Why Nepal's rare, deadly flood struck without warning

Thousands of people are missing and hundreds are dead after a flash flood in Nepal that struck with little warning earlier this week. Why it matters: The flood is exposing the gaps in warning systems that are designed to watch out for extreme rain and rivers — not for sudden collapses high in the mountains during good weather. Glacier-driven floods under clear skies — sometimes called "blue-sky floods" — can happen in the U.S. too, especially in Alaska and the Pacific Northwest, where sudden…