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.

"Let me be clear: This storm is large, unique, dangerous, and unlike anything our region has experienced in quite some time," D.C. Mayor Vincent Gray said.

The federal government, as well as the governments in D.C., Maryland, and Montgomery, Prince George's, Fairfax and Arlington counties, announced they would close Monday. Thousands had already lost power in Virginia and evacuations of some coastal areas began as Sandy approached the Mid-Atlantic. Residents braced for widespread, long-lasting outages as Pepco made 3,700 requests for mutual assistance from other utilities, the biggest request in company history, Pepco region President Tom Graham said Sunday.

The Washington Metropolitan Transit Authority suspended all rail and bus service beginning Monday "until further notice" in a rare move highlighting the expected severity of the hurricane. The Maryland Transit Authority announced that its MARC commuter train service wouldn't run, and the Virginia Railway Express canceled Monday service. Meanwhile, Washingtonians across the country were stranded as flights in and out of local airports were delayed or canceled.

In Maryland, Gov. Martin O'Malley issued mandatory evacuation orders for Ocean City and Worcester, warning that Ocean City could be ravaged in a way not seen since 1985's Hurricane Gloria.

"This is a serious, killer storm," said O'Malley. "It's important that everyone is vigilant."

He shuttered all public transit, including buses, for Monday, and told people to stay inside and off the roads. Early voting in Maryland on Monday was canceled.

All the local school systems closed Monday, with Montgomery and Fairfax counties canceling classes on Tuesday, too.

Grocery stores and gas stations were hollowed out Sunday by residents preparing for the storm with arm-fulls of canned tuna and soup, and carts full of bottled water. At the Food Lion in Rockville's Wintergreen Plaza, Sebastian Miller -- who had already stocked up on canned foods, batteries and extra flashlights -- grabbed a propane tank for his grill in case his home lost power.

"Before the derecho this summer I wouldn't have said we needed to do all this. But after losing power for a week [with two kids], you start to think differently," Miller said.

Twenty-three-year-old Lauren Samet said she had been a "cynical skeptic" of Sandy until she felt fierce winds while visiting a friend in Hollywood, Md., on Sunday. Arriving back in the District, she found her local Safeway to be out of flashlights and water.

"I got some Gatorade because that's all they had," Samet said.

Gray told reporters the recent derecho and last year's Hurricane Irene had been "something of a test run" for Sandy. The storm was expected to leave the Washington area by Wednesday afternoon. But Graham warned that residents could be without power for much longer, because crews couldn't safely venture out until the hurricane had passed.