Hurricane damage is caused by 3 factors: wind, storm surge, and rain. A Category 5 hurricane will have exceptionally devastating winds, and may also have an unusually high storm surge. The kind of damage these can do to a major metropolitan city was vividly described in the August 28, 2005 National Weather Service bulletin about Hurricane Katrina's imminent landfall near New Orleans. Yet Hurricane Katrina was only a Category 3 when it brushed by New Orleans. What happened there is not as bad as it could be if a Category 5 hurricane strikes a major metropolitan city.
The Saffir-Simpson Hurricane Scale is based on the expected amount of wind damage to fixed structures. In a Category 5 hurricane, the maximum sustained wind speed in the eyewall is 157 miles per hour or more. This is the equivalent of an EF3 tornado or stronger, over a much larger area and for a much longer time.
Winds at these speeds will cause catastrophic damage in any major metropolitan city, no matter how well prepared. When exposed to these kinds of winds, at least half of all well-constructed homes will have severe roof and wall failure. High rise buildings will sway and may collapse. Concrete block low-rise apartments will sustain some wall and roof failure, while wood-frame low-rise apartments will be torn down. All buildings built to lesser standards will probably be completely destroyed.
Nearly every tree exposed to the strongest winds will be snapped or uprooted. Every window which has not been boarded up will blow out, which means that nearly all highrise windows will be shattered.
All that broken glass and wood will become flying shrapnel, whipped around at high speeds. Other flying debris from roads and destroyed houses could be up to the size of household appliances and even light cars. The high winds will also shift and sometimes topple heavier vehicles, including trucks and trains. It is highly unlikely that any living thing caught outside shelter at this time will survive.
Many hurricanes also produce tornadoes. Most hurricane-generated tornadoes spawn from the outer bands in the northeast quadrant, where the sustained winds are otherwise light by comparison to the eyewall. Hurricane Katrina alone spawned 53 tornadoes, including 6 EF2s. This means that Category 5 wind damage to a metropolitan city is not limited to the passage of the eye.
Although wind speed is important in building storm surge, it is not the main factor in the final height of the storm surge. The local ocean depth and the direction and shape of ocean channels can magnify the storm surge or eliminate it completely, in a way similar to tsunami. Timing is also important. High tide will make a storm surge much higher than it would otherwise be. For these reasons, the effects of storm surge depend much more strongly on location and timing than on wind speed.
However, the threat of storm surge should never be treated lightly. Most hurricane-related deaths occur as a result of storm surge. People who are trapped on a low-lying island will have no chance at all against even a 6-foot storm surge, which is about the absolute minimum storm surge possible for a Category 5 hurricane. During Hurricane Katrina, the flooding at Gulfport Beach, Mississippi, reached 41 feet.
To make matters worse, the storm surge will reach far inland to cut off low-lying evacuation routes hours ahead of the hurricane's arrival. The flooding from the storm surge will remain for hours, sometimes days or even weeks, after the hurricane is over, complicating access, rescue, and repair work.
The storm surge may also be high and strong enough to overwhelm levees, as it did during Hurricane Katrina. This will cause extensive, long-lasting flooding. Repairing damage to levees will be difficult and time-consuming. It may also be complicated by later hurricanes during the same hurricane season.
Although all coastal cities are vulnerable to storm surge damage, some are much more vulnerable than others. Low-lying cities on long, shallow shorelines are particularly vulnerable. So are major metropolitan areas close to a channel of water which opens to the south or southeast and narrows towards the north. Finally, a heavy population distribution along vulnerable shorelines could make matters much worse.
= New York City =
The New York City metropolitan area has the worst of all 3 worlds. Its harbor and major rivers open southeast into New York Bay and the Atlantic Ocean. The harbor mouth has been artificially deepened, while the Hudson River becomes shallow quickly. It also has a very heavy population density along low-lying ground close to the Hudson and East Rivers.
