Hurricane Laura and the Wind Speed Dilemma

Reposted from the Cliff Mass Weather Blog

Thursday, 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.
_____________________________
Some excellent articles on the surprisingly low wind speed over land  during hurricanes, by meteorologist and writer Bob Henson, can be found here:

https://www.wunderground.com/cat6/hurricane-winds-landfall-measurement-challenge
https://www.wunderground.com/cat6/hurricane-winds-landfall-why-it-they-seem-fall-short___________________________
Will do a weather forecast and discussion tomorrow-Friday (video on my blog)

My blog on the KNKX firing is found here.
_____

80 thoughts on “Hurricane Laura and the Wind Speed Dilemma

  1. That is a very good summary. The only thing left out was tornadoes. Tornadoes are most prominent when the rotating winds of the bands come ashore. Much wind damage is due to those tornadoes but it is difficult to determine which winds caused a structure to fail as the evidence is destroyed.

    • I was also going to mention tornados. When the one and only strong band preceding tropical storm Isaias came through the Northern Neck of Virginia it spawned EF2 tornadoes that did incredible damage. While most tornadoes in tropical systems are short lived and relatively weak (EF1), they don’t always follow that rule. The tornadoes can be pretty far ahead of the eye, and in fact generally are mostly in the first strong band you see from an approaching storm. The real problem is they spin up in minutes so you don’t get a lot of warning.

    • I am always baffled by the use of the term”winds”…which implies that there is more than one wind at play; which begs the question: “where does one wind end and the next one begin?”

  2. Interesting posting of the effects on wind speeds in a hurricane. Yesterday I commented about a hurricane (typhoon) in went through 20 meters up (wind gauge about 23 meters) in an air traffic control tower in Vietnam, in 1969. This location was around 50 miles inland. The gusts were on the order of 70 to 80 knots, but once the eyewall approached the wind steadily increased to 111 knots and stayed steady for about 10 minutes as the eyewall slid (never saw the actual eye) past us. In my case the sustained was much greater that the gusts, and remember we were at twice the height above ground than the official sustained” measurement height.

  3. As a child, I was repeatedly exposed to story of The Boy Who Cried Wolf. We humans can be a deceitful lot, and lying or embellishing can cause other people to react in ways that furthers our agenda or simply amuses us. I have followed hurricanes for years, and the one thing you can always take to the bank………always take the wind speed with a grain of salt.

    • Well, you need a new fable now….

      Apparently all the hurricanes these days are mere breezes, the thick ice is transparent to satellite photos, the fires are just pleasantly warming…

      I honestly believe some people here would deny their own house was on fire…

      • non se·qui·tur
        /ˌnän ˈsekwədər/
        noun
        a conclusion or statement that does not logically follow from the previous argument or statement.

      • And rioters are protesters, while riots are love fests.
        Also, I know for a fact that many Liberals deny their own cities are on fire. So there’s that.

      • This hurricane wasn’t much compared to some that happened in the 50’s and 60’s. Hazel in 1954 did make Laura look like a breeze and Camille in 1969 put ocean going ships 3 miles inland. The Great Hurricane of 1935 had sustained winds of 185 mph and higher gusts. Do you know how much more energy winds at 185 mph has compared to 130? You think we never had a hurricane in the past?

        Hurricane landfalls in the US are trending down. There were no western Pacific typhoons for a MONTH during the peak of the season. How does that fit into the CAGW models?

        Take your tinfoil hat off please.

      • griff, I know that you are paid to make a fool of yourself, but do you have to be so good at it.

        Pointing out that hurricanes aren’t getting stronger or more common is the same as saying that hurricanes are just mere breezes.

        Pointing out the difficulties with using photos to measure ice is the same as saying it can’t be done.

        Pointing out that fires aren’t getting more common, is the same as saying they are just “pleasant warming”.

        If your house is not on fire, why should one believe that it is?

        • Mine’s not either. Neither are my neighbor’s. In fact, I don’t see any in any direction.

          If it were on fire, I would leave and call 911.

      • griff.. we all understand that you want to be able to take what we produce and earn and give to you and your indolent friends… And that everything you say or write is colored by your passion for those larcenous urges.

        Long story short we don’t care what you say or write, but find it perversely amusing.

