Hurricane rating system

KEW

KEW

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#1
How is it that as good as meteorologists have gotten at evaluating hurricanes (still plenty of room for improvement, but they know a lot before landfall happens),the best method we have for rating the severity of the hurricane is the "Category rating"?
As I understand it the Category is based solely on wind speed and ignores flood potential.

I have family and friends in SC and NC, so followed the projections with interest. I watched as Florence became a Cat 4 hurricane, then dropped to a Cat 1 hurricane by the time of landfall (IIRC) and it wasn't too long after that it was down-graded to a "Tropical Depression".
Now, if you paid attention to the weather forecasts, you knew there was going to be serious storm surge along with subsequent flooding, but if you only paid attention to the hurricane rating, you might have thought the storm had lost most of its severity as it approached land. I suspect many along the coast decided that if Category 4 Hugo didn't get them, Category 1/Tropical Depression Florence would be a cake walk!

Given the importance of understanding the danger of a hurricane and the tendency of people to want to truncate things to their simplest description, how is it that we have not developed a storm rating system that includes an assessment of wind as well as flooding?

I might imagine Florence being presented as "a hurricane with a severity rating of wind 1 and flood 4 for a combined severity rating of 5" or something along those lines!

What am I missing? This seems too obvious and there are plenty of smart people involved in meteorology and emergency planning!
 
everettT

everettT

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#2
It's hard to predict storm surge, which is a seperate association. Wind typically is far less of a gauge of destruction in most storms. Example would be New Orleans where a strong tropical systems rain has more damage potential than wind ever would. It would be near impossible for a storm like florence to slow down and maintain its wind strength.
 
Steve81

Steve81

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#3
As I understand it the Category is based solely on wind speed and ignores flood potential.
....
What am I missing? This seems too obvious and there are plenty of smart people involved in meteorology and emergency planning!
This might help:

https://www.nhc.noaa.gov/surge/
Storm surge is produced by water being pushed toward the shore by the force of the winds moving cyclonically around the storm. The impact on surge of the low pressure associated with intense storms is minimal in comparison to the water being forced toward the shore by the wind.

The maximum potential storm surge for a particular location depends on a number of different factors. Storm surge is a very complex phenomenon because it is sensitive to the slightest changes in storm intensity, forward speed, size (radius of maximum winds-RMW),angle of approach to the coast, central pressure (minimal contribution in comparison to the wind),and the shape and characteristics of coastal features such as bays and estuaries.

Other factors which can impact storm surge are the width and slope of the continental shelf. A shallow slope will potentially produce a greater storm surge than a steep shelf. For example, a Category 4 storm hitting the Louisiana coastline, which has a very wide and shallow continental shelf, may produce a 20-foot storm surge, while the same hurricane in a place like Miami Beach, Florida, where the continental shelf drops off very quickly, might see an 8 or 9-foot surge. More information regarding storm surge impacts and their associated generalizations can be found in the FAQ section.
 
KEW

KEW

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#4
Okay, so storm surge may be pretty complex - although once the location is established, knowing what shore is relevant may still allow assessment. They gave predictions of how many feet from the storm surge before Florence hit the shore.
However, the weather forecast talked about the slowing of wind speed (hurricane category) as Florence was getting heavy from picking up water and talking about 40" of rainfall in some areas.
Here is the forecast from 9/12 and they seem very aware that this will be an especially bad storm from a flood standpoint as it was downgraded to Cat 2. I am making the case for a way to communicate that in a simple and direct fashion.
Perhaps the problem is we can evaluate the wind speed with good precision so the Category is a precise measure of the storm while the flooding potential may not be so precisely evaluated (?).

https://abcnews.go.com/US/hurricane...roach-friday-night-landfall/story?id=57765962
 
Swerd

Swerd

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#5
How is it that as good as meteorologists have gotten at evaluating hurricanes (still plenty of room for improvement, but they know a lot before landfall happens),the best method we have for rating the severity of the hurricane is the "Category rating"?
No matter how good meteorologists get at evaluating hurricanes, people are equally capable of ignoring the predictions. Yes, I agree categories 1-5 based on wind speed is oversimplifying things. But there are many people whose eyes glaze over as soon as anyone starts talking to them with numbers. That's why we see weather forecasts that talk about heat index in the summer or wind chill temperatures in the winter as if they are real temperatures. Why not just say the air temperature and humidity, or the temperature and wind speed?

As mentioned above, a hurricane's storm surge is complex. You must factor in atmospheric pressure, tide status as the storm comes ashore, where some one is on the coast relative to the storm's eye (remember the counter-clockwise rotation),slope of the ocean floor just off the coast, etc. I don't see how a single number can cover all that.

