Last month, several
powerful storms hit California, causing widespread flooding in both Northern
and Southern California.
The storms
caused multiple rivers to flood in Northern California, including the Russian
River in Sonoma County. Southern
California experienced record-breaking rainfall in many places, including the
Los Angeles Airport. Throughout the
state, the storms prompted evacuations, forced thousands to flee their homes,
caused mudslides and rockslides, knocked down power lines, uprooted trees, and
flooded roads and freeways causing numerous accidents. At least eight people died. Many of the damages from the storms will
likely be covered losses under homeowners’ and commercial insurance
policies.
These storms
were atmospheric rivers
(ARs). This is a weather phenomenon
consisting of water vapor forming over the ocean and being transported in long
and relatively narrow regions of the atmosphere (on average 400-600 km wide).
ARs can create extreme rainfall and major flooding when they make landfall over an area. According to the National Oceanic and
Atmospheric Administration’s Earth System Research Laboratory, a strong AR can
transport an amount of water vapor equal to up to 15 times the average flow of
water at the mouth of the Mississippi River.
ARs are
particularly significant in the west coast of the United States, where it is estimated that they
generate about 30-50% of annual precipitation. A type of AR bringing water from the tropics
near Hawaii is commonly known as a “Pineapple Express.”
In the Golden
State, ARs have been responsible for numerous winter storms and multiple floods over the last two decades. For example, from late December 1996 to early
January 1997, a Pineapple Express struck Northern California, causing a major
flood that became known as the “New Year’s Day Flood,” and resulted in over $1
billion in damages. ARs are therefore
very significant events for the insurance industry.
While ARs can
lead to devastating floods, the precipitation they cause is critical to
California’s water supply. In addition
to heavy rain, last month’s winter storms also brought blizzard conditions in
parts of the Sierra Nevada mountain range in Northern California, and
significantly increased its snowpack, a key source of water for the state. These ARs have helped alleviate California’s six-year drought conditions – about 18% of the state is now free of drought.
ARs can now be forecasted. The National Weather Service has techniques
that can identify these phenomena and give advance warnings of their presence,
5 to 7 days before the ARs make landfall. Additionally, scientists within the ESRL have developed AR observatories
along the west coast, to monitor ARs and improve our understanding of them. It is expected that these advances will help
to mitigate the risk of major flood events, and at the same time improve water
management decisions in California and the other western states.
Nevertheless,
experts are concerned that, as a result of climate change, ARs are projected to
become more frequent and intense. This
can result in major insured losses in the west coast. The United States Geological Survey, Multi
Hazards Demonstration Project, has developed a megastorm scenario called “ARkStorm” (for
Atmospheric River 1000 Storm), studying the catastrophic impact of a series of
extreme ARs in California. We will
discuss this scenario, referred to by the USGS as “California’s other ‘Big One,’”
in an upcoming post.
Posted
by José Umbert
