It is time to discuss, once again, the falsehood known as “Hurricane Landfall.”

A hurricane is a whopping big thing. A hurricane can be bigger than some states. The physical region across which a hurricane is potentially deadly and damaging is very large, many tens of miles across, sometimes a couple of hundred miles across. The danger zones are often organized like this:

The central storm surge. A central region may have a strong storm surge caused by the low pressure of the storm. This may be dozens of miles wide, but the area of effect is determined as much by the shape of the coastline the hurricane is landing on as by the hurricane itself.

The Right Punch. To the right of the central storm surge area is a region where the counter-clockwise rotating storm bearing down on the coast and hinterland will have very strong winds combined with very low pressure to increase the storm surge even more.

The storm surge caused by the low pressure system and the right punch can be a very wide area, and it can affect a coastal region for a long period of time, as the “surge” itself maybe several miles “deep.” If a hurricane is moving slowly, if the winds are strong and the pressure low, and if tides are already high and coastal flooding is already underway because of hurricane caused rain that came through for several hours before the surge arrives, this flooding can be extensive.

In some cases, the initial flooding may be very bad but the secondary flow of flood waters can be worse. If a hurricane storm surge and rain-related floods fill up the lagoons behind barrier islands, and at the same time the outlets get clogged by debris pushed up by high waves, that flooding can break open the barrier beaches in new locations, which may or my not be underneath settlements, major roads, etc. (This was, by the way, the theme of the famous book “Condominium” by John D. MacDonald which had a lot of inaccurate or outdated science but still stands as a classic “Disaster” book.)

So to review so far, around the middle of the hurricane is very low pressure, and to the right of this middle are very strong winds (and low pressure) that can cause a big flood that can first run over the land and flood stuff, then run back to the ocean and do even worse damage. Well within this area is the “eye” of the storm. The area of storm surge is much, much larger than the eye.

Spin-off Tornadoes. The big giant hurricane may cause the formation of many small tornadoes which will essentially act as very intense ambassadors of the passing cyclone. The tornadoes actually form as a function off the weakening, or generally messing up, of the organized cyclonic hurricane. They tend to form in advance of “Landfall” by as much as two days, but are most common as the hurricane’s right front quadrant is over land, and occur over a very large area. (They can also occur long after the Hurricane is downgraded to a tropical storm and is mostly on land, days after “landfall.”)

Hurricane Winds. Around this area of storm surge is a region of hurricane- or near hurricane-force winds which, especially if the hurricane is moving slowly, can buffet an area for hours and hours of time. This is like one of those nasty thunderstorms with the ‘straight line winds” coming through and knocking down a couple of trees, but for several hours. The area of these winds is typically measurable in the hundreds of miles in all directions.

Flooding Rains. Usually, over a larger area than this wind zone there will be bands of rain falling. The region over which sufficient rain to cause flooding may fall is hundreds of miles in any one direction. Flooding may start long before the hurricane force winds reach the coast. After the hurricane is way inland and is no longer called a hurricane, it can still cause major flooding. If you live, say, in Virginia or Connecticut, don’t think the hurricane has missed you just because it hits the Gulf Coast. You may be in for some major flooding in a week or so.

The Landfall Fallacy. Now, there is this thing about scientists, even meteorologists. When dealing with time-based phenomena, they need to know when something starts and when it ends, so they can do things like measure when on average certain things start, how long on average they last, etc. Therefore, there has to be an agreed upon point in time when a hurricane starts to exist (this is when the winds reach a certain strength and the cyclonic storm reaches a certain degree of organization, all of which is actually kinda hard to measure but they do it anyway). There is also an agreed upon point at which the hurricane “hits land” … known as landfall. This is when the eye of the hurricane, which is usually still visible on satellite views, on radar, as well as on the ground, crosses the shoreline. That is the arbitrarily decided on moment when scientists say the hurricane is at a certain point in its life cycle.

This does not. Repeat, not. NOT. mean that a hurricane “hits land” or “arrives” or “becomes a problem” or “starts to do damage” on “landfall.” No. The hurricane arrived hours before landfall! The outer bands that brought the beginnings of flooding rain came way before. The occasional tornado spun off by a hurricane may have already done in a neighborhood hours before. The highly damaging winds arrived long ago. The storm surge may have even started to chew up cities, towns, and industrial areas along the coast before the eye wall crosses landward, depending on all sorts of different factors. Hell, it is actually possible for a hurricane to totally mess up a coastal region then move back out to sea with the eye never crossing the coast. No landfall, but big problems. Landfall is not arrival.

