A culmination of unique events led to the recent extensive flooding in the Yellowstone region. Water quickly rose in an area that was in an extended drought, forcing officials to race to save homes, roads and towns. Mountain snow accumulated in the late spring. During a warm stretch, rain fell and melted snowpack, causing a tremendous amount of water to flow down the mountains. The resulting flood raged higher and faster than the rare benchmark 500-year storm. La Nina conditions occur when parts of the Pacific cool and while those conditions keep the Southwest dry, they can lead to more rain and snow in the Northwest. It is another extreme weather event in a warming world.