A lethal wall of water in the Grand Canyon over the weekend wasn't just bad luck—it was the product of several forces converging over the desert. The flash flood, which killed at least one person, hit during Arizona's monsoon season, but forecasters say a souped-up atmosphere played a big role: a powerful El Niño and an intense Pacific marine heat wave have pushed atmospheric moisture 5% to 15% above normal this summer, per the Washington Post. That extra moisture doesn't guarantee heavier rain, but it raises the odds of extreme downpours when thunderstorms fire, as they did Saturday over steep, rocky, drought-parched terrain that sheds water instead of soaking it up.
The remnants of Tropical Storm Iselle added even more humidity, with 1.24 inches of rain falling in three hours near the flood zone, including 0.84 inches in a single hour, per the Post. CNN meteorologist Derek Van Dam describes up to an inch of rain in 30 minutes, including along the North Rim's 2025 burn scar, which couldn't absorb the deluge, triggering dramatic debris flows. Flood watches remain in place from Arizona into Utah and western Colorado, with the National Weather Service warning of "raging killer currents" in canyons and washes. Forecasters are also eyeing another tropical system that could approach California in about a week, a still-uncertain threat that highlights how unusually warm Pacific waters are priming the West for weather extremes.
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