Regarding the real storm hitting California right now, 30,000 are being urged to evacuate. There is more informaiton about that storm down at the bottom of the post.
Meanwhile, on the hypothetical worst case scenario for flooding in California…
If an ARkStorm were to come to the US West Coast, this is what the flooding in California could look like:
The ARkStorm (Atmospheric River k for 1,000 Storm, but it is possible there is a biblical reference of some kind in there) is both a theoretical possibility and a reality. It is a theoretical possibility because one major “pineapple express” sort of storm, or an extra large one, can cause major flooding. A second one, and maybe a third one, in a short time, can cause MAJOR flooding. If an early storm, or two, causes huge snowpack development, followed by a couple of really wet storms, that that can cause MAJOR MAJOR flooding. You add it up, you get an ARkStorm.
It is a real thing, and not just theoretical, because it has been observed. It happened during the winter of December through January 1861-1862. It flooded like in that map, but worse, because the storm also affected states and provinces all along the coast from north of Vancouver to south of the Mexican border, and interior US states.
It was known as the Great Flood of 1862, and there has never since been a flood of that magnitude observed in the region.
It caused a lake to form that was 300 miles north- to south, about 20 miles wide, and up to 30 feet deep. Some towns were washed away or badly damaged and required extensive rebuilding. Some towns, such as Eldoradoville, were washed away and never rebuilt.
The total economic impact of a similar flood, were it to happen today, would be greater (maybe double) the economic impact of any of the larger earthquake scenarios that planners plan for. Similarly, the morbidity and mortality would reach levels exceeding a major earthquake. Much of the existing flood control infrastructure in California is built with the ARkStorm in mind. That, of course, will help. But the Great Storm of 1862 happened when hardly anybody lived in California.
The ARkStorm is thought to be a once in a century, maybe once in two century, event. However, the kind of storm system that would cause an ARkStorm is probably more common today under climate change, so maybe we can expect to see more of them.
I’ll speculate, however, that the worse possible ARkStrom may be somewhat less likely today because the build up of snowpack over the course of a few storms during early and mid winter is less likely, with global warming. So that reservoir of water to be washed off the mountains by subsequent storms in the series would be less. So that’s something.
Want to read more about ARkStorm? Click here for access to the USGS report.
Later this week, a pineapple express type storm will strike Southern California. This is not an ARkStorm, but a storm such as this week’s is one component of such a storm. So it is a good reminder. However, while this storm is not going to turn the interior valleys of California into the great lakes they want so much to be, it may cause heavy rain in areas recently denuded by massive fires. So, there is the threat of landslides.
#AtmosphericRiver event just getting started. Very prominent moisture plume extends from deep subtropics directly to California. Lull in rain right now, but intensity should pick up dramatically tomorrow (esp. Central Coast) as large-scale dynamics act on moisture. #CAwx pic.twitter.com/ZXvfbkXXkx
— Daniel Swain (@Weather_West) March 21, 2018
From Climate Signals, an organization that watches these things: Record breaking rain is expected. Note that this sort of atmospheric river event is the main cause of flooding in thatregion. This is part of an overall pattern of extreme rain events increasing with climate change. See THIS LINK for more on atmospheric rivers.