Los Angeles faces rare ‘high risk’ of flooding as atmospheric river hits SoCal
Angels — Forecasters highlighted Southern California, including downtown Los Angeles, for a rare “high risk” of flash flooding on Sunday and Monday due to the imminent arrival of a powerful atmospheric river that promises to dump several inches of rain across the region.
“High risk” is the highest score on NOAA’s flash flood threat scale and is only issued under the most severe flood forecasts.
“Changes in previous forecasts were intended to extend the high-risk area to the east into the eastern portions of the transect where downslope flow persists longer than in areas farther to the west,” NOAA’s Weather Prediction Center (WPC) said. “The high risk has also been expanded southward to include all of Los Angeles County south of the transect in accordance with coordination with (the National Weather Service Los Angeles office).”
Up to 5 inches of rain is likely from the Central California Coast Range south to the Transverse Mountain Range and parts of the Los Angeles metro area, while some downhill areas of those mountain ranges could see more than 10 inches of rain throughout the event.
“Life-threatening flash floods and urban flash flooding are possible in the high-risk area,” the Environmental Protection Committee warned.
Why rare ‘high risk’ flood days should be taken seriously
The National Weather Service’s Los Angeles office continues to use strong language in its forecast, indicating a high threat of widespread and dangerous flash flooding through Monday. Meteorologists warned that flooding problems would not be limited to normally exposed areas in the foothills, mountains and burned areas.
“All areas, including densely populated urban areas, will be at risk for life-threatening flooding,” NWS Los Angeles meteorologists wrote in all caps for emphasis during a weather discussion Friday night. “Small streams and rivers, as well as the Los Angeles River through the San Fernando Valley and Los Angeles Metro, will rise rapidly and turn into very dangerous raging rivers. Many roads will be impassable due to flooding.”
The National Weather Service also warned that numerous mudslides and debris flows are expected.
“Due to the significant amounts of rainfall expected in this time period, 3-6 hour rainfall rates could be problematic, exceeding USGS thresholds which could lead to mud/debris flow issues as well as rapid water rescues in local rivers and streams ” the NWS office in Los Angeles wrote in a weather discussion Sunday morning.
Additionally, moist and unstable air will likely produce strong, scattered thunderstorms along and off the Central California coast. Some storms could produce plumes Sunday afternoon across central and southern coastal waters from the Santa Barbara Channel northward past Santa Cruz. A brief tornado or two are also likely when any hoses wash ashore, the National Weather Service said.
NOAA’s Storm Prediction Center has issued a severe weather risk level of 1 out of 5 for areas facing this low-level tornado threat.
“Extreme” rainfall totals are possible across Southern California
The low pressure area will target another atmospheric river toward the California coast through Monday and bring a mass of moisture directly from the tropics, according to the FOX Forecast Center.
Unlike the last regime, this storm will be in no hurry to leave. This will keep the atmospheric moisture river headed toward California for about three days with rain continuing into Tuesday.
What is an atmospheric river?
“The last weather river event we had, there was movement,” FOX Weather meteorologist Britta Merwin said. “This time, there will be less movement, which means heavy rain.”
While the entire state of California will get rain, it is becoming clear that the areas of greatest concern will be along the southern coastal ranges across the Los Angeles Basin. Heavy rain is expected to fall between Sunday and Monday, and rainfall rates are expected to reach their peak on Sunday night.
The Los Angeles metro area could see 5 inches or more by midweek. Los Angeles averages just over 14 inches of rainfall per year. As mentioned earlier, the surrounding mountains and foothills could see 10 to 12 inches of rain.
“This is the kind of rain they can’t handle,” Merwin said. “This is a guaranteed setup for flooding. There’s no way around it. We know it’s going to be bad, and there’s going to be massive impacts.”
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Evacuation orders issued for some Southern California neighborhoods
This storm comes on the heels of Thursday’s Pineapple Express storm, and soils in Southern California were already highly saturated. Thursday’s record 2.45 inches of rain in Long Beach flooded highways, including the busy 710 Freeway. Los Angeles also recorded a daily record of 2.49 inches of rain on Thursday.
“Everyone, especially those near or in south-facing mountains, should start preparing now for possible evacuations during or even before the storm hits,” Los Angeles meteorologists said.
Evacuation warnings were issued for parts of low-lying Santa Barbara County on Friday, and strict evacuation orders were issued for some campgrounds and coastal communities on Saturday.
Aside from the high-risk area in the Los Angeles metro and the mountains north of the city, the level 3 out of 4 flash flood risk extends all the way up the coast toward the San Francisco Bay Area.
On Monday, the Level 4 risk remains centered directly over the Los Angeles metro area, while the Level 3 risk extends from north of Los Angeles southward into the San Diego area.
“It shows you confidence that we know this is going to be a bigger storm,” Merwin said. “It has a lot of moisture. There’s a much greater threat of flooding in Southern California.”
Due to the threat, NASCAR Cup Series and NASCAR Mexico Series events scheduled for Sunday were postponed one day to Saturday to take advantage of the drier weather.
California’s ‘astronomical storm’: Historic 1,000-year floods from 1861 to 1862 marked by 8 weeks of atmospheric rivers
Why are high-risk days so dangerous?
High-risk flood forecasts are only issued on about 4% of days (including tropical and non-tropical events), but this risk category accounts for 39% of flood-related deaths and 83% of flood-related damage in the continental United States. According to research by meteorologists at NOAA’s Weather Prediction Center (WPC).
Furthermore, 46% of high-risk flood days cause at least one death or injury, and 62% of these days cause at least $1 million in damage. Greg CarbinHead of the Forecast Operations Branch at WPC, referred to in the research.
The last day of severe flash flood threat in the United States was August 20, 2023, which coincided with Hurricane Hillary moving inland across Southern California.
Several feet of snow is likely in the Sierra Nevada
The moisture plume will generate a major winter storm in the Sierra Nevada, with 4 to 6 feet of snow possible above 5,000 to 6,000 feet, the Fox Forecast Center said. Disturbances to daily life, including difficult to impossible travel conditions, are expected.
In the mountains of Southern California, snow levels are expected to reach about 7,000 feet, but above that level, 2-4 feet of snow is possible.
Damaging winds are likely around the San Francisco, Central California coast
In addition to the threat of heavy rain, damaging winds are expected to blow along the Central California coast south toward Santa Barbara. This includes parts of the San Francisco Bay Area.
High wind warnings have already been posted for wind gusts along the coast and coastal mountains of up to 60-70 mph. Wind warnings warn of possible wind gusts of up to 50 mph in the San Francisco Bay Area.
Stronger winds are likely in the mountains surrounding Southern California, reaching speeds of 70-80 mph.
Winds greater than 40 mph will likely extend inland into the Central Valley. These winds will have the potential to down many trees and cause major power outages.
On the coast, these winds will also send the Pacific Ocean into a frenzy with waves between 10 and 20 feet high likely to hit beaches up and down the state.
Moisture flowing behind the storm will keep spells of rain in the forecast through most of this week, although there will be longer dry spells between rain showers to shake them off.