  4. Looking at hurricane landfalls like this and flooding like they’re seeing in China makes one wonder at why cultures are so set on building cities next to oceans and rivers. Obviously, you need ports and landings for commerce. But other than those facilities, putting every other building next to water seems like you’re just asking for it. Relying on the kindness of nature and probability doesn’t seem to be doing these areas any favors.

    • The key is recognizing that nature is going to do what nature does, then plan and engineer accordingly. Believing you can control nature with dances, prayers, or taxes puts people in danger.

  5. No expert here, but I live in Florida and have been following hurricane issues for a long time. I think the winds quoted in the measurement of hurricanes (the Saffir-Simpson scale) are the winds aloft (higher than a few thousand feet) measured by the hurricane hunter aircraft. The winds on the ground are, in my experience, always less than those aloft. The winds aloft are the Hurricane (or Tropical Storm) Category winds reported by the NHC and the media. Before a hurricane reaches landfall, the NHC tries to estimate the ground-level winds by dropping wind metering equipment from the hurricane hunting airplanes.

    The take away is there is always a difference between the hurricane rating and the winds on the ground. Sometimes media outlets will show both, but seldom. My hope is the reporting will more often include the surface winds. Ocean buoys and shoreline weather stations (at https://www.ndbc.noaa.gov/) are a good way to gauge the surface winds, when the stations are reporting.

    It’s also good to see that the storm surge is getting more coverage. It’s hard to estimate. But a big storm surge is most often more dangerous than the hurricane winds — at least for hurricanes below level 4 or 5.

    My 2c.

    • Yesterday’s Daily Mail UK said definitively that it was category 4 and found some pictures of damaged structures to prove it.

      95% of the time we find the marine forecast here to overestimate wind speeds. We anchored in a bay open to the northwest last week on the basis of the sunset and spent a very quiet night, despite 15-20 knots NW forecast for overnight. Of course there was a plan for wind in the night.

    • What happened to cause the expected up-to-20 foot surge to come in at less than 10 feet?

      Another issue with aloft wind speeds vs ground level? I can’t remember any hurricane ever meeting o exceeding forecast water surge. Maybe Katrina due to bad levees?

      • Fetch has a lot to do with it. If the storm spins up close to the shore it doesn’t have time to push the water ahead of it. If it is stronger farther out the waves build on one another for a longer period of time piling the water up.

      • The tide gauge closest to Cameron, LA topped out at 9 feet, instead of the catastrophically predicted 20 feet. Ike produced 19-22 foot surges but it was a much larger storm with much more time to push the water in front of it. A 20 foot prediction for Laura always sounded like the worst-case scenario and unrealistic. The storm was only a three or four for 24 hours, not long enough to really get a decent storm surge accumulated in front of it.

    • The standard elevation for measuring sustained wind speed of a Hurricane is 10m above land. No aircraft need apply.

    • Take a look at the data from the weather buoys in the path of the last 3 hurricanes to make landfall in the Gulf Coast region and you will see that the sustained winds came nowhere near the wind speed of the category the hurricanes were classified as. Granted these buoys are usually 3-4m above sea level as opposed to 10m but I would still expect the recorded wind speeds to be closer to the claimed wind speeds than they are.

      https://www.ndbc.noaa.gov/?lat=31.970804&lon

  6. It is nice to see others questioning what happened with Laura. When Michael came ashore, it literally erased the beachfront housing, but when Laura came ashore it left many buildings mainly intact. Not saying that the visible damages to buildings and infrastructure were not there, it just wasn’t what we experienced with Michael. Category 4 hurricanes are supposed to cause damage extensively throughout the landfall area and to points well inland. Many weeks (months) later Michael was upgraded to a Category 5 hurricane at landfall. Will Laura be downgraded to a Category 3 or even a Category 2 hurricane?

    • Michael literally erased the beachside neighborhood; everything across the street was still standing with damage not consistent with a cat 5. And the beachside neighborhood was wiped out by the surge, not the winds.

      • Michael took out the clapper board stuff on the seafront. The modern building codes housing behind it was completely undamaged; they managed to find one building with a corner of its roof pulled back, that was all. Not a single broken window.

        A genuine storm surge has all manner of junk in it that gets hurled against buildings as the surge breaks over or against it, and that breaks windows. As you say, the Michael storm surge struggled to get off the beach, a big nothing in hurricane terms.

