Such events are indicated by a strong warming of equatorial waters in the western Pacific Ocean that spreads eastward to the Western Hemisphere.
Events occur every few years, and some events are much larger than others.
We have not included the below normal contours in the combined maps, since dry conditions are not conducive to landsliding. In the digital National Landslide Overview Map, landslides are characterized as any downward and outward movement of earth materials on a slope. Susceptibility is not indicated where it is the same as or lower than incidence. The susceptibility categories are largely subjective because insufficient data were available for precise determinations. These predictive maps, prepared in cooperation with Dr. Robert E. Livezey of NOAA's Climate Prediction Center, show contours of precipitation anomalies and zones of landslide susceptibility and incidence for large areas of the country.