It seemed to me that Florence's slow traveling speed accounted for much of the flooding. It rained heavily for days instead of hours, without moving far from Wilmington. It wasn't the rate of rainfall, it was the total rainfall that caused such severe flooding.

The predictions for Florence were very good and the warnings were clear – evacuate. Not all people listen. I could shrug and say Darwin was right, but I don't know how weather forecasters can do better.
 
TLS Guy

TLS Guy

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#6
Okay, so storm surge may be pretty complex - although once the location is established, knowing what shore is relevant may still allow assessment. They gave predictions of how many feet from the storm surge before Florence hit the shore.
However, the weather forecast talked about the slowing of wind speed (hurricane category) as Florence was getting heavy from picking up water and talking about 40" of rainfall in some areas.
Here is the forecast from 9/12 and they seem very aware that this will be an especially bad storm from a flood standpoint as it was downgraded to Cat 2. I am making the case for a way to communicate that in a simple and direct fashion.
Perhaps the problem is we can evaluate the wind speed with good precision so the Category is a precise measure of the storm while the flooding potential may not be so precisely evaluated (?).

https://abcnews.go.com/US/hurricane...roach-friday-night-landfall/story?id=57765962
The real issue, is how wide the storm is and how fast it moves when it makes landfall. There has to be some common sense in the public. I know this is not common. But you know the thing is full of water, and the speed after landfall was very accurately predicted. So a lot of water in the hurricane and slow movement means a lot of water in one place, versus the same water over a larger area in a faster moving storm. The same thing happened in Houston last year.

Now any hurricane is going to be over saturated with water. The global temperatures and oceans are warming on average. There is no debate about that, as you can easily measure it. Now that means unless there us sudden cooling all hurricanes are going to carry more water per cu. cm or by what ever yardstick you want to use.

So that means the water dumped over a given area is proportional to the area covered by the hurricane and how long it is over a given area. In other words it is inversely proportional to its speed of progression. I guess the solution is more formal education of the public.

I have to say the warnings in all aspects of the Florence event were highly accurate. So anybody taking a proper interest, which they certainly should, would have been duly forewarned.

Having lived through floods though I can tell you that home owners are almost universally ignorant of their elevation in relation to surrounding water. Everybody needs to be familiar with their local flood plain map.

As I mentioned in the other thread of Swerd's, these official maps are produced by the US Army Corps of Engineers. These are frequently out of date, but will tell you your elevation above significant bodies of water.
So if you know your elevation above a river or lake expected to leave its banks, and know your elevation you can work out if you will flood or not before or at any projected crest. This is an area were people and I might say city officials need more training. It is not difficult and I used these calculations to protect our neighborhood in the flood of 1997. The problem is the crest was not predicted accurately until far too late.

However along with a patient of mine, who was a civil engineer, we made some observations, and probably by luck rather than judgement made a very accurate prediction of the crest. I used our judgement for preparation as building chairman of our medical facilities and not the Corp or city officials to start primary protection measures early.

When I got our neighborhood together, and started our serious defenses only one resident did not take heed and got a flooded home. The city engineer even went on the radio to say my neighborhood preparations were not necessary. Then as we were evacuating the city a few days later he was diking his home in his edition, and I was pretty sure he did not need to and he didn't. As for us, our ring home dikes were just high enough. They were well built and did not fail or get topped, but the latter was close. I went to obsessional measures to protect and keep the neighbor hood road side power transformers dry. The power company did an inspection and agreed they would keep the power on as long as homes did not start to flood. This allowed home sump pumps to keep running in our area and we could use electric pumps on the dry side of the dikes for any under seepage, and not be dependent on gas pumps. We evacuated on the Saturday afternoon, as the city center was ablaze with the water too deep for the fire trucks to fight the blaze. The sky was full of helicopters plucking people of their roofs. We left two neighborhood volunteers behind to keep an eye on things. We did not have a a single fatality in the city though, which was very fortunate.

The Corps of course made blunder after blunder. They tried fruitlessly to do their defenses too close to the river, and did not protect the high ground. So when their defenses failed without backup protection on the high ground water quickly flowed over 85 to 90% of the city.

So knowing your elevation above problematic bodies of water is something all home owners and neighborhoods should know. That is job 1 when it comes to flooding, which unfortunately is far too common and takes home owners by surprise.
 
KEW

KEW

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#7
Thanks guys!

Dr. Mark,
Ever thought about moving down here?
I live in a pretty nice subdivision and, while flooding is not a concern, it'd be great to have you around to help set up neighborhood defenses for when the Zombie-pocalypse comes, or the drone-wars, or whatever dystopian future may pan out!!!:eek:

Seriously, an interesting read! It is disappointing to hear that our Corp of Engineers does not have their act together any better than it does. It is becoming more and more obvious that flood management is becoming more important as the Earth heats up.
 