    • LAURA took almost the same path as Hurricane RITA. Most of the buildings that remain from Laura were rebuilt to higher wind ratings and higher elevations due to RITA. Would not consider buildings left standing as a good measurement of strength of the storm.

  7. It is rare that the maximum sustain wind would ever be observed with an anemometer because it isn’t the “typical” sustained wind, it’s the “maximum” sustained wind, which is at some unknown location but inferred to probably exist based upon other measurements. As the hurricane makes landfall, you would have to have an 30 ft high anemometer tower every several hundred feet across 50 miles of coastline to capture it. Whether “maximum sustained wind” is a useful measure to give to the public is certainly debatable.

  8. The strongest wind might have been expected on the Rockefeller Wildlife Refuge south of Grand and White Lakes (between 92.5 and 93 degrees W). They should have weather information, had considerable wind during Rita. This area to the E was outside of the eye when it crossed the coastline. Highest winds in Harvey where I live were N of Rockport on the Lamar Peninsula based on the tree damage, again outside (N) of the eye, portable gauges destroyed. Watched it closely on radar, appeared highest in Copano Bay where about a 3 meter surge occurred to the S. There may have been a seiche as the N end had a meter rise, not sure of timing. Closest coastal Laura wind station shown above to the E was at Cypremort Point, there is also a coastal Marsh Island Wildlife Refuge, the large island just to the south and the State Wildlife Refuge on the western shore of Vermilion Bay, Audubon Refuge adjacent to the west.

    On the maps above the larger Calcasieu Lake was not named as were the smaller ones to the west, wind station at the Pass. There also is the Cameron Prairie National Wildlife Refuge on the SE shore of the Lake, the green like the named Sabine Refuge on the western shore, might also have information.

  9. The various views of modeled winds as the storm moves inland are very informative. I have never seen such an illustration before. Thanks, Mr. Mass.

  10. So, based on the “Damage” part of the Saffir-Simpson scale, what would it appear that Laura was? What I’m seeing on google images suggests it was probably a catagory 3.

  11. Another way to describe gusts are as turbulent flow. Turbulent flow is what causes erosion/entrainment of sediments in water as well as what causes most structural damage from wind. It’s why wind can tear off a roof while leaving a picnic table in the backyard unmoved. Turbulent flow is increased by the uneven boundary layer between the fluid and the surface below it, which is why gusts hold out longer than sustained winds (laminar flow) as the storm moves from water to land.

    • I would bet if you calculated the Reynolds number, you’d find that the sustained winds were turbulent as well as the transition from laminar to turbulent flow occurs are relatively low velocities. The only difference is that the flow regime for sustained winds is well-developed (i.e., fairly uniform) but at the boundary layers, as you pointed out, could be quite chaotic.

  12. The damage photos of the Capital One Tower are very telling. Those widows were ballistically strengthened to survive 132 mph winds, they didn’t, why?

  13. 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.

    When I was a kid we used to leave a kite up after dark, and it would be dead calm where we tied it to a post, but the string was pulled tight by the wind aloft. Two or three hundred yards of fishing line was enough.

    • At the foot of the Olympics in Aberdeen, Washington, in rolling hills of farmland and conifer forest 30 or so miles inland, the locals use a weather term “high wind”. This unusual condition could last for hours. I could stand in my 5 acre hay field surrounded by trees, light a match and hold it over my head and it would not blow out. At the same time, 75′ to 100′ up, tree tops would be bent over and branches breaking off and blowing out.

  14. As a micrometeorologist, I make measurements near the surface (which is likely the kind of measurements you are using over land). I am not aware of any common, research-grade instruments that are capable of making reliable measurements of winds faster than about 60 mph (which are quite uncommon). I would assume/hope that the aircraft measurements made by the hurricane researchers (pitot tube based) can do better. They routinely need to measure winds much greater than 60 mph.
    So the fact that land-based measurements don’t support measurements from hurricane researchers is not surprising.

    • Pitot tubes are generally quite precise at subsonic speeds, but they must be corrected for position error, local air pressure, true aircraft speed relative to the ground and the angle between the aircraft movement vector (note NOT the aircraft course) and the wind direction.

      In short, do not expect extreme precision.