Even if a mere Category 2 ever strikes Staten Island, the storm surge will be pushed from lower New York Bay into upper New York Bay and right up the Hudson and East Rivers towards Brooklyn, Queens, Manhattan, and parts of Jersey City. All that water will have nowhere to go but up, causing a storm surge up to 15 feet high. A similar hit by a Category 3 hurricane could put John F. Kennedy International Airport under 19 feet of water.
All train and subway stations for a couple of miles in all directions will be completely flooded out, including Grand Central and Penn Stations. So will the roads and tunnels. All subterranean power plants and communications wiring will be shorted out. Most warehouse food will be damaged beyond salvage. Clean water will be compromised when sewers overflow, to say nothing of all the rats which will be trying to find safe ground above the water.
Currently, a Category 1 hurricane striking a direct hit on New York City is thought to be a 100-year event. However, rising sea levels will increase the chances of disastrous flooding to a 35-year event by 2080.
A Category 5 hurricane will not necessarily produce more rain than a Category 1 hurricane or even a tropical storm. However, in a major metropolitan city such as NYC, each inch of rain represents a billion gallons of storm water. It cannot sink through concrete and asphalt. If too much rain falls, the storm sewers won't be able to handle it, and there will be flash flooding in every part of the city touched by the hurricane.
In the Adirondacks and other northeast mountain ranges, the soil is very thin over the bedrock. It cannot hold much water before it starts to run off into the streams and local rivers. This kind of heavy rain and flooding washes away bridge supports and roadbeds.
Under these conditions, even mild hurricanes and tropical storms can quickly touch off raging freshwater flash floods. As a result, coastal metropolitan areas in the Northeast could be hit by 2 waves of flooding: the first from the coastal storm surge, and the second from inland areas after the storm surge recedes.
After the raging floods ebb, a lot of stagnant water will still be standing around. This will breed clouds of mosquitoes over the coming weeks. Although most mosquito-borne diseases are not endemic in the continental U.S., the warm weather and disaster conditions after a Category 5 hurricane could create the perfect conditions for an outbreak.
All incoming roads will be blocked by floodwater, fallen trees, abandoned or wind-blown cars, and other debris, especially underpasses, old bridges, and coastal roads. Docks will be inaccessible, heavily damaged, or completely destroyed. These transportation arteries could take days to clear. During that period of time, nothing will leave or come into the city except by air drops and helicopter rescues.
Electricity will certainly be cut, and may remain out of service for weeks or even months. This is because the power poles and even transmission towers will be snapped or toppled, and most transformers in the region will be destroyed. Everything that depends on electricity, from city water pumps to nuclear plant cooling, will have to change over to generator power. However, the generators will soon run out of fuel.
Cell phone towers will be destroyed as well. The only possible communication will depend on direct satellite radio links. During 2012 through 2014, these could be disrupted by solar storms during the solar maximum.
All city water will probably be contaminated. This would normally result in a boil-water advisory, but there will be no electricity to heat the water. Instead, there will be a severe clean water shortage. If the water situation cannot be improved quickly, disease may result.
Natural gas lines may also be cut. Sparking from gas leaks could result in localized fires. Without water or transportation, there will be no way to put them out. However, depending upon the amount of flooding, they may not spread.
The largest metropolitan cities are highly dependent on a just-in-time food delivery structure. With existing stores of food destroyed and roads cut, food shortages will start in less than 48 hours.
Hospitals in the immediate area will be on their own for at least the first 24 hours, until medical drops can be made. Even if they remain undamaged from wind or flooding, they will be trying to cope without power, sanitation, or any way to replenish medical supplies. With adequate hurricane warnings, no patients should be in surgery when the hurricane hits, but many critically ill patients will die when the equipment keeping them alive loses power.
Long term damage
If a Category 5 hurricane struck a major metropolitan city, it will probably never completely recover. Even today, the population of New Orleans is less than 75% of its pre-Katrina size. However, about a quarter of that decline is probably due to previous population trends in New Orleans.
The size of the disaster eliminated thousands of businesses, along with the jobs they created. People who had been left destitute by Katrina had little or no money to spend locally on the remaining businesses. In turn, loss of jobs has permanently reduced the tax base for the region. Finally, insurance rates for the region are now permanently higher.