Hilary has caused record rainfall in California, but it may not mitigate the risk of wildfires

SAN DIEGO — Another flood was struck in Southern California this year as Tropical Storm Hilary swept through the region over the weekend.

While the storm broke precipitation records for this time of year in some parts of the year, experts believe that precipitation at the peak of the season may not have a major impact on local wildfire danger.

After Sunday's downpour, Cal Fire officials said the fire threat could ease somewhat in the short term as vegetation reacts to the extra moisture. However, they still expect the wildfire season to proceed normally, rolling into the driest and most fire-prone months in the state.

“Right now it's a pretty normal September and October,” Cal Fire's public information officer Matthew Cornette told “The rain could help slow the fires a bit until the fuels dry up again and we're back to where we were before the storm.”

According to Cal Fire, California's hydrology and the speed at which Hilary was moving through the region are one of the reasons authorities don't expect significant changes in wildfire danger in the coming months.

Experts believe that the heavy rainfall in California's drought-stricken soil makes it much more difficult to soak up the heavy rainfall over a short period of time, as prolonged dry spells cause soil deposits to dry out and become more compact. This in turn makes it difficult for the soil to absorb water.

Once the soil reaches the point of saturation that it can tolerate, additional water accumulates directly above ground due to sustained rains, causing flooding that is channeled back to the sea via streams and creeks.

Faster, more violent storms don't give the soil the time it needs to regenerate before absorbing more water, Cornette explained, limiting the amount of moisture that could prevent vegetation from drying out and becoming fuel for wildfires.

“If we have a longer storm for about a week that triggers lighter rain, that gives the ground a lot more time to absorb the water and increase the fuel moisture significantly,” he added.

As of August 21, the state fuel moistureAccording to the National Oceanic and Atmospheric Administration, smaller vegetation — like grasses and foliage — appeared to be recovering in numbers.

However, this type of vegetation can lose the moisture it absorbs relatively quickly as it reacts to changes in the soil and atmosphere.

Graph showing the fuel moisture threshold for the 10 hour time delay.  According to NOAA, the 10-hour time lag represents the effect of humidity changes on smaller vegetation such as grass and leaves. (Courtesy NOAA)
Graph showing the fuel moisture threshold for the 10 hour time delay. According to NOAA, the 10-hour time lag represents the effect of humidity changes on smaller vegetation such as grass and leaves. (Courtesy NOAA)

Experts say this will be important in the coming weeks to better understand the risk of fire in areas of heavy rain.

“The main thing is how much water seeps into the soil,” said Dr. Pat Abbott, professor emeritus of geology at San Diego State University, on FOX 5 Sunday night. “Those are the things that we're sensing over time.”

This is particularly striking, he said, when looking at recent California developments.Megafires“, a nickname used to describe wildfires that spread about 100,000 acres or more.

In 2020, there were three megafires in California – the Dolan and Creek wildfires in Northern California and the Bobcat fire in Los Angeles County. The next year, in 2021, the Windy Fire burned approximately 97,528 acres near the Sequoia National Forest.

As he explained, all of these major fires occurred during a record-breaking drought in the Golden State, causing even more vegetation to dry out and provide fuel for the blaze.

That changed in 2022, when there were no megafire-sized wildfires. While wildfires still raged, relief from the aggressive wildfires, he said, came from the remnants of Tropical Storm Kay, which replenished some moisture in the ground after dumping sunlit rain over Southern California.

“It's no coincidence that we didn't have any major fires (last year),” Abbott said. “This is the time of year when vegetation dries out the most and is therefore most susceptible to fire.”

The most recent forecast fire outlook used by Cal Fire suggests that the extensive rain that Hilary has inflicted across the region is unlikely to spark significant fires for at least the next week.

In addition, there remains a high level of uncertainty as to how the rains in Hilary could affect wildfire risk later in the season.

“That threat still exists,” Cornette said. “The storm will help a little, but overall … it won't have that much of an impact on fuel dryness.”

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