Daniel Swain

ARkStorm Fears Recede in California Despite Flooding; Anomalous Lack of Rain-Shadowing Explained by Weather West

Article-Grid, Northern California, Physical Geography / By

Fears of an impending ARkStorm in California have receded, although much of the state has been receiving prodigious amounts of rainfall and the forecast remains wet for the next 10 days. In the most recent storm, the heaviest rains have fallen in the Santa Barbara and Ventura areas, northwest of Los Angeles. The map posted below shows total precipitation amounts of up to 14 inches in a 24-hour period; other reports indicate that a few areas have received more than 18 inches. As would be expected, floods and mudslides have hit the region, causing considerable damage and taking several lives. For the state as a whole, however, the damage has been less than has been reported in many sensational news articles. I have read stories and seen videos that describe California as being “devastated,” “drowned,” and “underwater.” Despite the localized destruction, which should not be minimized, the recent storms have been beneficial for the state as a whole, washing away a devastating drought, at least temporarily. Even in some of the hardest hit locations, some rain enthusiasts posting on the Weather West blog are joyful for what they have received.

From a climatological perspective, the most interesting feature of the map posted above is not the torrential rain in places like San Marcos Pass, which periodically receive heavy and extended downpours. More unusual are the relatively high figures in inland areas that are rain-shadowed by mountains, such as the Cuyama Valley. Cuyama is extremely dry, receiving only 8 inches of precipitation a year on average. As bands of rain typically move from the south or southwest to the north and east, they dump most of their moisture over the coastal highlands as the air rises and cools; as the air descends and warms on the lee side, precipitation rates plummet. This dynamic is especially noted under conditions of an atmospheric river, which brings a relatively shallow but extremely wet airmass streaming in from the subtropics. As the recent flooding rains in the Santa Barbara area came from a stalled atmospheric river, the relatively high level of rainfall in Cuyama was unexpected.

A more pronounced precipitation anomaly was found further to the north, just to the east of the Sierra Nevada crest in east-central California. A high range, the Sierra rings most of the water out of winter storms. And as result, the Owens Valley, lying just to east of the southern Sierra, is extremely dry. The town of Bishop in the northern valley receives less than 5 inches of precipitation in the average year. Yet over the course of a mere 24-hour period on January 9th and 10th, 2023 – under conditions of an atmospheric river – Bishop received over 3 inches. And the downpour continues; just 11 minutes ago, commentator Unbiased Observer noted in Weather West that Bishop is closing in on 4 inches. This oddity demands an explanation.

Fortunately, such an explanation was made available, again by Unbiased Observer, on the Weather West blog, run by meteorologist Daniel Swain. I have posted the pertinent information below from the blog’s discussion forum. Such sharing of information among a devoted community of weather watchers is one of the many reasons why Weather West is such a valuable resource.

Should California Be Bracing for a Possible ARkStorm?

Article-Grid, Northern California, Physical Geography / By

The storm currently hitting California has not produced as much precipitation as was anticipated, irritating some Weather West readers (see yesterday’s post) while reducing flood concerns for the present. But the forecast remains extremely wet over the next week and beyond. As the maps posted below show, rain in the lowlands and snow in the mountains could fall in prodigious quantities, which would probably cause extensive flooding (note the 256 inches of snow over roughly two weeks forecasted on one of the maps posted below). If the current seven-day forecast verifies, and if the wet pattern remains entrenched, California might even experience what is known as an ARkStorm, an event that occurs on average once every several hundred years. In an ARkStorm, much of the Central Valley, California’s agricultural heartland and home to millions of people, could be inundated for weeks.

As noted in the previous post, California has been locked in a persistent drought, experiencing only two wet years out of the past 12. An abrupt end to a long-term drought by devastating floods would not be unprecedented. Indeed, this is precisely what happened in the mid 19th century. As reported by EarthDate:

In the 1840s and 1850s, California was exceptionally dry, so by the fall of 1861, California ranchers were hoping for rain in late November they got what they were wishing for and – then some. It didn’t stop raining for 43 days and by January 1862 the Central Valley was filled with an inland sea.

The Great Flood of 1862 was that an extraordinary event, one that affected much of the western United States. The Wikipedia article on it provides a good summary. As it notes:

The event dumped an equivalent of 10 feet (3.0 m) of water in California, in the form of rain and snow. .. An area about 300 miles (480 km) long, averaging 20 miles (32 km) in width, and covering 5,000 to 6,000 square miles (13,000 to 16,000 km2) was under water over a period of 43 days.

