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.
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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:
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.
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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!
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.
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.
You should ask Dr. Mass that question on his site.
Here is how the alarmist press hypes it:
https://insideclimatenews.org/news/27082020/laura-rapidly-intensified-over-super-warm-gulf-only-storm-surge-faltered?utm_source=InsideClimate+News&utm_campaign=3932fbc440-&utm_medium=email&utm_term=0_29c928ffb5-3932fbc440-327531733
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.
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.
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