In August, Hurricane Hilary brought drenching rain to much of the desert Southwest. The map below shows radar-estimated rainfall totals in southern California; the spots of pink and white show mountainous areas where 4"-6" of rain fell, with even more at the highest elevations. It was a rare occurrence for a part of the country that averages less than 10" of rain in an entire year to get so much rain in just a 3-day period.
However, Hilary was just the latest storm to bring record rainfall to the West. From December 2022 through March, a series of atmospheric rivers carved through communities along California's coast, breaking rainfall records from San Luis Obispo to Los Angeles. In San Diego, more rain has fallen so far this year than in all of 2021 and 2022 combined- and the "wet season" hasn't even started yet. While these staggering statistics have brought devastating impacts to millions of Californians, the long-term impact to the Golden State's water supply is the silver lining. 2023's rain has essentially ended the drought in both California and Nevada, and even arid Arizona has seen big improvements.
The drought relief has been most profound in California and Nevada. A year ago, nearly every square mile of these states was experiencing at least Moderate Drought conditions. Today, drought conditions are nearly nonexistent there.
The 2023 deluge has also been a boon for California's reservoirs. California is by far the most populous state in the country, and it is also the USA's #1 supplier of produce. Maintaining this water supply is absolutely crucial, and the situation was bleak at this time last year.
As mentioned earlier, a series of atmospheric river events at the beginning of the year brought catastrophic flooding to California, especially its central coast. But, the deluge fell in the form of snow in the highest elevations of the Sierra Range, providing a slow-deploying source of fresh water for the state's reservoirs.
Snow is by far a better moisture source for the water supply, not only because it can slowly melt into reservoirs for months, but also because liquid rain is more likely to run off ending up in the ocean where it can't be drank or used for irrigation. So, what's responsible for this stunning change in fortune- and how long will it stay wet out West?
Last winter, the La Niña phase of ENSO was still firmly in place. While El Niño is much better known for delivering rain to Southern California, the entire ENSO cycle is only capable of slightly nudging the primary storm track of the West Coast. So even in La Niña, the opposite phase of El Niño, it's still possible for a storm track to impact California's coastline, which extends over 3,000 miles.
Image from NASA
And now, southern California's "wet season" is on the horizon. Nearly all of Los Angeles's average annual rainfall is recorded from October to April. And, El Niño conditions are expected to continue in the equatorial Pacific through the winter. This warm phase of ENSO is favorable for more wet weather in the region. If you're old enough to remember the 1997-1998 El Niño episode, which brought mudslide-inducing rainfall and coastal erosion that forever changed the California coastline, then the thought of another ENSO warm phase may be alarming. But it's important to remember that no two ENSO events are exactly alike, and the impacts can vary. Wasn't it just last winter when the La Niña phase was supposed to keep California bone-dry? Well, that didn't happen. Another example: the "Super El Niño" of 2015-2016 was even longer and stronger than the 1997-1998 event, but rainfall was below average in southern California during this time. For more on ENSO and its varied impacts, including the affects that climate change may be having on this oscillation, check out this blog post. And check out my post from May on the potential impacts of El Niño in the Tri-State area.
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