A “killer” storm that local officials and experts are describing as potentially unprecedented in its intensity has shut down Metro and the federal government as the Washington area braces for devastating rains and winds that are expected to last well into Tuesday.

The potential collision of Hurricane Sandy with a nor’easter is expected to flood the region, knock down power lines and cause up to $1 billion in damages, forecasters and local officials said. Peak winds sustained at 35 to 45 mph, with gusts of up to 60 mph, and rain at a rate of up to two inches per hour, were expected to hit Washington at noon on Monday and last all night.