Sandy is no ordinary hurricane.
A remarkable configuration of weather systems are tugging on the storm system as it slogs up the eastern seaboard, slowing and widening the cyclone. A full moon will drag tides up the shore just as the storm whips the sea. And an area of low pressure is expected to pump in cold air from the west that could lead to several inches of snow in Western Maryland.
"It's a very, very rare storm, and the way it is going to interact, the confluence of things coming together late in the season, is something we'll be taking a look at for years to come," said Steve Zubrick, science and operations officer at the National Weather Service's Sterling, Va., office. "But what people need to know is, it's a very dangerous storm. We're going to see some incredibly high winds and people need to be as cautious as they can be."
High winds and intense rains will herald the approach of the storm late Sunday and early Monday. It is expected to make landfall late Monday and linger over the mid-Atlantic region through Tuesday evening.
"It may only be a Category 1 hurricane, but it's a very broad system, and it's impact will be felt for hundreds of miles," said Kevin Witt, a meteorologist with the National Weather Service.
Unlike most hurricanes, which revolve around centers of warm air, a high pressure system from Eastern Canada is pumping colder, drier air into Sandy. The mix of warm and cool air provokes especially strong winds, Witt said.
Two weather systems to the west — one from Canada and one from the Midwest — are affecting the storm's trajectory and intensity.
The high pressure system from Canada had been pushing the storm away as it traveled up from the south, but is now drawing the hurricane toward the coast, Witt said. The position and intensity of the high pressure system would determine exactly where and when the storm would turn west to strike the coast, he said.
Witt said residents of the Baltimore area should brace for the heaviest rains and winds Monday afternoon through Tuesday evening.
Meanwhile, a low pressure system hanging over the Ohio Valley will add to the amount of precipitation — and could drop several inches of snow in the mountains of Western Maryland, Witt said. As much as three feet of snow could fall in the West Virginia mountains and two feet could accumulate in elevated areas of Southwestern Virginia, Kentucky, North Carolina and Tennessee, Zubrick said.
"I don't think any meteorologist around here has seen snow as part of the forecast for a tropical system," Zubrick said.
Snow is unlikely in most of Maryland, where heavy rains are expected to cause flooding both inland and in coastal areas.
"Practically every creek in Baltimore City and Baltimore County is going to be out of its banks," said Jim Lee, a National Weather Service meteorologist. Water had already covered some roadways in St. Mary's County Sunday evening, Witt said.
Meteorologists are predicting 6 to 10 inches of rain in the Baltimore area, with more falling in some areas.
Monday's full moon is expected to add to the storm surge, especially if water levels rise as tides are at their highest. Surge-related flooding was expected to peak at 1 to 2 feet in the upper and middle portions of the Chesapeake Bay, while reaching 4 to 8 feet in Ocean City. The severity of flooding will depend on when the surge comes in the tide cycle, the hurricane center reported. Witt said the highest storm surge was expected to hit near the mouth of the Potomac.
There is one bright spot for Baltimore, Witt said. Winds from the north and northwest should blow water from the Inner Harbor toward the Bay, minimizing flooding in the downtown area.
The storm's most damaging effect in Central Maryland will likely be caused by winds, which are expected to reach 30 to 55 miles per hour Monday evening with gusts up to 60 to 70 miles per hour.
Under leaden skies, winds were sweeping into the Baltimore area from the northeast Sunday afternoon.
By noon Sunday, winds had reached 35 knots at the Thomas Point Lighthouse south of Annapolis and gusts were measured at 35 miles per hour at Baltimore-Washington International Thurgood Marshall Airport, Lee said.
"You're seeing the winds pick up as Sandy moves closer," he said.
Jim Lee, a National Weather Service meteorologist, warned that such winds would topple many trees and limbs, as in last summer's derecho storm. He cautioned residents to seek shelter in the lower floors of their homes. Three people were killed by falling trees in the derecho as they slept in second-story bedrooms, Lee said.
It remained unclear whether Sandy would be classified as a hurricane or a tropical storm when it makes landfall, but Lee said the effects would be the same.
"The impacts are going to be the same — very heavy rains and extremely strong winds that are going to lead to a lot of tree damage," Lee said.
Baltimore Sun reporter Chris Korman contributed to this report.