KEW

KEW

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#9
The predictions for Florence were very good and the warnings were clear – evacuate. Not all people listen. I could shrug and say Darwin was right, but I don't know how weather forecasters can do better.
I agree, I thought they did an exceptional job of telling us what to expect.

However, your example of wind chill or heat index metrics are excellent examples of converting data into a very relevant number. Knowing it "feels like" 113 degrees is much more useful to the average person than knowing it is 95 degrees and 60% humidity

As mentioned above, a hurricane's storm surge is complex. You must factor in atmospheric pressure, tide status as the storm comes ashore, where some one is on the coast relative to the storm's eye (remember the counter-clockwise rotation),slope of the ocean floor just off the coast, etc. I don't see how a single number can cover all that.
It doesn't have to cover all of that with accuracy, it just needs to be better than the current Category rating which has often been erroneously used as the only relevant metric of hurricane severity/hazard. Or just have a second number based on the weight of water being carried by the storm.
It seems these events are becoming more common and I feel a better system for communicating the severity is in order.
However, given that three of the AH members I trust/respect most think it is not a reasonable prospect, I will assume that my perspective has a lapse (it is not the first time)!
 
KEW

KEW

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#10
I was down on Tybee Island with the inlaws, which reminded me, I saw this handy marker by the beach there:
View attachment 25839
Excellent!
That is great information to present to better quantify storm severity.
The only access road to Tybee tends to get underwater 3-4 times a year these days (without hurricanes),so they are well aware of the need to get off the island when the getting is good!

I hope you toured Fort Pulaski (either this trip or a previous one) the story of their defense against the civil war assault and the first military use of rifling in cannons (I believe, maybe Europe?) is one of the most thought provoking I have heard.
 
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TLS Guy

TLS Guy

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#11
Thanks guys!

Dr. Mark,
Ever thought about moving down here?
I live in a pretty nice subdivision and, while flooding is not a concern, it'd be great to have you around to help set up neighborhood defenses for when the Zombie-pocalypse comes, or the drone-wars, or whatever dystopian future may pan out!!!:eek:

Seriously, an interesting read! It is disappointing to hear that our Corp of Engineers does not have their act together any better than it does. It is becoming more and more obvious that flood management is becoming more important as the Earth heats up.
Not a chance. All our children and Grandchildren live in the Twin Cities metro, and so do most of our close friends..

As far as your number about rain there really is one, and that is predicted rate of rise and crest of rivers. That is all you really need to know about rain fall. Also I clearly remember the warning from the hurricane center to ignore the falling category ratings as reassurance.
 
KEW

KEW

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#12
Also I clearly remember the warning from the hurricane center to ignore the falling category ratings as reassurance.
I never got that warning, although listening to the forecast made it quite clear that this was going to be a severe storm despite the declassification.
However, I was sitting in the comfort of home watching the weather and not desperately trying to find/make sand bags and "stem the tide"!
 
Steve81

Steve81

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#13
I hope you toured Fort Pulaski (either this trip or a previous one) the story of their defense against the civil war assault and the first military use of rifling in cannons (I believe, maybe Europe?) is one of the most thought provoking I have heard.
I'll have to catch it on the next go round. This was my first time visiting the area (my wife has some family down there); by in large when we weren't on the beach, we were checking out Savannah. Did climb the stairs to the top of the Tybee lighthouse and enjoy the view though.
 
KEW

KEW

Audioholic Warlord
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#14
Savannah is a cool place. If you missed it, take your wife to Lulu's Chocolate Bar for an interesting dessert-drink experience!
They do have excellent chocolate if you/she don't drink, but the array of chocolate drinks are the true novelty!
Also, if you like jazz, look up their jazz bars on-line!
 
Swerd

Swerd

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#15
However, your example of wind chill or heat index metrics are excellent examples of converting data into a very relevant number. Knowing it "feels like" 113 degrees is much more useful to the average person than knowing it is 95 degrees and 60% humidity
I guess its true that wind chill or heat index values are examples of what you would like to see done with hurricanes. For hurricanes, I can imagine a "Get out your wet suits & snorkels" scale. It would not be a linear scale.

I have a pet peeve with heat index or wind chill numbers. I know people, who are otherwise intelligent, who always seem to take a heat index or wind chill number and speak of it as a real temperature. To me, it's simple enough to know two numbers, temperature and humidity.

Lately I've learned to like dew point temperatures better than relative humidity. In the graph, air temperature is red, dew point temperature is dark green, and relative humidity % is light green. The relative humidity line varies throughout the day as the temperature rises & falls. The graphed lines of temperature and relative humidity are 'out of phase' with each other. Dew point changes only as the amount of water vapor in the air changes.

