On Dec. 7, 2022, endless rain besieged the Atlantic coast of Portugal. Flash floods hit Lisbon in just three hours, and the event caused at least one death.
A new study published in Weather and Climate Extremes suggests that the vast storms that cause these types of floods are predictable. The storms likely to cause the most damage are also the ones we can most easily anticipate.
“The irony is that the most dangerous events are often the ones the atmosphere signals most clearly,” said the study authors, from the Karlsruhe Institute of Technology and the Hebrew University of Jerusalem, in the release. “When the large-scale structure is strong and organized, the system becomes more ‘readable’.”
Sky Rivers and Extreme Weather Events
The new study focuses on heavy precipitation events (HPEs) across the western Iberian Peninsula. Many of the most extreme weather events originate in atmospheric rivers, which are huge concentrations of water vapor carried in long channels of moisture over great distances. These rivers carry moisture across oceans and onto coastal areas.
These atmospheric rivers greatly increase rainfall in subsequent storms. The team found that the increase is linked to altered wind patterns that bring moisture over the Peninsula, rather than to wetter air overall.
“It’s not just how much water the atmosphere holds. It’s how effectively the system delivers that water to the ground,” said the study authors.
Read More: Landslide Causes 1,500 Residents to Evacuate Small Sicilian Town
Modeling Rivers in the Sky
This initial finding is useful for researchers seeking to understand what causes HPEs’ intense rainfall, but the paper’s real impact lies in its modeling work.
The team modeled how these sky rivers change atmospheric conditions. They looked at alterations in both the upper and lower atmosphere. The team noticed that the most predictable storms were marked by a distinct signature: deep cyclones forming over the north Atlantic, roughly 250 miles (400 kilometers) southwest of Ireland.
These systems caused stronger pressure changes from less predictable storms. It was more obvious how these cyclones would interact with the jet stream and how they moved through the atmosphere.
Predicting Devastating Storms
On average, when these predictable storms affected Iberia, they produced levels of rain 80 percent greater than those created by unpredictable storms.
The authors showed that the damaging storm that hit Portugal in 2022 began when an atmospheric river collided with a cyclone and jet stream. Future storms of this type could be more easily predicted. Similar storms that affect coastal regions globally could also be predicted using the same techniques, including the western coasts of North and South America, and those of southern Africa.
“Enhancing the ability to quantify the predictability of such events has clear value for impact mitigation and risk management, especially under a changing climate,” the authors wrote in their paper.
Read More: Submarine Landslide May Have Contributed to This Devastating 1957 Tsunami
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