooding/debris flows. Peak rainfall rates 0.50-1.00 inch per hour. Make preparations now and follow directions of local officials. #CAstorm #LArain http://pic.twitter.com/C0iYz6nBjS
— NWS Los Angeles (@NWSLosAngeles) January 7, 2018
The same concern applies to the San Francisco Bay Area’s northern counties, which faced devastating wildfires in October and now faces its “strongest storm of the season thus far,” the NWS said.
“Heavy rain on the North Bay burn scars could result in debris flows, mudslides, and flash flooding” starting Monday night, the agency warned Sunday.
Concern about flash flood and debris flow after wildfires encapsulates the multifaceted face of climate change in California, which already has the most naturally variable climate in North America, scientists say.
“Burn scars” on land recently parched by wildfires make the soil hydrophobic, meaning it struggles to absorb water, and triggers runoffs and floods. While there’s a risk of such flooding and debris flow after any prolonged period of drought, it’s especially pronounced after severe wildfires, water experts say.
But it’s not just the hydrophobic land that poses a threat. Wildfires often burn down the trees and vegetation crucial for slowing rain’s path into the ground or down hillsides, the NWS says.
Scientists warned Sunday that those factors would likely be at play as rain approaches the counties devastated by recent blazes.
The storm predicted for Southern California following the wildfires is a “recipe for flash flooding and debris flows,” meteorologist Alex Lamers tweeted.
Big rainfall signal in the transverse ranges of Southern California Monday and Monday Night. Unfortunately, many recent (and large) burn scars there, such as #ThomasFire. http://pic.twitter.com/PYd8J8Jz3L
— Alex Lamers (@AlexJLamers) January 7, 2018
Flash flood watch issued for most of SoCal Monday afternoon to Tuesday night. Rainfall rates greater than .5" an hour possible and if that happens we'll get burn scar flooding. #NBC4You #CAstorm http://pic.twitter.com/fZCtH4f6yl
— Anthony Yanez (@AnthonyNBCLA) January 7, 2018
This is why there is so much concern re: upcoming storm near #ThomasFire burn scar. Vast regions w/steep slopes have been completely denuded of vegetation, w/ an accumulation of ash that can be mobilized in wet cement-like debris flows. #CAwx #CAfire #Ojai #Ventura #SantaBarbara https://t.co/suWIPCi4pO
— Daniel Swain (@Weather_West) January 6, 2018
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