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In article <t15rrk$2qu36$
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governor.swill@gmail.com> wrote:
Flood drown fags. Let it rain, let it rain, let it rain rain rain.
It hasn’t happened since 1862, but California is due for another
one
A mention of California might usually conjure images of
wildfires and droughts, but scientists say that the Golden State
is also the site of extreme, once-a-century “megafloods” — and
that climate change could amplify just how bad one gets.
The idea seems inconceivable — a month-long storm that dumps 30
inches of rain in San Francisco and up to 100 inches of rain
and/or melted snow in the mountains. But it has happened before
— most recently in 1862 — and if history is any indicator, we’re
overdue for another, according to research published Friday in
Science Advances that seeks to shed light on the lurking hazard.
“This risk is increasing and was already underappreciated,” said
Daniel Swain, one of the study’s two authors and a climate
scientist at the University of California at Los Angeles. “We
want to get ahead of it.”
In such an event, some in the Sierra Nevada could end up with 25
to 34 feet of snow, and most of California’s major highways
would be washed out or become inaccessible.
Swain is working with emergency management officials and the
National Weather Service, explaining that it’s not a question of
whether a megaflood will happen but when.
“It already has happened in 1862, and it probably has happened
about five times per millennium before that,” he said. “On human
time scales, 100 or 200 years sounds like a long time. But these
are fairly regular occurrences.”
His paper built on the work of other scientists, who examined
layers of sediment along the coastline to determine how
frequently megafloods occurred. They found evidence of extreme
freshwater runoff, which washed soil and stony materials out to
sea. Those layers of material became buried beneath years of
sand. The depth of the layers and the sizes of the pebbles and
other material contained in them offer insight into the severity
of past floods.
“It hasn’t happened in recent memory, so it’s a little bit ‘out
of sight, out of mind,’ ” Swain said. “But [California is] a
region that is in the perfect area … in a climatological and
geographic context.”
On the West Coast, there commonly are atmospheric rivers, or
streams of moisture-rich air at the mid-levels of the atmosphere
with connections to the deep tropics. For a California megaflood
to occur, you’d need a nearly stationary zone of low pressure in
the northeast Pacific, which would sling a succession of high-
end atmospheric rivers into the California coastline.
“These would be atmospheric river families,” Swain said. “You
get one of these semi-persistent [dips in the jet stream] over
the northeast Pacific that wobbles around for a few weeks and
allows winter storm after winter storm across the northeast
Pacific into California.”
The paper warns of “extraordinary impacts” and reports that such
an episode could transform “the interior Sacramento and San
Joaquin valleys into a temporary but vast inland sea nearly 300
miles in length and [inundate] much of the now densely populated
coastal plain in present-day Los Angeles and Orange Counties.”
The effects of a month-long barrage of soaking storms could be
disastrous, but Swain notes that it’s possible to have advance
warning.
“This is something we’d see coming three to five days out, and
I’d hope a week and perhaps even two weeks out, with a
probabilistic type of prediction,” Swain said. “We’d have a
decent amount of warning for it.”
Swain’s simulations showed the odds of a megaflood occurring are
far greater in winters dominated by El Niño than in winters
influenced by La Niña. El Niño is a large-scale chain-reaction
atmosphere-ocean pattern that can dominate the atmosphere for
several years at a time, and it usually begins with higher-than-
normal sea surface temperatures in the eastern tropical Pacific.
“When you look at the top eight monthly precipitation totals in
simulations, eight out of eight occurred in El Niño years,”
Swain said.
The influence of human-caused climate change also plays a role:
Swain says it boosts the ceiling in a megaflood.
“We have multiple scenarios. The future one is much larger,
consistent with [climate change],” he said. “In the historical
scenario, the lesser one, certain parts of the Sierra Nevada see
50 to 60 inches of liquid-equivalent precipitation … but in the
future event, some places see 70 to 80 and a few see 100 in a 30-
day period. Even places like San Francisco and Sacramento could
see 20 to 30 inches of rain, and that’s just in one month.”
An independent study published in Scientific Reports on Friday
concluded that human-caused climate change will intensify
atmospheric rivers and could double or triple their economic
damage in the western United States by the 2090s.
A warmer atmosphere has a greater capacity to store moisture. In
the absence of storms, that means the air can more quickly dry
up the landscape — hence California’s prolonged drought — but
should rain occur, the deck is stacked to favor an exceptional
event.
“Moisture isn’t the limiting factor in California,” Swain said.
“There’s plenty of moisture around even in the drought years.
The absence is a lack of mechanism. It’s a lack of storms rather
than moisture.”
Alan Rhoades, who is an expert on atmospheric rivers and was not
involved in either study, said the research highlights the “the
importance of not forgetting about major flood events, which are
also central to California’s history."
“The major worry is how much climate change will alter the
frequency of these event occurrences and how much it will fuel
and amplify the impacts of the next record-setting [atmospheric
river] event,” wrote Rhoades, a hydroclimate research scientist
at Lawrence Berkeley National Laboratory, in an email.
He added that compared to previous megafloods in the late 1800s,
“California has vastly expanded its rural, urban, and
agricultural sprawl, which could lead to more potential for loss
of life and property.”
While researchers can’t say when the next California megaflood
will strike, forecasters are confident that it will happen.
There’s a 0.5 to 1.0 percent chance of it happening in any given
year.
Swain said one goal of his work is to push officials to prepare.
He suggested working with the National Oceanic and Atmospheric
Administration to “run through simulations as a real tabletop on
the ground disaster scenarios.”
“We’ll work through where the points of failure would actually
be, because one of the things we want to do is get ahead of the
curve,” he said.
https://www.washingtonpost.com/climate- environment/2022/08/12/megaflood-california-flood-rain-climate/
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