August 27, 2020

Hurricane Laura and the Wind Speed Dilemma

Last night, Hurricane Laura made landfall on the southwestern coast of Louisiana, bring heavy rain (6-8 inches),  strong winds (gusting to 132 mph at one location), and a coastal storm surge (roughly 10 feet at the most vulnerable locations).

The NWS Lake Charles radar image at midnight central time showed a well defined eye as the storm was making landfall.


Now the dilemma and interesting part.  Based on reconnaissance aircraft and other information, the National Weather Service's National Hurricane Center had estimated that Laura was a Category Four hurricane just prior to landfall, and according to the official Saffir-Simpson scale, that means the sustained surface (10-m) winds, averaged over a few minutes, were between 130 and 156 mph (see below).  Not gusts, sustained winds.

Saffir-Simpson Hurricane Categories

But here is the issue.  What were the maximum sustained winds that occurred last night as Laura made landfall?   Looking at all available stations, the highest sustained wind was 98 mph at Lake Charles Airport.  The map below shows the sustained winds at 1 AM, when the storm was just moving inland (wind barbs show sustained winds, with gusts in red).  The blue arrow indicates Lake Charles Airport.


Looking at the sustained winds, one would conclude that Laura was only a weak category two hurricane (96-110 mph).

And then there are gusts.  Gusts are not used as part of the Saffir-Simpson hurricane scale, but, let's face it, gusts are very important.  The big damage in most storms are done by the gusts.

Below are the maximum gusts of Laura. Two locations are extreme: Calcasieu Pass on the coast and Lake Charles, a few miles to the north (127 and 132 mph gusts, respectively)


Such strong gusts are consistent with the destruction of the NWS radar dome at Lake Charles Airport--they are rated to handle up to about 135 mph. (see the before and after below).  








So what is going on?  How strong was the storm?  Category two or four?

A key issue is friction and drag, which is much greater over land (with trees, hills, buildings, etc) that over the aerodynamically smooth water.   As a result of this surface drag, winds decrease VERY rapidly over land, even if the hurricane remains relatively intact aloft. 

Let me illustrate this visually, by showing you a forecast by the state-of-the-art NOAA/NWS HRRR model as Laura made landfall.  These plots show surface (10-m) surface wind in knots (1 knot=1.15 mph)

Before landfall (9 PM PDT), a nice hurricane structure is apparent, with some winds getting to 90 knots in the eyewall.


But then as the storm makes landfall (1 AM PDT), you can see a profound weakening of winds over land.


And by 5 AM PDT, with the storm completely over land, the fastest winds are gone.


So even if the storm had category four sustained winds near the surface while it is offshore,  the sustained winds decline precipitously when the store goes onshore.

But yet the storm can still remain very, very dangerous in the hours after landfall.
Why?

First, even the reduced sustained winds (e.g., 90-100 mph in this case) can produce great damage.

But there is more.  Gusts don't necessarily decline as rapidly as sustained winds as the storm moves over land.

To illustrate this, here is a plot of the  predicted gusts as the storm made landfall.  Not as much a decline over land as for sustained winds.  Gusts are caused by the intermittent mixing down of faster (higher momentum) air from aloft down to the surface.  So even if winds are slower down lower, sometimes air from aloft...where the winds are still blowing hard...can be mixed to the surface.  So gusts can hold out longer than sustained winds as a storm makes landfall.


The bottom line: a storm that was category four over water can still maintain a real "punch" over land, even after it nominally declines to a category two. Strong, damaging gusts can remain, even when the sustained winds decline.
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Some excellent articles on the surprisingly low wind speed over land  during hurricanes, by meteorologist and writer Bob Henson, can be found here:


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Will do a weather forecast and discussion tomorrow-Friday (video on my blog)

My blog on the KNKX firing is found here.
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10 comments:

  1. Very interesting explanation of what causes wind gusts - thank you.

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  2. If you read about other catastrophic US hurricanes in the past, you really appreciate the modern weather forecasting that we currently enjoy. The Galveston hurricane of 1900 springs to mind, since it wasn't too far away from this hurricane's landfall. Truly horrific loss of life, we've come a long way.

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  3. Hi Cliff. Is it just me, or does the news call every substantial hurricane that makes landfall “the most powerful ever”?

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    Replies
    1. It's not just you. Everything that happens must be exaggerated. The biggest, smallest, best, worst EVER! It's fascinating to think what sociological conditions produce this neediness. This is not a trait found in a healthy society.

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    2. Actually it seemed to me that the news coverage of this hurricane was rather subdued, and I did not hear any superlatives being used to describe it. Once upon a time when there was no virus and American politics were far sleepier, August was notoriously very "slow" with respect to news, so a hurricane was a good chance for the news networks to go full throttle with coverage . . . you know, give Dan Rather or Tom Brokaw or Peter Jennings a raincoat and send them to the beach. Those days, I believe, are over, at least until the virus is over and, one hopes, normal, calm, non-hyperbolic discourse returns to American politics.

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  4. So what do you call it? Cat 2 or 4? Maybe split the difference...3? Thats seems consistent with the wind damage in aerial and ground photos,

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  5. Uneven land surfaces will reduce wind forces at ground level - laminar flow of water will impede wind forces much less - fluids dynamics in action - NWS is trying to find the knife's edge - not overplay the destructive nature of these storms while not endangering life by underplaying - how do you define a win in a game like this?

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  6. Storm surge should be a factor in ranking hurricanes if the purpose of the ranking is to determine emergency response and resources allocated to the response.

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  7. I just found your blog thanks to your post on WUWT. This is something I've been investigating (as much as I can) after a few hurricanes that came close to or hit my city in Central Florida. I'm just an engineer trying to understand these things, but at least that leaves me comfortable with numbers.

    When I've looked closely, I've seen exactly the things you report on. Winds reported by buoys or other sensors never match the winds given by the NHC.

    In Hurricane Matthew in '16, NHC reported winds at 150 mph, but I never found a buoy that showed above 100. In '17, Irma came up north over the Keys and went over Naples (SW Florida). I could watch a buoy near Naples and could see when the eye went overhead - the winds dropped to under 10 knots and the barometric pressure dropped to its minimum 27.75 inches. The highest wind before the eye was 49.4 mph. The strongest wind after the eye passed was 64.3 mph, an hour after the eye. Was Irma even a hurricane at that point?

    I'm fully comfortable with saying the storms are enormous, complex systems that aren't well described by one number, but it comes across as the NHC routinely rating the storms as more severe than they really are. In the case of Laura, it seems that both the wind and storm surge predictions were very overstated.

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  8. All of this reminds me of the Columbus Day "Big Blow" that hit the Puget sound area big time in 1962...It was the equivalent of a Category 2 when it came from Oregon, and slowly dissipated to a Category 1 by the time it hit Bellingham...then it broke down in B.C. to just another storm...nontheless, it was the largest "storm" to hit this area in over 100 years...I could go on...but please go to a website called "Storm King". run by a professor up at UBC in British Columbia...he has studied all of the major storms to hit the NW over the last 100 years or so, and his accounts are fascinating...and very detailed...I was 15 when the Columbus Day storm happened, and will never forget what it was like...50mph sustained winds in Seattle, with Gusts to 80, and Renton had a 100mph gust!..this storm lasted for four hours, and was relentless!...I know, as I was outside, trying to deliver newspapers when it hit!

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