Los Angeles as seen from Griffith Park on January 10th, 2019, just days before an Atmospheric River moved into the region. Source: Gerardo Diaz Jr.
After spending a few days soaking in the Southern California sun, I was about ready to head back up to my home in the Midwest. That being said, my last morning in LA was anything but sunny. The sky was grey and traffic was backed up for miles on several of the freeways as rain slowly moved its way into the normally picturesque blue skies of the famed Golden Coast. Indeed, these conditions had only just begun to roll into the region after months of historic wildfires that left many parts of the State in ruins, including well-known towns such as Malibu. And while the region is renowned for having near-perfect weather all year round, these sudden and often abrupt rain events are an integral piece to the region’s climate.
Grey skies over the Santa Monica Pier. Source: Gerardo Diaz Jr.
Atmospheric Rivers, or ARs, are the transport of moisture from the tropics into higher latitudes. In the case of California, moisture tends to be transported over from the warm tropical waters of the West Pacific. As the moisture is carried over a landmass, the continental air mass will become saturated, resulting in the release of moisture in the form of precipitation. This process is so frequent and comes at such a varying degree of intensities on the West Coast that it is colloquially referred to as the Pineapple Express. Overall, ARs bring both benefits and challenges to the State, all while playing an important role in supplying and maintaining the entire region’s water sources.
Cloudy skies overlooking downtown Santa Monica on the morning of January 14th, 2019, as more and more moisture was advected onto shore. Source: Gerardo Diaz Jr.
In terms of their benefits, ARs are responsible for providing much of the annual precipitation for places like Southern California, along with the vast majority of the snow pack for higher elevations all along the entire stretch of the West Coast. The improvement of snowpack following a severe drought is critical for ski resorts all along the State. The lack of snow over much of the Sierra Range was a major cause for concern for resorts all across the State, especially in North and Central California. Thankfully, much of the region as of the time that this article was released has experienced a rejuvenation in its snow pack thanks entirely to the most recent AR. In fact, according to NWS Sacramento, many parts of the Sierra and outlying towns hare now at or even above average precipitation levels for the first time in months. As such, ARs can significantly help in alleviating drought conditions and can bring an immediate and sudden end to wildfire seasons, as has been the case over the last couple of years in California. The most recent and devastating fires across the State were essentially washed away by the introduction of moisture into the region via ARs.
The average amount of water that has fallen over parts of Central and Northern California this season. Source: National Weather Service, Sacramento.
With that all being said and done, moderate to strong ARs can have dramatic and extreme impacts to unsuspecting residents, especially if they occur immediately following a prolonged dry-spell. In fact, during this most recent rain spell alone, LA has seen over half of the rain it typically experiences in a single year, placing it in a much better situation than where the region was during the end of last year in terms of drought conditions but in a much more serious risk for flash flooding and landslides. Indeed, an immediate and large introduction of moisture to such a concentrated region just weeks after it experienced severe fire damage is a recipe for new challenges, including mudslides, and landslides, if said areas are introduced to large amounts of precipitation. This was exactly what occurred on January 17th, 2019, as reports of cars being either washed away or struck directly by mud and debris were reported all across the Pacific Coast Highway in Southern California. Simply put, the dried and burnt up mountains and hills in the region simply could not absorb the large quantities of precipitation that fell in the region during such a short amount of time. It also should be noted that Northern California is just as susceptible to extreme moisture introduction following heavy drought and fire weather conditions.
Damage along the Pacific Coast Highway due to a heavy rain-induced mudslide. Source: Courtesy Caltrans(@CaltransDist7).
It is an understatement to say that this atmospheric process is an important part of the climate system of the region. Without ARs, the state of California, and indeed the entire West Coast of the United States, would experience a much more arid climate and subsequently would struggle to maintain its current population. Nevertheless, when strong ARs bring large quantities of moisture are advected into the region in a short amount of time, the consequences can be extreme, especially in those areas that experience drought conditions immediately before the moisture advection. As such, understanding ARs is important for both meteorologists who forecast them and for those who live along the West Coast and experience their effects first-hand.
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© 2018 Meteorologist Gerardo Diaz Jr.