When the skies opened over western Portugal in December 2022, rivers swelled, streets flooded, and communities were left picking up the pieces long after the rain stopped. To many, the storm felt like pure chaos—a reminder of a climate growing more volatile by the year. But new research suggests some of the most intense rainstorms may be hiding a surprising secret: they are, in fact, among the most predictable weather events on Earth.
A study led by Ehud Bartfeld and Dr. Assaf Hochman of the Hebrew University of Jerusalem, together with Dr. Alexandre M. Ramos of the Karlsruhe Institute of Technology, has found that the fiercest downpours hitting Portugal often follow atmospheric patterns that are not only identifiable but intrinsically more predictable than gentler storms. Their findings, published in the journal Weather and Climate Extremes, offer an unexpected glimmer of hope in a warming world.
At the heart of these intense storms are atmospheric rivers—long, narrow corridors of moisture that transport vast amounts of water vapor across oceans into coastal regions. When these rivers are involved, rainfall in the western Iberian Peninsula becomes 36% stronger on average. Yet the added punch does not come from more moisture in the air overall. Instead, it comes from stronger low-level winds that funnel that moisture more efficiently into the region. "It's not just how much water the atmosphere holds," the researchers noted. "It's how effectively the system delivers that water to the ground."
To answer the question of when extreme events become predictable, the team used a novel dynamical systems approach, examining how atmospheric conditions evolve before and during storms across both lower and upper atmospheric layers. What emerged was a striking divide. The most predictable extreme rainfall events were tied to well-organized, deep extra-tropical cyclones forming over the North Atlantic near 50°N and 15°W. These systems showed pressure anomalies roughly twice as strong as less predictable counterparts, along with clearer jet stream interactions and more coherent large-scale wave patterns.
In practical terms, the difference is dramatic: highly predictable events produced rainfall intensities about 80% greater than less predictable storms. "The irony is that the most dangerous events are often the ones the atmosphere signals most clearly," the researchers said. "When the large-scale structure is strong and organized, the system becomes more 'readable.'"
The December 2022 disaster in Portugal provided a vivid case study. The researchers showed how the alignment of an atmospheric river with a powerful cyclone and structured jet stream created both extreme rainfall and relatively high forecast confidence—a combination that, if better understood, could dramatically improve early warnings.
The findings suggest that integrating atmospheric river detection with dynamical systems analysis could sharpen forecasting tools, not only in Portugal but in vulnerable coastal regions worldwide. As climate change continues to amplify rainfall extremes, the distinction between chaos and predictability may prove critical. The atmosphere, it turns out, does not always hide its intentions. Sometimes, it signals them loudly, coherently, and in ways scientists are only now beginning to decode.