Rare "high risk" threat for flooding in Southern California, NOAA says

Key Concepts

• Atmospheric River: A concentrated band of moisture in the atmosphere, responsible for significant precipitation.
• Burn Scars: Areas affected by wildfires, where soil is unable to absorb rainfall effectively, increasing flood risk.
• Flash Flooding: Rapid and intense flooding occurring within a short timeframe, often triggered by heavy rainfall.
• Debris Flow/Mudslides: A rapid movement of a slurry of water-saturated rock, soil, and debris down a slope.
• Evacuation Orders/Warnings: Official instructions to residents to leave or prepare to leave areas threatened by hazardous conditions.

Intense Flooding on the West Coast - A Detailed Report

I. National Overview & Risk Assessment

The West Coast is currently experiencing intense flooding due to a powerful storm and an atmospheric river. The National Oceanic and Atmospheric Administration (NOAA) has identified a rare high risk for floods across California. Nearly half of California, encompassing over 19 million residents, is under a moderate or high risk of potential flooding as of Christmas Eve. This storm follows a similar atmospheric river event that impacted Northern California just days prior, prompting officials to prepare for potentially severe consequences.

II. Situation in Altadena, California

CBS News reporter Andreas Gutierrez reported live from Altadena, north of Los Angeles, detailing the immediate impact of the storm. The area is particularly vulnerable due to recent wildfires, specifically the “Eaton Fire.” The burned areas exhibit reduced soil absorption capacity, leading to significant runoff and localized flooding.

• Rainfall Intensity: Torrential rain began shortly after midnight, intensifying around 4:30 AM, creating substantial runoff.
• Mitigation Efforts: Heavy machinery is being used to clear debris and prevent clogging of storm drains.
• Evacuation Procedures: Mandatory evacuation warnings are in effect for over 120 homes in Altadena and Pacific Palisades. LA County Sheriff deputies are conducting door-to-door notifications.
• Resident Experiences: One resident, having experienced a mudflow after the Eaton Fire, proactively evacuated to an Airbnb. She noted the lack of barriers after the fire initially left her home completely caked in mud, but expressed hope that newly installed barriers and sandbags will provide protection this time. The use of sandbags, typically uncommon in Southern California, highlights the severity of the current situation.

III. West Coast Weather Forecast & Regional Impacts

CBS News Bay Area meteorologist Zoe Mintz provided a comprehensive forecast, outlining widespread weather alerts across the entire West Coast.

• Wind Warnings: High wind warnings are in place, with gusts reaching upwards of 80-90 mph already recorded.
• Snowfall: 5 to 8 feet of snow is expected at higher elevations in the Sierra Nevada, creating treacherous travel conditions for skiers and snowboarders. Chain requirements for tires are strongly advised.
• Rainfall Totals: Central and Southern California are forecast to receive 6 to 14 inches of rain within the next 24 to 36 hours. This is a significant amount of precipitation in a short period.
• Burn Scar Impacts: Burn scar flash flooding is a major concern, leading to likely mudslides and debris flows. Rainfall is expected to continue throughout the morning and into the early afternoon, with further waves of rain anticipated throughout Christmas Day.
• Travel Disruptions: Messy travel conditions are expected across the entire West Coast.
• Contrasting Conditions: While the West Coast faces severe weather, the central part of the country is experiencing record-breaking warm temperatures, with Denver, Colorado, expected to reach 70°F on Christmas Day.

IV. Technical Considerations & Terminology

• Atmospheric River: These are essentially long, narrow regions in the atmosphere that transport massive amounts of water vapor, often leading to extreme precipitation when they make landfall.
• Burn Scar Flash Flooding: Wildfires destroy vegetation and alter soil structure, reducing its ability to absorb water. This results in rapid runoff and increased risk of flash floods, even with moderate rainfall.
• Debris Flow: A highly destructive mixture of water, sediment, and debris that flows rapidly down a slope, posing a significant threat to life and property.

V. Synthesis & Main Takeaways

The West Coast is facing a significant weather crisis characterized by intense rainfall, high winds, and heavy snowfall. The combination of an atmospheric river and burn scars from recent wildfires is creating a particularly dangerous situation, with widespread flood warnings and evacuation orders in effect. The forecast indicates continued hazardous conditions throughout Christmas Day, emphasizing the need for caution and preparedness. The contrasting weather patterns across the US – severe storms in the West and record warmth in the central region – underscore the complex and dynamic nature of weather systems. Proactive measures, such as evacuations and debris management, are crucial in mitigating the potential impacts of this extreme weather event.