California has recently been inundated by heavy rains from a pair of atmospheric rivers, causing widespread disruptions, including mudslides, floods, and major weather impacts. But what exactly is causing such intense weather, and why is it affecting the state so severely?
The Pineapple Express is the term for a powerful type of atmospheric river, a weather system commonly seen along the US West Coast and in parts of Canada. These systems derive their name from the tropical moisture they carry, which originates near Hawaii, an island chain famed for its pineapples. While “Macadamia Nut Express” might have a similar origin, it doesn’t quite carry the same weight!
Atmospheric rivers are long, narrow bands of concentrated moisture that can extend over 2,000 miles (3,220 km) but are only a few hundred miles wide. These systems typically form ahead of cold fronts, where warm, moist air is lifted into the atmosphere, condensing into rainclouds as it rises. This process can produce significant rainfall, especially in mountainous regions.
Atmospheric rivers play a crucial role in California’s climate, accounting for roughly 50% of the state’s annual precipitation. While they are vital for replenishing water supplies, they can also bring significant hazards. The potential for extreme flooding, mudslides, and property damage is a growing concern, especially with stronger atmospheric river events.
These weather systems can intensify in different ways:
- Prolonged rainfall: If an atmospheric river stalls over a region, it can bring heavy, sustained rainfall, increasing the risk of flooding.
- Rapid ascent over mountains: If the system is forced to rise quickly over mountain ranges, the moisture is squeezed out more rapidly, leading to more intense rainfall.
- Increased moisture levels: More moisture and stronger winds within the atmospheric river system can result in more severe weather.
Atmospheric rivers are not exclusive to the US and Canada. The UK has also seen extreme weather from these systems. A prime example is the Cumbria flood of 2009, where an extraordinary 12.5 inches (316.4mm) of rain fell in just 24 hours in Seathwaite, Borrowdale.
This latest Pineapple Express storm has been especially impactful for California. One of the primary reasons is the storm’s persistence over highly populated regions. Over the course of Sunday and Monday, Los Angeles saw a staggering 203mm of rainfall, more than half of the city’s annual average. The surrounding hills have been even wetter, receiving over 300mm of rain in just 48 hours.
This overwhelming rainfall, combined with snowmelt, has caused dangerous flooding and mudslides in some areas. Although the atmospheric river is expected to move out of the region by Tuesday, showers will persist briefly before the system clears out.
As climate change continues to warm the atmosphere, studies by the National Oceanic and Atmospheric Administration (NOAA) predict that atmospheric rivers will become more frequent and intense, particularly at lower elevations in the Western US. With warmer temperatures, more moisture will evaporate from the oceans, increasing the overall moisture available for these systems. This could lead to longer, wider, and more dangerous atmospheric rivers, making flooding and storm-related damage more common and severe.
In addition to increased rainfall, warmer ocean temperatures could shift these systems, intensifying the threat to regions already prone to flooding. As California braces for more extreme weather events, proactive measures will be crucial to minimize their impact on vulnerable communities.
