ABOUT EL NINO: El Niņo is a cyclical warming of the ocean waters in the central and eastern tropical Pacific that generally occurs every three to seven years, usually around the end of the calendar year. It is associated with changes in air pressure and the movement of high-level winds, and can affect weather worldwide. In the United States, El Niņo normally results in warmer-than-normal temperatures across the northern and western states. Wetter conditions result in the south, with dry weather across the Ohio Valley and Pacific Northwest. El Niņo typically peaks during the winter months. It alternates with La Niņa, the cooling of ocean waters in the same region of the Pacific.
WHY THIS EL NINO IS DIFFERENT: The newly-recognized version of the phenomenon is called El Niņo Modoki (from the Japanese word meaning "similar, but different"). Instead of forming in the Eastern Pacific, it forms toward the center of the ocean, which affects ocean circulation and weather patterns and may lead to higher frequency of strong hurricanes in North America.
RATING HURRICANES: Hurricanes are categorized according to the strength of their winds according to the Saffir-Simpson Hurricane Scale. They are rated from a Category 1 with winds between 74 and 95mph to a Category 5 with winds over 155mph. But even lower category storms can cause a great deal of damage, mostly from storm surges and the resulting flooding. The worst devastation from hurricane Katrina, for example, occurred when flooding caused the New Orleans levees to fail.