  15. When Hurricane Hugo hit SC the maximum actually-measured sustained winds were something like 120 mph, but that was on a ship (suggesting an anemometer some distance above the ground) in Georgetown, SC, harbor. In Downtown Charleston, to my recollection, maximum sustained winds never exceeded 90 mph (though gusts were well over 100). And the eye of the storm passed directly over Downtown Charleston! The worst part of the storm went over Awendaw and McClellanville between Charleston and Georgetown, but there were no instruments to measure it there. I would guess maximum sustained winds at treetop height must have been close to the 140 mph NWS estimated for the storm right before landfall, because pretty much all the pine trees in the Francis Marion National Forest where the heft of the storm hit were broken in half about halfway up the trunk – it looked like a forest of 4s. That was certainly more tree damage than I would have expected from 90 mph sustained winds.

    I suspect that in Laura, like Hugo, the heft of the storm went over a very sparsely populated area (I shudder to think what Hugo would have been like for Charleston had the eye gone over, say, Edisto Island instead of Peninsular Charleston so that the heft of the storm went right up Meeting Street . . .). Perhaps if there were anemometers in Grand Chernier we might have seen winds closer to the NWS estimates. As it is I am glad Laura hit a sparsely populated area rather than downtown Houston. I feel for the people of the Lake Charles Area (been through it myself), but it truly could have been so much worse!

  16. So does this mean that historical hurricanes where all we have are ground speeds were in comparison much stronger than we thought (over the ocean)? This would add more fuel to the fire that hurricanes are NOT increasing in intensity – it’s just how we measure their strength that has changed.

  17. NOAA had planned to have a spare NexRad radar to in case one was destroyed as happened now at Lake Charles (site ID: KLCH). The antenna is destroyed. The production line was long ago shutdown. That part of southern Louisiana now has a huge gap in radar coverage.

    Cliff Mass pressured NOAA though via Congress to use that spare NexRad radar for his personal vanity project so he could watch birds. The spare went to Langley Hill WA (KLGX) installation.
    No way for NOAA now to replace it, at least easily.

    Click here: https://radar.weather.gov/Conus/southmissvly.php
    to see the Cliff Mass Memorial Gap in radar coverage in Louisiana.
    Thanks Cliff, you screwed Louisiana and the Gulf Coast.

  18. Does anybody know if there are any wind farms or wind power electrical generators in this area? Are there any solar farms in this area? If so, do we know if there are any assessments on how or if wind and solar generators survived the hurricane.
    See in the photographs of the before and after of that radome caused me to wonder about the weather dependent powered generators and how they managed in the storm.

  19. Two comments …

    1. NHC uses 1-minute average for sustained wind while surface observations use a 10-minute average. So surface obs will always be less than NHC winds.

    2. NHC uses implied surface winds from flight level winds. I’m not sure of the accuracy of this conversion algorithm.

  20. I’ve noticed the wind discrepancy for a few years now. It even happens when a storm goes over islands, which should have minimal effect on wind speeds. I’m convinced that they are either using wind speed at altitude, or they are using models to guess what surface wind speed is supposed to be.

    This issue definitely deserves some investigation though

  21. 3 years ago NOAA started to use models instead of actual measurements and hurricanes became 2 categories stronger overnight. I have been telling this since the infamous Texas 2017 hurricane Harvey.

  22. Back during Hurricane Andrew, I was living in the NOLA area, and drove into the impacted area weekly for my job. Customers and workers had all sorts of scary tails of riding it out. Especially those down in Dulac and one in an old house in Berwick.
    Anyhow, I recall the official wind speed for Morgan City during the storm is lower than what the winds actually were because the official wind speed sensor was broken by the winds at the speed listed, but unofficial sources measured winds higher, and gusts well over 200.

  23. Like many others, I’ve noticed that the modern model winds, (gust or sustained), almost never verify. E.g. someone on AmWx posted the euro showing expected Laura gust of 199 at Lake Charles. I knew that wasn’t gonna happen.
    NHC is so fond of naming storms, I think they should all be called “Wolf”.

  24. “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.”

    Pure rationalization while ignoring the ‘problem’!
    The problem is the storm metric are measurements of “sustained winds”.
    That is, wind speed as measured over time; in Saffir-Simpson terms the average wind speed for one minute of time.

    A metric designed to ignore gusts!

    A Category 4 hurricane has minimum sustained wind speed of 111mph (178kph).
    As measured at landfall, Laura was a Category 2!

