Windspeed calculations, Mets/exp trackers please chime in.

[Pebbles]

There is a discussion going on in regards to how windspeed is calculated in relation to the ground. As it was off topic of the previous thread, and I thought this was an important discussion, would like to start a specific thread. Is the forward motion of a hurricane figured into the windspeeds of a hurricane/tropical storm? Please back up your answer with an explanation. It seems this answer has been given both ways and there is quite a bit of confusion on this matter.

Here's a copy of some beginning discussion:

They figure windspeed without forward motion included in it ...she's definitely cat 4...

Nope, had a long discussion with a professional met about this during Dennis. The windspeed reported in the public updates is the actual (if measured) or estimated (if not measured) windspeed that is on the ground, which means inclusive of forward speed winds and cyclonic winds. They even mention in the NHC discussion that the highest winds (125 kt) were in the NE quadrent.

What this means is you DO NOT need to ADD forward speed to the public advisory in order to get actual wind speed on the ground, but that also means you would need to SUBTRACT forward speed in order to get cyclonic wind speed, which, IMHO, is indicative of the actual storm strength. This explains why Mobile, AL got such light effects from Dennis as compared to Panama City Beach - Mobile had winds of less than 80 (120 reported at Navarre Beach minus 20 for forward speed to get cyclonic wind speed, and minus another 20 for being on the west side of the storm where forward speed and cyclonic winds are in opposite directions. )

Coincidently, I was using this same idea in the opposite direction to forecast Dennis being Cat 4 about 1 hour before it went public. My reasoning: Max flight level winds were reported by RECON at 105 kt in the SW quadrent (about 110 mph on the ground,) and Dennis was moving forward at 15. Because this is the quadrent where forward speed and cyclonic speed are opposite, you need to add twice the forward speed in order to get ground windspeed in the NE quadrent. Do this and you get a flight level windspeed of 135 KT, or a 145 mph Cat 4 on the ground. Sure enough, on the next public advisory (Advisory 24,) windspeed was adjusted up to the exact speed my math had predicted, using the same RECON data the NHC uses.


See, the math backs me up every time, and I stand my my original assertion: Emily is a fast moving Cat 3 with Cat 4 winds in the NE quad.

[Huckster]

This is confusing to me. Let me see if I understand what this guy is saying. When the advisory is given, the windspeed they list as sustained incorporates the forward motion of the storm? So a cat. 1, 75 mph hurricane moving at 15 mph is really a 60 mph tropical storm? Somehow that doesn't seem to make much sense to me. I don't think the atmosphere is that rigid. It's always been my perception and belief that a hurricane's central pressure and the environmental pressure determined it's windspeed primarily. Using that forward motion theory, we could make the 1938 hurricane a tropical storm if we just made its movement stationary. Am I following this reasoning correctly?

[NC George]

I'm here. And yes, what you said is what I'm saying.

I think it's important to note the distinction between two hypothetical storms, both reported as Cat 4 with 145 winds, both with the same diameter eye and overall diameter: Storm 1 is a Cat 4 145 mph storm moving a 5 mph, and Storm 2 is a cat 4 145 mph storm moving at 25 mph (Storm 2.) Storm 1 will have much worse effects on the area it hits, and the damage will be more widespread. People need to know this difference.

Some of the differences off the top of my head: Windspeed on the left side of Storm 1 will be 135 mph, and for a long period of time. Windspeed on the left side of Storm 2 will be 95 mph, and for 1/5 the time. Storm surge on Storm 1 will be higher, as high seas take both fetch and time to develop (fetch is distance the wind is blowing over the water.) A slow moving storm will have more time to develop the high seas, and more time pushing water against the shore, while the fast moving storm will not.

Another result of this: What we characterize as a 'Cat 4' on satellite (as in oh, it looks like a Cat 4) may in fact be a Cat 3 cyclonically, which would explain why on slow moving storms we are always stating that the storm 'looks like' a Cat 4 even though it is a Cat 3. This also explains why some fast moving Cat 1's look so poor (they are TS's with added windspeed due to forward momentum) and some slow moving Cat 1's look much more organized (they have true cyclonic winds of hurricane strength.)

Again to recap, the windspeed reported includes both the forward component and the cyclonic component.

[NC George]

Using that forward motion theory, we could make the 1938 hurricane a tropical storm if we just made its movement stationary. Am I following this reasoning correctly?

No, it would still be a Cat 2 with 90-100 mph winds, forward speed was around 40-50 mph at landfall winds of up to 150 reported on the right side, NYC, on the left side, had max winds of 60 at ground level (higher on top of the Empire State Building.)

[Wthrman13]

I'm here. And yes, what you said is what I'm saying.

I think it's important to note the distinction between two hypothetical storms, both reported as Cat 4 with 145 winds, both with the same diameter eye and overall diameter: Storm 1 is a Cat 4 145 mph storm moving a 5 mph, and Storm 2 is a cat 4 145 mph storm moving at 25 mph (Storm 2.) Storm 1 will have much worse effects on the area it hits, and the damage will be more widespread. People need to know this difference.

Some of the differences off the top of my head: Windspeed on the left side of Storm 1 will be 135 mph, and for a long period of time. Windspeed on the left side of Storm 2 will be 95 mph, and for 1/5 the time. Storm surge on Storm 1 will be higher, as high seas take both fetch and time to develop (fetch is distance the wind is blowing over the water.) A slow moving storm will have more time to develop the high seas, and more time pushing water against the shore, while the fast moving storm will not.

Another result of this: What we characterize as a 'Cat 4' on satellite (as in oh, it looks like a Cat 4) may in fact be a Cat 3 cyclonically, which would explain why on slow moving storms we are always stating that the storm 'looks like' a Cat 4 even though it is a Cat 3. This also explains why some fast moving Cat 1's look so poor (they are TS's with added windspeed due to forward momentum) and some slow moving Cat 1's look much more organized (they have true cyclonic winds of hurricane strength.)

Again to recap, the windspeed reported includes both the forward component and the cyclonic component.

Essentially correct. You can indeed envision cases where a storm would be a different category altogether if it were moving faster or slower, and you are correct when you say that for storm organization at least, it's what you call the "cyclonic" component of the winds that is more important. Another term for it is "storm-relative winds", that is winds relative to the storm's forward motion. Actually, whenever you talk about wind speeds, it's always, either implicitly or explicitly, relative to some reference frame. Most of the time, when we talk about winds in meteorology, we are implicitly taking the reference frame to be the earth itself. However, sometimes it is useful to talk about storm-relative winds, or other storm-relative quantities. In the case of tropical cyclones, it matters as far as storm organization goes, but what matters to us folks on the ground is obviously the ground-relative winds, so that is what is reported, and that is what is used to determine the category of the storm. There are tradeoffs to both ways (ground-relative vs. storm-relative) of looking at things. The trick is to keep them consistent in your own mind and when discussing them.

[wxman57]

Winds that are reported/measured are inclusive of storm motion, as some have mentioned. Therefore, a hurricane moving westward at 20 mph with 105 mph winds in the northern semicircle has just that - 105 mph max winds. You do not add the forward speed into the wind measurement. However, on the south side, winds will most likely be less due to the quick westerly motion of the storm.

[caneflyer]

Keep in mind that tropical cyclones move with a deep-layer mean environemntal wind, not with the surface environmental wind. So this notion of subtracting twice the motion to get the winds on the left hand side would only be valid if there were no vertical shear. And even then, there are other things going in a tropical cyclone, particularly at landfall, that distort this simple asymmetry.