Although this was the biggest flood in California’s recorded history, geological evidence shows that even larger floods have occurred over the past several thousand years. Of particular note were the years 440, 1418, 1605, and 1750. The largest flood was that of 1605 (± 5 years). As noted in a 2017 Quarternary Research article, this event may have even produced a large lake in the Mojave Desert that lasted for several decades. The Quarternary Research article claims that this flood may have been linked to a global cooling cycle associated with this so-called Little Ice Age. As the authors note, “This cooling was probably accompanied by an equatorward shift of prevailing wind patterns and associated storm tracks.”

The current concern is that global warming will lead to increased flood risks in California – along with increased drought risks. As Xingying Huang and Daniel Swain wrote in an August 2022 Science Advances article:

Despite the recent prevalence of severe drought, California faces a broadly underappreciated risk of severe floods. Here, we investigate the physical characteristics of “plausible worst case scenario” extreme storm sequences capable of giving rise to “megaflood” conditions using a combination of climate model data and high-resolution weather modeling. Using the data from the Community Earth System Model Large Ensemble, we find that climate change has already doubled the likelihood of an event capable of producing catastrophic flooding, but larger future increases are likely due to continued warming. We further find that runoff in the future extreme storm scenario is 200 to 400% greater than historical values in the Sierra Nevada because of increased precipitation rates and decreased snow fraction. These findings have direct implications for flood and emergency management, as well as broader implications for hazard mitigation and climate adaptation activities.

(See the map made by Xingying Huang and Daniel Swain posted blow to compare historical ARkStorms and those predicted for the future.)

 

Daniel Swain’s avid followers at Weather West have noted how the current situation follows Swain’s recent ARkStorm article:

Many researchers are concerned that California is not doing enough to prepare for the possibility of devastating floods. One proposal for dealing with extreme precipitation events is to allow rivers to occupy more of their natural floodplains, as outlined in a New York Times article published today. Such an approach would also help recharge California’s aquifers, many of which are severely depleted. But as the author observes, this would be an expensive solution that would generate pronounced opposition. From an environmental perspective, however, it makes a lot of sense.

 

The Weather West Blog Community and the Possible End of the Great California Drought

Article-Grid, Cartography, Northern California, Place Names / By

One of my favorite blogs is Weather West: California Weather and Climate Perspectives, run by meteorologist Daniel Swain. Posting once or twice a month, Swain focuses on current and upcoming weather events and conditions. He delves into meteorological complexities but writes in an accessible manner that can be easily understood by non-specialists. More important for the concerns of GeoCurrents, Swain’s posts are always illustrated with informative and often striking maps. For those who appreciate the aesthetic properties of cartography, it can be difficult to beat meteorological mapping. I often find the patterns and colors almost mesmerizing.

Equally impressive is Swain’s devoted readership. Each of his posts receives thousands of comments. Many are deeply informed, and they are also often illustrated with useful maps and dramatic photographs. For weather enthusiasts such as myself, the cloudscapes that are periodically posted on Weather West are reason enough to follow the blog.

What I most appreciate about the Weather West community, however, is its idiosyncratic perspective on precipitation. Here we find a group of devoted people who love rain and fully understand just how essential it is. Given California’s seemingly interminable drought – 10 of the past 12 years have been dry, the last two exceedingly so – one might expect this attitude to be common in the state, but in my experience it remains rare. Even National Weather Service (NWS) forecasters in California sometimes write about the “threat of rain” during times of dire drought. A few years back I was so frustrated by such mindless wording that I wrote a letter to the NWS urging them to replace “threat” with “promise” under drought conditions. I was surprised to receive a reply, but it turned out to be defensive and entirely non-apologetic. But some people understand. The best birthday present I ever received was a CD put together by my wife filled with rain-positive song in many genres and from several countries. One of the most memorable was Luke Bryan’s raunchy country tune called “Rain Is a Good Thing.” As Bryan emphasizes, farmers certainly understand. As his song opens:

My daddy spent his life lookin’ up at the sky

He’d cuss kick the dust, sayin’ son its way to dry

It clouds up in the city, the weather man complains

But where I come from, rain is a good thing

When rain does come to California, the Weather West community exults. They post their own precipitation numbers with pride, and bitterly complain when their own locations are stinted, ending up in the dreaded “donut hole.”  Some tend toward pessimism and sometimes find themselves gently chided by those more hopeful about coming storms. Overall, they seek to teach and inform each other, and thus form a model blog-focused community. (“Model” is used as something of a pun here, as Weather West readers often urge each other to beware of “model riding,” or giving too much credence to particular meteorological model outputs. This is especially the case when the output in question refer to “fantasyland,” or the time beyond the period of relatively reliable forecasting.)