1537330364671.png
 
TLS Guy

TLS Guy

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#16
I guess its true that wind chill or heat index values are examples of what you would like to see done with hurricanes. For hurricanes, I can imagine a "Get out your wet suits & snorkels" scale. It would not be a linear scale.

I have a pet peeve with heat index or wind chill numbers. I know people, who are otherwise intelligent, who always seem to take a heat index or wind chill number and speak of it as a real temperature. To me, it's simple enough to know two numbers, temperature and humidity.

Lately I've learned to like dew point temperatures better than relative humidity. In the graph, air temperature is red, dew point temperature is dark green, and relative humidity % is light green. The relative humidity line varies throughout the day as the temperature rises & falls. The graphed lines of temperature and relative humidity are 'out of phase' with each other. Dew point changes only as the amount of water vapor in the air changes.

View attachment 25847
That is a really important point Swerd. Dew point is a much better guide to the water vapor in the air than relative humidity.

It also tells you how low the temperature can drop, very useful up here in the "tundra"! The reason is that once the temperature tries to go below the dew point the water condenses and gives back its latent heat of vaporization to the atmosphere holding the temperature. The heat given back as the water condenses is huge: 2,260 kJ/kg. Of course when the sun vaporizes the oceans to create the hurricane, then energy flow is reversed. So I think you can see how hurricanes get their enormous energy. You can also see from those numbers how increase in temperature adds to the energy of adverse weather events, such as wind, rain, tornadoes and hurricanes.

I will say it again there is not nearly enough science taught in schools and far too much spent on social study BS.
 
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Swerd

Swerd

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#17
That is a really important point Swerd. Dew point is a much better guide to the water vapor in the air than relative humidity.
Thanks, I'm glad someone appreciated it.

For those interested, I get those graphs from a weather web page called Intellicast. You can customize them to show the details you want.
http://www.intellicast.com/National/Wind/JetStream.aspx

This site also shows a simple diagram of each day's Jet Stream winds over most of North America. Those high altitude wind patterns are (to oversimplify a bit) what drives/guides the weather patterns, those high and low pressure centers as well as the various fronts between them. I look areas where the usual high speed wind pattern breaks down and is replaced by whirls & eddys. I marked them with red arrows.

1537375782247.png
 
Swerd

Swerd

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#18
That graphic I pasted in above must be very big. I couldn't edit it so I continued my text here…

The high speed jet stream winds, the roughly parallel lines running west to east in the diagram, are pretty typical for North America. They drive the weather patterns in a general pattern from west to east. When those wind patterns vary is when something difficult to predict happens. Instead of parallel lines, look for whirls or eddies in the jet stream winds. On the diagram, there are two that I marked with red arrows.

Those whirls & eddies can be signs of trouble, but it also depends what weather prevails on the ground. When Hurricane Florence was approaching NC, there were standing whirls & eddies in the jet stream over the southeastern US. The hurricane stayed put over NC for all that time because the high pressure zones to the north and west prevented it from moving on quickly. And those high pressure zones weren't moving because the jet stream wasn't pushing them along.

I enjoy getting geeky about weather :cool:.
 
D

Drunkpenguin

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#19
Do we really need a better system tho? I've never lived thru a hurricane or had to evacuate due to one, but watching tv from afar and it seems pretty obvious to me that if one comes, you leave. Doesnt matter the wind speed or the predictions or the size of it. All of them cause floods. Why don't people who live in those areas see it the same way?
 
KEW

KEW

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#20
Do we really need a better system tho? I've never lived thru a hurricane or had to evacuate due to one, but watching tv from afar and it seems pretty obvious to me that if one comes, you leave. Doesnt matter the wind speed or the predictions or the size of it. All of them cause floods. Why don't people who live in those areas see it the same way?
Trust me, anyone living in Cape Hatteras as out of there (or a complete idiot).
However, there are no well marked boundaries around the periphery of the "danger zone" and that is where a good method of evaluating how dangerous a storm will be becomes part of a complex decision.
We used to have a pace on Edisto Island and it was up on stilts (like a real beach house). The one hurricane that hit there pretty hard took out 3 homes that had an enclosed lower level. I would never try to ride out a hurricane because those stilts (essentially telephone poles) flex quite a bit when the wind hits hard.
However if you lived in-land in a home on a decent hill with some elevation you are more worried about wind (and trees) than rain and storm surge, so knowing both the wind and the flood potential helps assess whether to stay or go.
It sucks for the friends and family of the 32 that have so far died, but I think on a relative scale that is a pretty low death toll given the magnitude, so most people in the wrong place did a good job of recognizing where the wrong place was (or the authorities did a good job of telling them?).
 

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