The reason I mention this is that the “landfall fallacy” was one of the two Great Stupidosities that happened in relation to Hurricane Katrina, the anniversary of which is coming soon, and I fear that this fallacy remains in place (or has made a comeback) as we approach the visitation of Hurricane Isaac, now entering the Gulf of Mexico. (The other fallacy is that Katrina did not breach the dikes in NOLA, that they were breached by a flood. Which must have happened at the same time as the hurricane. But really was the hurricane… yes, you can imagine that these two Stupidosities, both perpetuated by a combination of ill intentioned politicians and not very well trained weather reporters and other journalists, are related.)

In a way, Hurricane Isaac is already affecting the Gulf coast because people are reacting to it! But really, it may be hours before “landfall” when we see our first serious flooding, wind damage, tornadoes, and all that, wherever Isaac ends up going. The area across which it will affect things will be much, much broader than the spot it makes landfall. When Katrina hit, Mississippi really took it in the neck. Huge areas of coastal Mississippi were wiped out. But, since “landfall” was in New Orleans, it didn’t make a lot of sense to report much of that in the early days of that disaster.

The hurricane hits where it hits, and it is a Big Giant Thing. Isaac may very well hit BOTH Mississippi and New Orleans, and a bit of Florida, just like Katrina did.

Do not fetishize the landfall, grasshopper.

Comments

  1. #1 Bacopa
    August 26, 2012

    Maybe this will help folks understand what a hurricane is like. Greg’s got a great summary of all the issues here but I thought I might add a point about the wind. Here’s the only way I can describe it from personal experience: Imagine the biggest supercell thunderstorm ever. Now imagine one of those really strong gusts of wind from that kind of storm. Now imagine that lasting a few hours rather than a few seconds. Then it gets even stronger for a few more hours. This is serious stuff. You would have to experience it to believe it.

  2. #2 Greg Laden
    August 26, 2012

    Yeah, exactly. I’ve been lucky to have experienced my hurricanes from structures that easily withstood the storm, and never more than a Cat I. Imagine what it is like living in a village with wattle and daub huts and palm trees flying past.

  3. #3 Jared Cormier
    United States
    August 26, 2012

    We’ve already filled up tanks, prepped the generators, moved the animals indoors, stocked up on groceries, water, etc. and got plywood for windows. All I have left to do is put up the plywood and test the generators under load.

    As far as Katrina; the entire area from Venice to Pascagoula was a wasteland. The political idiocy and mismanagement leading up to and following the storm didn’t help much…

    Also, IIRC, the levees were over topped, which was partially due to the closure of a pumping station and led to rapid erosion and catastrophic failure. In any event, I’m surprised they survived as long as they did; they were rated for an indirect Cat 3 storm.

  4. #4 MadScientist
    August 26, 2012

    The best one I’d been in (in south-east Asia) had steady winds at sea of just over 200kts and wind over land at about 150kts. Only a fool would go out in such conditions, but there’s no shortage of them.

  5. #5 Birger Johansson
    August 27, 2012

    Since Baal -the god of storms- is a rival to Yahweh, it was plain stupid to have a political event featuring thousands of fundamentalists smack in the middle of hurricane territory.
    Floridans should sue GoP for putting their state in harm’s way.
    I am aware of the legal concept “act of God” but it is usually understood that the god in question is Yahweh, not Baal, so it should not interfere with a class-action suit.

  6. #6 Greg Laden
    August 27, 2012

    Yes, exactly. The whole “Act of God” thing makes total sense until you have to assign an “act” to a particular god, then it can get very tricky. Thor is not the only one with lightning at his disposal.

  7. [...] Hurricane Landfall: What is it and don’t be stupid about it. [Greg Laden's Blog] (scienceblogs.com) [...]

  8. [...] far I’ve not seen any reporters falling into the Landfall Fallacy, but on the other hand, I’ve not been watching the news much. Huxley and I have been busy [...]

  9. […] This seems to happen a lot with hurricanes. When the Bush Administration wanted to avoid taking responsibility for a poor response to Katrina, someone actually said that the major damage done to New Orleans was not due to Katrina, but rather, to flooding. This idea was bolstered by noting that the hurricane had made landfall at a different time and place than the flooding. That, in turn, was based on the idea of “landfall” being related to the location of the eye of the storm; but the eye of a hurricane is tiny compared to the entire storm, which may be hundreds of miles across. […]

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