SAN DIEGO — Yet another deluge was dropped on Southern California this year as Tropical Storm Hilary moved through the region over the weekend.
While the storm broke rainfall records for this time of year in some parts, experts say the rainfall might not have much impact on local wildfire risk moving into the peak of the season.
After Sunday’s downpour, Cal Fire officials said the potential for fires might ease slightly in the short-term as vegetation responds to the added moisture. But, they are still expecting wildfire season to proceed as normal moving into the state’s driest and most fire-prone months.
“As of right now, it’s kind of a near-normal September and October,” Cal Fire Public Information Officer Matthew Cornette told FOX5SanDiego.com. “The rain could help slow down the fires a little bit until the fuels dry out again and then we’ll be back to where we were before the storm.”
According to Cal Fire, California’s hydrology and the speed at which Hilary moved through the region are part of the reason why officials are not anticipating significant change to wildfire hazards in the coming months.
With intense rainfall occurring over a short period of time, experts say it’s generally much harder for California’s drought-stressed ground to soak up the heavy precipitation, given that prolonged dry spells cause soil deposits to desiccate and become more compact. This in turn makes it more difficult for the ground to absorb water.
Once the ground reaches the point of saturation it can tolerate, additional water accumulated due to continued rainfall just pools above ground, creating flooding that gets channeled back to the ocean by streams and creeks.
Quicker, intense storms do not give the ground the time it needs to reset before absorbing more water, Cornette explained, thus limiting the amount of moisture that could prevent vegetation from drying out and becoming fuel for wildfires.
“If we have a long storm over a week or so that is lighter rain, it gives that ground a lot more time to absorb the water and increase the fuel moistures quite a bit,” he added.
As of Aug. 21, the state’s fuel moisture, which is a measurement used to understand the potential for wildfires, for smaller vegetation — like grasses and leaves — appeared to be rebounding, according to the National Oceanic and Atmospheric Administration.
However, this type of vegetation can lose that moisture it gained relatively quickly as it responds to changes in the ground and atmosphere.
Into the coming weeks, experts say this will be important to keep monitoring to better understand fire potential across areas that got lots of heavy rain.
“The main thing is how much water soaks into the ground,” professor of geology emeritus at San Diego State University Dr. Pat Abbott said to FOX 5 Sunday night. “Those are the things we feel over time.”
This is particularly noticeable, he said, when looking at California’s recent “megafires,” a moniker used to describe wildfires that scorch through about 100,000 acres or more.
In 2020, California had three megafires — the Dolan and Creek wildfires in Northern California, as well as the Bobcat fire in Los Angeles County. The next year, in 2021, the Windy fire burned through about 97,528 acres near Sequoia National Forest.
As he explained, all of these massive fires occurred during record-breaking drought conditions in the Golden State, leaving more vegetation to dry out and become fuel for the flames.
That changed in 2022, which saw no wildfires of megafire size. While there still were wildfires, the relief from the aggressive megafires, he said, came from the remnants of Tropical Storm Kay, which replenished some moisture in the soil after it dropped sunstantial rain over southern California.
“That’s not a coincidence that we didn’t have megafires (last year),” Abbott said. “This is the time of year that the vegetation is drying out the most, getting the most fire prone.”
The most recent predictive fire outlook used by Cal Fire indicates that the widespread rain brought on by Hilary throughout the region will likely preclude significant fire activity for at least the next week.
Beyond that, there remains a high level of uncertainty about how rains from Hilary might impact wildfire risk across the rest of the season.
“We’re still in that threat,” Cornette said. “The storm will help a little bit, but in the big picture … It’s not going to have that great of an impact on the dryness of the fuel.”
