Large parts of California were recently drenched by intense rains, as an atmospheric river weather system made landfall across the state. While bringing much-needed precipitation to alleviate drought conditions, experts warn the heavy rains also raise the risk of flash floods and mudslides.
Understanding Atmospheric Rivers
Atmospheric rivers are categorized as narrow regions of high water vapor transport that extend for thousands of miles from the subtropics to the U.S. West Coast. Specifically, they consist of:
- Length: Can extend for over 5,000 miles
- Width: Typically 400-600 km wide
- Water vapor transport: Over 15 g/kg on average
- Structure:Tall, narrow plume-like moisture corridors
When atmospheric rivers make landfall, they drive episodes of intense horizontal precipitation, contributing significantly to California’s rainfall.
- California atmospheric river landfalls per year: 7-10 average
- Contribution to annual CA precipitation: 30-50%
- Consequences of strongest systems: Flash floods, mudslides risk to life and property
Overall, while mostly beneficial contributors to water supply, especially strong atmospheric rivers can overwhelm infrastructure capacity and require emergency planning.
Key Details on Atmospheric Rivers
| Parameter | Details |
| Formation | Extend thousands of miles from subtropics to West Coast of North America |
| Structure | Long, narrow regions of high water vapor transport |
| Scale | Can be hundreds of miles long but less than 100 miles wide |
| Strength | Categorized into weak, moderate, strong and extreme atmospheric rivers based on vapor transport and precipitation |
| Impact | Contribute 30-50% of annual rainfall in California when make landfall |
| Frequency | Average 7-10 atmospheric river landfalls per year in California |
Recent Extreme Weather Event
An intense atmospheric river made landfall in central California starting January 28th, 2023, resulting in:
Record 24-hour rainfall totals
- Santa Barbara:46 inches
- Los Angeles:46 inches
Additional precipitation measurements
Up to 14 inches in southern CA mountains over 48 hours
- General range across region: 4 to 7 inches
- Categorization: ‘Pineapple Express’ atmospheric river
- Origin: Subtropical Pacific near Hawaii
- Mechanism: Tapped plume of deep tropical moisture
Consequences
- Flash flooding through multiple CA counties
- Rising of rivers to flood stage
- Mudslides in wildfire burn scar areas
- Over 12 million residents under flood watches
This atmospheric river delivered all-time record single-day rainfalls to locations across central and southern California – leading to hazardous flooding, landslides, and demonstrating how tropical moisture corridors can yield extreme precipitation.
Impact of Recent Atmospheric River Event
- Record single-day rainfall of 5.46 inches in Santa Barbara on Jan 28
- Up to 14 inches of rain in some southern California mountains in 48 hours
- Flood watches issued for over 12 million residents
- Over $1 billion in estimated flood damage so far
- At least 18 deaths attributed to the storm across California
The extreme rainfall comes amidst the state’s severe and prolonged drought, providing welcome precipitation. But consequences like flash floods, landslides, falling trees and power outages also endangered lives and property throughout the region.
Link to El Niño Climate Cycle
The potential link between the recent atmospheric river event and climate cycles like El Niño is being analyzed by scientists.
- El Niño : It refers to a periodic warming of tropical Pacific sea surface temperatures by >0.5°C
- Typical frequency: Every 2 to 7 years
- Effects: Disrupts global weather patterns
- Impact on U.S. West Coast: Increased precipitation and storms
Proposed El Niño influence on California atmospheric rivers
- Number per year: Additional 1-4 landfalls
- Strength increase: More moisture transport
- Driver: Shifting of subtropical jet streams
Uncertainities Loom Large
- Current 2023-24 El Niño categorized as weak-moderate (+0.8°C ocean warming)
- Not all prior El Niños strongly boosted landfalling atmospheric rivers
While possibly an ‘amplifying’ factor, experts advise against concluding El Niño was sole contributor to recent extreme event, pending further analysis. Overall, relationships require deeper study regarding warming impacts on atmospheric river behavior.
Impact on California’s Drought
The recent atmospheric river delivered extreme rainfall across drought-stricken California:
- Prior conditions: 95% of state faced extreme-exceptional drought at end of January 2023
- Precipitation estimate: 8 trillion gallons, potentially supplying 15 million Californians for 1 year
- Initial impacts: Rapid reservoir storage level rises; flood risks emerge
However, hydrologists caution against prematurely declaring the drought over, as continued above-average rain and rebuilt snowpack is needed through Spring 2024.
Key next steps
- Ongoing precipitation monitoring
- Analyzing links to climate change patterns
- Planning for flood risks and emergency preparedness
- Investing in long-term drought resilience
This impactful atmospheric river event provided urgent drought relief yet lasting implications remain uncertain pending further data. Preparedness for both water shortages and extremes remains key for California communities.
