Amid firestorms and extended red flag warnings, the idea of a weekend rainstorm is a balm for Southern California’s soul. And the rain could help with the firefight, but forecasters say it probably will not end what has been a devastating fire season.
The rains will bring “a very beneficial wetting rain,” but “it will not end the fire season,” said National Weather Service meteorologist Alex Tardy.
Because the rain is expected to be light, the risk of debris flows in burned areas is low, noted Ryan Kittell, with the NWS. But the chances of snuffing out the fire season also are low.
“If we get one more little dry spell, it’ll pretty much negate any benefit from this rain,” meteorologist Kittell said.
That dry spell may be just around the corner. The longer-term outlook suggests that, on the heels of this storm, there could be more weeks of dry weather.
“More likely than not,” Kittell said, “we’re looking at dry conditions to follow this weekend, going into early February.”
Light rain is likely
Forecasters are confident there will be some rain and mountain snow this weekend. They also expect that precipitation to be light — probably less than half an inch for the three-day rain event, from Saturday to Monday.
The most likely forecast would bring as much as half an inch to San Diego, San Clemente and Covina; two-fifths of an inch to downtown Los Angeles and Long Beach; one-third of an inch to Anaheim, Redondo Beach and San Clemente; and one-quarter of an inch to Santa Clarita and Canoga Park.
If those totals are correct, they would snap a record dry streak for downtown Los Angeles. It’s been 262 days, and counting, since downtown L.A. last saw more than one-tenth of an inch of rain on a single day — that was May 5.
The previous record was 253 consecutive days, from Feb. 25 to Nov. 3, 2008.
Downtown L.A. has received almost no rain since the water year began Oct. 1. Only 0.16 inches of rain has fallen since then — one of the driest starts to the water year on record. Just 3% of the average rainfall for downtown has accumulated at this point in the water year, which is 6.19 inches. For the entire water year, downtown L.A. averages 14.25 inches of rain.
Elsewhere across Southern California, this has been the driest start to the water year on record.
“We’ve never been in this territory before. We’ve never seen a mid-January with these numbers. Never,” said weather service meteorologist Tardy. The lack of precipitation and the Santa Ana winds — five wind events this month alone — make for a brutal combination.
The forecasted timing and totals of the upcoming rain event are still uncertain, Kittell noted. Less rain or as much as an inch in some locations is possible. And it could rain at any point from Saturday through Monday, but the best chance is Saturday night into Sunday morning.
Small chance of thunderstorms and landslides
There is also a 10% to 20% chance of thunderstorms and, with it, the chance of isolated but brief, heavy rain.
As a result, there’s a slight chance of landslides, debris flow and mudflows near recently burned areas, Kittell said.
With a thunderstorm, “heavy downpours, with rates maybe approaching a half-inch per hour” are possible, Kittell said.
“The vast majority of areas will not see this kind of situation,” he added, but if there are thunderstorms, “most likely we’ll see a spot — or two or three — that do get conditions like this.”
“It is not an atmospheric river,” Tardy said. “It is a cold storm. That is good for our burn scars.”
There is also a moderate risk of small hail across the region, Kittell said. Snow levels could fall to an elevation of 3,500 to 4,500 feet above sea level. Southern California’s ski resorts could get 10 inches of snow, Tardy said.
There’s a chance a thunderstorm could emerge directly over a recently burned area — creating a risk of landslides — but it’s not likely.
“It’s a possibility, but the risk is low,” Kittell said. Still, “it’s something to think about.”
For recently burned areas, he said, the rains could serve as a practice run in preparing for risks that are likely to remain for the next one to two years, after which the risk of debris flows and other landslide risks is substantially lowered.
Recently burned areas are at risk of landslides when subjected to intense rain over a short amount of time.
Hillsides are vulnerable to landslides after wildfires because the fires make the soils repellent to water, and instead of being absorbed, rain flows downhill and picks up rock and debris.
A “landslide” is an all-encompassing term that can describe any movement of rock, dirt or debris downhill. There are various types of landslides, including a mudflow, in which water rushes down with only mud, and is generally less than 15 feet deep.
During a debris flow, water not only picks up mud as it rapidly flows downhill but also rocks, branches, trees and sometimes boulders. This is considered a type of shallow landslide.
Debris flows can be deadly. In January 2018, 23 people died and at least 130 homes were destroyed when a river of mud and rock flowed through coastal Montecito, which had been burned less than a month earlier in the Thomas fire.
Rainfall rates need to be at around half an inch per hour or more to start causing debris flows of significance, Kittell said. Rates that are lower — like a quarter of an inch per hour — are less significant, “maybe some muddy water moving over some roads,” he said.
Before the rains, more fire weather ahead
In the meantime, red flag fire weather warnings — which began Monday morning across Southern California — will continue through much of Southern California through Thursday night.
Gusts earlier this week were quite strong. A wind gust hit 88 mph in the western San Gabriel Mountains Monday morning, on the Magic Mountain Truck Trail, said weather service meteorologist Lisa Phillips.
In the mountains east of San Diego, a wind gust of 102 mph was recorded at Sill Hill, nearing the all-time record of 105 mph recorded there on Feb. 26, 2020, Tardy said.
There will be a lull in winds coming out of the northeast for much of Wednesday. But they’ll expand and strengthen Wednesday night through Thursday morning, and will start lessening Thursday night, Kittell said.
The air is forecast to be extremely dry, with relative humidities under 10% through Friday. The meteorologist warned that vegetation is extremely dry, raising the risk of fire if ignited.
“Even if there are areas without a lot of wind, the risk for fast fire growth and large fire — it’s very large,” Kittell said. “And put any wind on it, it just makes it worse.”
Peak gusts of 30 to 50 mph are expected to be common Wednesday afternoon through Thursday night in Los Angeles and Ventura counties. There could be isolated gusts of 50 mph to 65 mph in the mountains and foothills.
Peak gusts could reach 30 mph in Oxnard and Anaheim, 33 mph in the western San Fernando Valley, 36 in Mira Mesa, 37 mph in Fillmore, 39 mph in Escondido, 40 mph in Thousand Oaks, 43 mph in Santa Clarita, 47 in Irvine, 54 mph in Ramona and 55 mph in Beaumont.
As of Tuesday afternoon, the weather service was not planning a “particularly dangerous situation” enhancement of the red flag warnings for later this week.
“Even though you don’t have a PDS,” Kittell stressed, “the red flag warning for this is still a very significant event.
“It’s definitely not a situation where we let our guard down.”