A fast-moving atmospheric river is heading toward California this week and could pack a punch, threatening periods of heavy rain and possible flooding and debris flows in recently burned areas.
After arriving in Northern California on Wednesday, the storm system is expected to land in Southern California on Thursday, where it could remain all the way through Saturday. It could produce the most rain downtown Los Angeles has seen in at least a month, and possibly since February.
“There is a chance for burn area flash flooding and debris flow issues. A flood watch will be possible for burn scars,” Rose Schoenfeld, a National Weather Service meteorologist in Oxnard, said Tuesday afternoon.
While the storm brings some concerns about flooding, it could also mark the end of the autumn fire season. Rainstorms in Northern California have already lessened the seasonal fire danger there considerably, and this storm could do likewise in the Southland.
The National Weather Service office in Oxnard called the forecast storm “potentially significant,” with roadways expected to flood in spots, rockslides possible on canyon roads and a chance of mudflows.
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The storm could snarl traffic in Los Angeles and Ventura counties for the Thursday afternoon and Friday commutes, bringing ponding of water on roadways. The L.A. metro area could be hard hit, with localized freeway flooding in low spots, forecasters warned.
In the L.A. area, there could be 1 to 2 inches of rain on the coast and in the valleys, with 2 to 4 inches possible in the mountains and foothill areas. “We’re looking at moderate to locally heavy rain rates with this system,” Schoenfeld said.
Between Thursday and Friday, Long Beach could get 1.08 inches of rain; downtown Los Angeles, Redondo Beach, Pomona and Pasadena, about 1.3 inches; Oxnard, 1.51 inches; Santa Clarita, 1.75 inches; Westlake Village, 1.98 inches; Santa Barbara, 2.22 inches; and Ojai, 2.88 inches.
Winds also could be an issue, with peak gusts of 50 mph along the Grapevine section of the 5 Freeway and in the Antelope Valley. Winds could hit 21 mph in downtown L.A., 24 mph in Long Beach, 26 mph in Santa Clarita, 32 mph in Redondo Beach and 39 mph in Lancaster.
Rain is expected to fall at widespread rates of 0.25 to 0.5 inches per hour, and 0.5 to 0.75 inches per hour in the mountains. Some areas could see rain fall at a rate of an inch per hour, with the Santa Ynez Mountains in Santa Barbara County and mountains in western Ventura County at highest risk.
Rainfall rates of half an inch per hour or greater can trigger significant debris flows — a risk that’s particularly pronounced in recent burn areas.
“If models stay consistent, it’s likely that a flood watch will be issued for the most vulnerable areas, including the most recent burn scars,” the weather service said.
The trajectory of the storm will bring winds from the south and southeast, Schoenfeld said, but it is possible they could shift by Friday to come from the north. The winds could create difficult driving conditions, especially for taller vehicles.
This isn’t the type of storm where officials are urging most people in the L.A. area to stay off the roads. However, people may want to avoid driving if they live in flood-prone areas such as canyon and mountain passes, or in the northwestern corner of San Luis Obispo County.
The coming storm could make a difference in California’s fire season “if we get the kind of storm totals we’re anticipating,” according to David Gomberg, NWS fire program manager. It will take a couple of weeks to assess how vegetation responds, he said, but the expected precipitation “definitely should start to make a big difference in the fire season.”
Officials generally like to see 3 to 4 inches of widespread rain in lower elevations to declare an end to the fire season. Downtown L.A. already received 1.38 inches of rain on Oct. 14. Before that, the last calendar day when downtown got more than an inch of rain was March 13, when 1.03 inches fell.
The wettest calendar day so far this year was Feb. 13, when 2.8 inches of rain fell on downtown L.A.
Temperatures are expected to plunge as a result of this week’s storm. Downtown L.A. hit a high of 92 degrees on Monday — one degree short of the record for the calendar day — but was expected to reach only 61 degrees by Friday. Anaheim’s high of 88 degrees Monday was expected to fall to a high of just 64 degrees by Friday. In San Diego, Monday’s high of 89 degrees at Brown Field Municipal Airport was expected to fall to a high of 61 degrees by Friday.
In San Bernardino County, wind gusts of up to 60 mph could hit the mountains and whip into the deserts.
There should be a break between the rainfall expected to end Friday and further rain showers that could hit L.A. starting Saturday.
For Saturday, “it might be something that you need to play it a little bit by ear and see how things are looking as we get closer,” Schoenfeld said. “That Saturday precipitation amount is pretty uncertain.”
Part of the issue with forecasting Saturday’s rain is the development of a “cut-off low pattern” — when the mass of low pressure driving storm activity is cut off from the jet stream — which is so notoriously difficult to predict it’s often called “weatherman’s woe.”
“With that low not tied to the bigger, larger weather features, it is unpredictable exactly where that center will end up” Saturday, Schoenfeld said. “It can have a lot of implications for exactly how much rain we’ll end up seeing on Saturday for the area, if any.”
There is also a chance of thunderstorms across a large swath of California, bringing the possibility of small hail, gusty winds, heavy downpours and lightning strikes.
In the San Francisco Bay Area, the most intense rain is expected late Wednesday and Thursday. Most of the region will see between half an inch and an inch of rain, with higher amounts in the mountains. The region could see gusts of up to 60 mph, and “downed trees and power lines may result in power outages,” according to the weather service office in Monterey.
Sacramento could get up to 2 inches of rain.
The storm could also bring heavy snow to the Sierra Nevada, and meteorologists were already discouraging travel between Thursday morning and Friday morning. Donner Peak could get 12 to 18 inches of snow, and the California Highway Patrol is expected to require motorists to use chains to cross California’s mightiest mountain range.
A winter storm warning has been issued from 1 a.m. Thursday until 4 p.m. Friday for elevations above 7,500 feet in Mono County, according to the weather service. Snow could accumulate between 1 and 2 feet on the Sierra Crest and between 4 and 12 inches along Highway 395.
Hazardous conditions could affect driving Thursday morning and evening, according to the weather service. Winds could also damage power lines and trees.
Wind gusts in the Sierra could exceed 100 mph along the ridgetops, the weather service office in Reno said.
Mammoth Mountain was celebrating the earlier-than-expected start to the ski-and-snowboarding season. Opening day was moved up to Saturday due to the incoming snowstorm. “Winter’s loading,” the resort wrote on its website. “Get ready for a stronger start to the season!”
Closer to Lake Tahoe, opening day for Northstar and Heavenly is set for Nov. 21; with Palisades Tahoe following on Nov. 26. At Palisades Tahoe, one problem has been the warmer air being pulled up from the south. “We’ll continue to search for bigger snowstorms that could give us an early season boost,” the resort wrote on its website.
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