Currently, the Weather West community it very excited but also worried. California’s long-term drought has just broken, at least temporarily. December precipitation was pronounced over almost the entire state, and January looks to be wetter still. Swain’s most recent post, of January the 2nd, is titled “Major Norcal Storm Wed.; Potential High-Impact Storm/Flood Pattern to Continue for 10 Plus Day. Wet Antecedent Conditions Set Stage for Future Flood Risk.” Even the blog’s most rain-besotted commentators are now concerned that they may get too much of a good thing. Some are even sheepishly admitting that they are now hoping for a mid-winter ridge that would produce a spell of dry weather.

California’s abrupt transition from dry to wet this winter was not expected. Until quite recently, mid- and long-range models predicted yet another rainy season of little rain. As almost all the state’s precipitation falls between November and March, this is a crucial matter. Driving these dry forecasts was the fact that the Pacific Ocean is still in La Niña* conditions, which have persisted for the past two years. In La Niña winters, far Northern California often gets ample precipitation, but the rest of the state is generally dry. In these years, the jet stream is typically displaced to the north and must ride over a large high-pressure ridge somewhere in the eastern Pacific. If the ridge is displaced too far to the east, California is hard hit by drought. If the high-pressure zone is instead pushed westward, cold storms can ride over the ridge and produce moderate rain and decent amounts of mountain snow. Under the contrasting El Niño** regime, a different winter pattern typically prevails, with the jet stream ripping directly across the Pacific. El Niño years usually bring abundant precipitation, especially to Central and Southern California. What makes the current situation so unusual and perhaps even inexplicable is that California is now experiencing an El Niño pattern in a La Niña year, with one relatively warm storm after another lined up across the Pacific. Meteorologists are trying to figure out what is going on, and undoubtedly much more will be written on the subject.

If the current forecasts through January pan out, California could end up with full reservoirs and a very healthy snowpack in the higher elevations. But that does not mean that drought conditions will not necessarily return before the wet season ends. Last year, heavy precipitation in December was followed by a parched period stretching from January through March, generally the wettest time of the year. By the end of the summer, the state’s crucial reservoirs were frighteningly depleted.

But even if this February and March are dry, fears of a disastrously water-short summer of 2023 are currently being washed away. Indication for the 2023-2024 wet season also look promising, as La Niña is dissipating and El Niño looks like it might return. But El Niño sometimes fails to produce the predicted downpours, as was the case in the winter of 2015-2016. As U.C. San Diego Scripps Institute of Oceanography reported:

Most long-range forecast models predicted a potentially drought-ending deluge in California from the climate pattern known as El Niño in winter 2015-16, but the actual precipitation was far less than expected. … “Comparing this El Niño to previous strong El Niños, we found big differences in the atmospheric response across the globe, including California,” said Nick Siler, lead author of the study that was published in the Journal of Climate, and a postdoctoral scholar in the research group of co-author Shang-Ping Xie at Scripps. “We found that these differences weren’t all random, but rather were caused by tropical sea-surface temperature anomalies unrelated to El Niño.” … The results of the study suggest that El Niño events might not have as strong an influence on California precipitation as previously thought. They also suggest that recent warming might have had a hand in making El Niño drier. The Indian Ocean is known to be warming faster than other ocean basins

Climate change seems to be intensifying California droughts, just as it might be undermining El Niño rains. But it might also be making wet periods wetter, particularly those produced by so-called atmospheric rivers. As a result, the chances of a devastating “arc storm” are increasing. As we shall see in tomorrow’s post, Daniel Swain is one of the leading experts on this topic.

*Wikipedia Definition: “During a La Niña period, the sea surface temperature across the eastern equatorial part of the central Pacific Ocean will be lower than normal by 3–5 °C (5.4–9 °F).

**Wikipedia definition: “[El Niño] is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific.