https://www.ksl.com/article/50268162/bomb-cyclones-and-atmospheric-rivers-
west-coast-braces-for-weather-whiplash
ATLANTA — Imagine a long river of water vapor in the sky coming into the
West Coast. It is how Marty Ralph, the director of the Center for Western
Weather and Water Extremes at the Scripps Institution of Oceanography in
San Diego, described the storm event threatening California at the moment.
The storms are called "atmospheric rivers," which are narrow bands of
concentrated moisture in the atmosphere emerging from the warm waters of
the Pacific Ocean, cruising more than two miles above the sea. An average
atmospheric river transports more than 20 times the water the Mississippi
River does, as vapor.
Throughout the weekend and into next week, parts of the West Coast will go
from extreme drought to facing a series of bomb cyclones and an associated
atmospheric river. The weather whiplash may unleash rains, flash floods,
debris flows, and potential hurricane-force winds, particularly in the
Pacific Northwest and Northern California.
"Wherever the storms hit shore on the West Coast is where the heaviest
precipitation occurs, and that can be very beneficial in areas that often
don't have enough water — and we have the drought going right now," Ralph
told CNN.
"And then there are times when there's too much and it can create
flooding," he added. "A few of these storms really make the difference
over the course of the year."
Human-caused climate change has increased the potential for this weather
whiplash, where dramatic shifts in periods of drought and high
precipitation can to occur more often. Scientists say the chances of
sudden transitions from severe drought to atmospheric river events will
become more common in California in the coming decades.
Much needed rain, but too much of a good thing?
In 2019, Ralph led the development of the system to categorize atmospheric
rivers by strength, much like hurricane categories. In the scale, AR4
translates to 'extreme,' while AR5 — which is what he projects this storm
to be — means 'exceptional.'
"AR's 4 and 5 are mostly hazardous, but they can also be very beneficial,
as we're seeing in this case, where it's coming on the heels of a serious
drought," he said. "And largely the impacts are probably going to be
beneficial because it's moisturizing the soil, restoring some water in the
rivers, and a little bit of the lakes."
But this storm, according to Ralph, is a rare event. AR5 storms are rare
in California, especially during October. Analyzing a 40-year period, his
team found only a total of 10 AR5 storms occurred in 40 years, and only
one happened in October.
The impacts also vary depending on the region. Smaller watersheds in urban
areas, for instance, may trigger flash floods, while regions where
wildfires have left burn scars may experience some debris flows.
Transportation, Ralph adds, could also be impacted with the wet roads,
cautioning drivers who may travel during the storm event.
Ultimately, experts like Ralph say the atmospheric river is needed more
than ever to replenish the unrelenting drought in the West.
In California, dry conditions this summer were the most extreme in the
entire 120-year record. It drained reservoirs and triggered historic water
shortages. Governor Gavin Newsom recently expanded a statewide drought
emergency proclamation.
Climate researchers say two major factors contributed to this summer's
severe drought: a lack of precipitation and an increase in evaporative
demand, also known as the "thirst of the atmosphere." In this case, the
coast did not experience enough storms to quench the thirst of the
atmosphere, as well as the drying landscape.
The historic drought in the West also created the perfect landscape for
wildfires to spark and expand.
In the short term, Julie Kalansky, a climate scientist at Scripps
Institution of Oceanography in San Diego, said the storm will help
alleviate the dry landscape as well as reduce the potential for fires to
spread and ignite.
"In the long term, it's really hard to tell," Kalansky told CNN. "Just
because we have a wet October or have a big event in October, it doesn't
necessarily mean the rest of the season is going to be wet, so the rest of
the season is still really to be determined."
California's Mediterranean climate, where dry summers and wet winters
provide the perfect conditions for a robust agricultural economy, makes it
vulnerable to drastic shifts in weather events. But after a long dry
period, Ralph said many farming communities would benefit from a high-
precipitation event.
"It seems to me this is a storm that's going to help return some moisture
to the soil in a significant way that should green up the hills and the
fields a bit more than otherwise," Ralph said. "And then, a big picture is
that this is a storm that's going to set the stage for the watersheds to
produce more runoff when the next storm comes, which is going to start
filling the depleted reservoirs that they depend upon."
Climate change is making storms wetter, stronger
Scientists have linked the climate crisis to an increase in the amount of
moisture the atmosphere holds, meaning storms, such as the one impacting
the West Coast now, will be able to bring more atmospheric moisture inland
than it would without climate change, which in turn leads to an increase
in rainfall rates and flash flooding. Meanwhile, the intensity of storms
is also increasing.
This weekend's bomb cyclone, named for the speed at which it intensifies
or "bombs out," is expected to near or set records for the lowest pressure
for a storm in the Northeast Pacific Ocean. The strong system is forecast
to produce winds of hurricane strength (75 mph or greater), though the
strongest winds should remain over the ocean, according to the latest
forecast.
And since the storm is coming earlier in the season than usual, Kalansky
said preparedness is crucial for an event like this.
"It is a very extreme event for this early in the season, and people might
not be prepared for something like this because it doesn't typically
happen," she said. "I always advise people that it is really important
that they stay up to date with what the National Weather Service is
predicting in terms of hazards."
These rapid changes in climate extremes — from drought to high
precipitation — could make it harder for societies and communities to
mitigate and adapt. In some cases, it could be destructive: according to a
2019 study, atmospheric rivers created an annual average of $1.1 billion
annually in flood damage across the West.
But one thing is clear: as the planet warms, extreme weather events such
as this will only get worse.
"The research has shown that atmospheric rivers are projected to become
more extreme in the future," Kalansky said. "For California, they're
projected to contribute more to the overall annual precipitation. Our
climate models are suggesting that atmospheric rivers will become
increasingly important as the climate changes."
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