Brace yourself. The U.S. space agency cautions that the phenomenon could be as severe as history’s worst 17 years ago. That means unusually warm weather for the Northern Hemisphere and extreme weather events, such as California being inundated with rain as it was in 1998. The periodic condition involves normally warm central Pacific waters expanding eastward. So far, it’s being blamed for the worst floods in a half-century hitting South America and high water in northern Britain — and aid agencies warn that it may curtail food supplies for famine-hit populations in Africa.