Please join us in taking a moment to extend your love and best warm wishes to all those in the path of Hurricane Sandy—now “officially the largest tropical cyclone ever.”

There are probably a number of engineers and meteorologists who can go in to greater detail and cover things I’ll miss, but I’ll take a wack at detailing the most significant impacts.

Hurricanes have three primary destructive forces you have to worry about. Rain, Wind, and Storm Surge. There are others, such as tornadoes, and lightning, but they are on a much smaller scale.

Lets go in order of destructive potential, least to greatest.

Outside of the most powerful hurricanes (realistically, Category 3 and up), the wind is not in and of itself particularly destructive, and the highest winds tend to be extremely localized around the eye. The 80mph wind Sandy is likely to have when she comes ashore, in any smaller scale (a thunderstorm, for instance), would be little more than a passing annoyance. It would knock down some trees and powerlines, maybe rip shingles and siding off some houses, perhaps even break a few windows from flying debris. Cleanup from a thunderstorm like that would generally take a few days at most. However, when that wind covers an area of hundreds or thousands of square miles, instead of tens, you get some new challenges. The sheer scale of this relatively moderate damage makes restoring power and opening roads a major task that can take days. This is doubly true in a very densely populated area like the DC/NY/Boston corridor. More infrastructure means more infrastructure that can be damaged, and in the case of restoring power, things have to be done in a particular order, which complicates and slows the entire effort. You aren’t looking at hundreds of square miles of buildings flattened like we saw with Katrina and Andrew. Just a lot of people without power, possibly without water. The most destructive wind will generally start to wane within about 100 miles of the storm coming ashore, especially with a weaker storm like Sandy.

Next up is rain. Hurricanes are big, and big weather systems rarely move quickly. Hurricanes also hold an enormous amount of moisture aloft. Sandy can reasonably be expected (and is forecast) to produce rain fall rates exceeding 1 inch per hour. Because it’s moving slowly, it is realistic to expect some areas will see total rainfall exceeding 12 inches. Except for the very highest ground (and even then, only high ground with excellent drainage) or areas very near the shore, this is going to cause flooding. There is simply no avoiding it. Ditches will turn in to small streams. Small streams will turn in to raging white water rapids, and both will have a tendency to jump banks, which can cause property flooding and road closures. Large rivers could rise several feet, flooding areas near their banks, and areas that are poorly drained will suffer serious ponding of water that will also close roads and damage property. This can be expected to reach up to 250 miles inland. This sort of flooding tends to be relatively short lived, and major property damage will occur in pockets in the most flood prone areas. This is the first threat that has the potential to displace large numbers of people for prolonged periods of time, but the sporadic distribution makes it easier to deal with. Again, you aren’t going to see hundreds or thousands of square miles rendered uninhabitable like we saw with Katrina’s flooding of New Orleans. The amount of steady rain that the PA/NY/MD area is receiving in advance of this storm isn’t going to help though. Here in western PA, we’ve already received about an inch of rain in the last 36 hours, and the ground will likely saturate long before Sandy actually arrives.

The last, and most worrisome risk with Sandy is the Storm Surge. Hurricanes move a hell of a lot of ocean water around, by sheer force of wind across the surface. They also create a sort of ‘bulge’ of water directly below them from the low air pressure, and the inward and upward air motion at the center of the storm. The pressure induced surge is very close to the eye. The wind driven surge tends to be on the northwestern corner of the storm, when it’s making landfall from the east like Sandy is. This is because of what they call the ‘onshore fetch’. As the winds rotate around the center of the storm, the point where they impact the shore closest to a right angle tends to be where you get the largest surge. When these high water events coincide with a normal high tide, bad things happen. Luckily, right now, the NHC is expecting landfall somewhere between midnight and 4am on Tuesday. This is good, this is near low tide, but they still expect tides to be 6-11 feet above normal.

If you think about how much of the area around NYC and coastal New Jersey are only a few feet above the beach, or exist along the rivers, which are even closer to sea level, you can get an idea of how much of the region is likely to suffer significant flooding. Storm surge tends to destroy buildings right along the coast through wave action, and render buildings further inland, especially near coastal rivers, uninhabitable just through water damage, and the long term complications of that. Mold, mud, and limited availability of building supplies and manpower make recovering from widespread storm surge events a protracted process.

If the storm were to slow down by about 7 hours, and make landfall around 9am Tuesday, at its current projected landfall in central or northern New Jersey, it could be a worst case for NYC, because that maximum storm surge on the northwestern corner will drive water right in to the Hudson, at high tide.

All of that said, it boils down to this. This is not going to be a Katrina caliber storm. There’s a hell of a lot of infrastructure in the region that is going to ensure a rapid response from emergency workers and the government. They’ve got a lot of warning. Katrina intensified rapidly right before landfall, Sandy isn’t likely to. Also, the area isn’t apt to flood and then stay flooded like New Orleans did.

You’re going to see a huge chunk of real estate without power, probably in to the weekend, but it should rapidly recover after that. You’re going to see a huge chunk of real estate under water, but it should subside by Thursday. The number of people who live above street level in that region, and the cold weather behind the storm, should keep the number of displaced people to a manageable number. There will still be thousands. Maybe tens of thousands, but you aren’t going to see the hundreds of thousands of people that are rendered homeless long term like you saw on the Gulf Coast. The fact that there is high ground readily accessible from these areas (looking at topo maps of coastal NY and NJ, virtually nobody is more than 10 miles from land higher than 100 feet above sea level, and nobody in the low areas is very far from buildings higher than 3 stories) you aren’t going to see the massive humanitarian crisis that delayed evacuation and rescue caused in the Gulf Coast.

It’s going to suck. It’s going to shut down the NYC area for probably close to a week. It’s going to cause miserable weather and widespread power outages reaching well in to eastern PA, New Jersey, eastern MD, and southern New York. It’s going to ruin trick-or-treat. It isn’t going to kill thousands. It isn’t going to leave hundreds of thousands homeless for months or years. It isn’t going to render large swaths of urban areas uninhabitable.

It is going to give an election weary public and media something else to talk about for a week.

Qualifications: 20 years as a storm chaser and amateur meteorologist, including 8 years chasing hurricanes in the Gulf States. 10 years doing disaster prep and relief volunteer work. 5 Years as a volunteer firefighter, numerous responses to storm damage calls.

Edit: Added a sentence to the Wind paragraph for clarity.

Edit 2: bstone99 correctly pointed out that I erroneously stated the peak storm surge would be the northeastern corner of the storm. In this case, it will actually be the northwestern corner, north of the eye. With current tracks, somewhere between northern New Jersey, and Long Island.

Edit 3: Thanks for all the upvotes and great questions folks. So far, it’s my second most popular comment ever, and certainly the one that drew the best response from other users. But… I have a huge test in the morning, and I’ve been using this as an excuse to avoid studying. I’ll try to answer additional questions tomorrow, but I’m betting there are folks in this subreddit far better qualified than I to answer many of them.