When you usually think of a hurricane, your fears might involve high winds ripping out trees and torrential rains swelling rivers to bursting. But though Hurricane Sandy will bring fierce winds and sheets of rain and snow, the bigger danger that many researchers expect from her is massively high tides that could cause flooding throughout the eastern seaboard.

On Monday morning, New York Governor Andrew Cuomo said at a press conference that Sandy-related storm surges had already matched the levels caused by last year's Hurricane Irene, even though the massive storm is still hundreds of miles offshore.

Hurricanes primarily cause storm surges because their powerful winds whip up the ocean's surface, piling water up higher than normal. Low pressure at the center of the storm can also contribute to higher sea levels. And Sandy has winds and low pressure in ample supply.

Sandy is a huge storm, with tropical force winds whipping up the sea over an area more than 1,000 miles across. Such a broad range of high winds is a recipe for a record storm surge, according to Weather Underground meteorologist Jeff Masters.

  1. "That's a huge area of motion in the ocean, and when that piles up against the coast, it's got nowhere to go," Masters said in a phone interview.

  2. Sandy also has a very low central pressure, down 5 percent from regular atmospheric pressure, according to Masters.

  3. "That acts like soda straw, sucking up on the water," he says.

The storm is also predicted to make landfall late Monday night, close to a high tide that will be especially high thanks to the full moon. Exactly when the storm hits the coast – right at the high tide mark, or just before or after – could mean extra storm surge and more flooding.

As it stands, Sandy is likely to break New York City's record storm surge of 10.5 feet, set in 1960 by Hurricane Donna, according to Masters.

Klaus Jacob, a Columbia University researcher who specializes in climate change and disaster preparedness in New York City, says there's about a 50 percent chance that Sandy's storm surge could flood subway tunnels in the Big Apple.

“Under certain circumstances, it may take only 40 minutes for the entire subway system to flood,” Jacob said in a phone interview.

Though the Metropolitan Transit Authority has had to deal with minor amounts of flooding from other storms, there has not been a complete flooding of the subway tunnels in recent memory, according to an MTA spokesman.

New York City's subway system has to deal with a lot of water even when the weather is clear. Because many parts of the system are underground and below the water table, the MTA has to remove between 13 and 14 million gallons of water a day, even when there's not a drop of rain.

The MTA has three pump trains and hundreds of pump rooms to whisk water away. But if the subway tunnels under the rivers completely flooded, Klaus estimates that it would take 100 pump trains to remove all that water within a week. Without obtaining extra trains from other states, pumping the water out could take several weeks.

And getting the water out would just be the first step. Then the tunnels have to be inspected for damage. Parts have to be removed, repaired, dried, and tested. With New York City responsible for about $4 billion of economic output per day, a long subway stoppage could add up quickly, according to Jacob.

“If you have half of the city's economic capacity curtailed for four weeks, that adds up to damage in the range of $40-50 billion,” Jacob said.

The MTA has already taken precautions to try and stave off flooding by sandbagging and covering up ventilation grates in Southern Manhattan, where the Hudson and East Rivers have already begun to creep up into the Financial District. Shutting down the subway system on Sunday may have caused some headaches for evacuees and those foraging for supplies at the last minute, but ensured that commuters wouldn't be stranded or in harm's way.

But to truly protect against the risk of flooded subway tunnels, Jacob thinks the city needs a much more comprehensive overhaul. Sidewalk ventilation grates in low-lying areas would have to be done away with completely and replaced by forced-air vents. Those vents would require the construction of fan plants to push the air through the tunnels. In certain tunnel entrances, the MTA could install specially engineered gates that would expand like balloons to seal them off. (However, such gates aren't actually available yet and would have to be designed, Jacob says.)

Such preventative measures come with a high price tag, though, and would certainly mean the state legislature loosening its purse strings and straphangers swallowing steep fare hikes – not an especially likely scenario.

“But if we had already spent $10 billion or $5 billion, we could potentially prevent a $50 billion loss,” Jacob says.