    “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.”

    Complete and utter bollocks!

    Hurricanes spin off tornadoes and have thunderstorms that generate strong downbursts.
    Both tornadoes and downbursts are significant causes of damage over very narrow areas.

    However, watch any video where severe hurricane damage is occurring and it obviously is not caused by gusts, but by the sustained wind speeds!

    Face it, there is no defense for labeling a Category 2 storm as a Category 4 just to keep media, NOAA and NHC happy.

    Hurricanes lose power and energy as they approach shore and especially after they move onshore.
    When Laura approached the coast, it was easy to see on satellite and radar images that the eye got much larger and then misshapen.
    The storm’s circulation slowed and the wind field expanded.

    • You are off by a whole Category. Cat 3 starts at 111 mph, Cat 4 starts at 130 mph.
      Face it, there no defense for not checking this before you post.

  25. I read the article and down through the thread and didn’t see anyone define “sustained”! So for those that are wondering “Sustained” in reference to Saffir-Simpson is a minimum period of 1 minute.

    I would like to thank Mr. Rotter for posting Cliff Mass’s article here. Time and again I have observed that the highest maximum sustained wind speed recorded by a land station does not come close to reaching the minimum speed required for the category of hurricane that has been declared when the storm is approaching landfall. Now I understand why and it makes perfect sense to me. That being said I have a question. Were there any buoys just off shore that the eyewall passed over providing windspeed data?

    I ask this question because in some previous cases where there was buoy data available the wind speeds recorded were also well under the minimum for the reported category of the storm that passed over them.

  26. The wind speed charade has been going on for ages at NHC. They have been over-hyping hurricanes for at least 20 years since I’ve been watching them. Every tropical store near 70 mph is over-classified as a hurricane. They hype the off shore winds constantly, until the storm hits land and real, measured speeds are reported. Almost always lower, and usually much lower. A 4 lands as a 2 or lower. Why NHC does this is perplexing. NHC needs a good shaking up, and should be required to report publically, after every storm, a comparison between their forecast and actual windspeeds when hurricanes hit shore. This is basic accountability. NHC has absolutely none today.

    • There is an incentive for an official agency like the NHC to overhype hurricane strength, because it gives them a margin of safety in the public’s eye. It they gave more accurate and lower likely strengths, and a storm exceeded them, they’d be blamed for not alerting the populace. Sins of omission are less costly, optically.

      Another incentive is a desire to scare the public into boarding up or evacuating, which it would do in lower numbers if forecasts were less alarming. But, over time, the public becomes cynical about these cries of wolf and takes LESS action.

  27. Our town was visited by Rita and Laura. Rita broke few trees and damage a few roofs. Laura has broken or uprooted hundreds of trees and it seems like each one wanted to fall on a power line instead of a house. While a few houses were hit by falling trees, most were not but many had close calls. Our city park, “West Park” has lost at least a third of its tall pine trees. The roof damage from wind was not wide spread and was light. Our city is called DeRidder, it is about 30 miles north of DeQuincy which is about 20 miles north of Lake Charles. Rita caused a lot of damage up to DeQuincy and started calming down as it headed north from there. Laura did not calm down until after passing thru DeRidder. Rita caused us to be without power for three weeks. Laura looks like it will be 1 to 2 months before power is restored to everyone. My house fared well, we lost the top of our brick chimney. The wind took it. The roof didn’t lose a single shingle and no trees fell on the house because we removed all of the trees that were close to the house years ago. However we did have a tree fall onto our dog-yard fence and many trees fell or broke along the property line. A lot of chainsaw action coming up to clean up that mess.
    I can’t quantify the difference between the two storms, but our experience tells us that Laura wins in the trouble that hurricanes cause after they come ashore.

  28. Currently NHC uses 4 sec average winds to determine hurricane strength sampled as the peak winds observed by hurricane hunter aircraft from the Stepped Frequency Microwave RADAR on those planes. They used to use buoys and observation stations which used 8min to 10min average winds. That is the discrepancy that you see over the last 15 or so years. They have not published anything that suggests this however if you read the discussion of every forcast for at least the last 6 or so they specifically state that the speed is set by observed SFMR at the peak point normally found in the NE quadrant of the storm.
    v/r,
    David Riser

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