Rita Downgraded?

[Derek Ortt]

wind does start to take over in the cat 4 range.

had katrina have hit as a 4 or a 5, the wind would have been noticible, even with the surge. The last 3 4's and 5's to hit the USA have been nearly 100% wind events... and the effected areas were leveled

[MiamiensisWx]

Andrew would be a good example of that, Ortt.

[JtSmarts]

Andrew would be a good example of that, Ortt.

Charley too

[wxman57]

Question. So an area that is affected with surge in a CAT 5 surge area , according to a MEOW map, from a CAT 3 storm, is still only considered a CAT 3 surge?

The Saffir-Simpson scale deals with maximum 1-minute winds only. There is no such thing as a Cat 3 surge or a Cat 5 surge. Therefore, a Cat 3 hurricane can produce nothing other than a "Cat 3 surge", a "Cat 3 surge" being a surge produced by a Category 3 hurricane. There are a number of factors that affect surge size, as I've discussed above. The maximum 1-minute wind in a small section of a hurricane is a minor factor as far as surge production is concerned.

Stop thinking of storm surge as a function of peak intensity. Sure, that's a small part of what produces storm surge, but more important is the areal extent of the higher wind radii (all else being equal).

[wxmann_91]

wind does start to take over in the cat 4 range.

had katrina have hit as a 4 or a 5, the wind would have been noticible, even with the surge. The last 3 4's and 5's to hit the USA have been nearly 100% wind events... and the effected areas were leveled

Yes but in a Cat 5 storm making landfall most of the Cat 5 winds will be over water, and legit, bonifide, and sufficiently long-lasting Cat 4 winds will be only felt in isolated areas. Surge OTOH can reach areas well to the east of the center, especially in large canes. So in those isolated areas yes wind damage takes over, but it really is the surge that has the most impact overall.

[Derek Ortt]

even so, the winds overland will be at least at F-2 onshore, whic will devastate almost anything

[wxman57]

Of course, much depends on the horizontal structure of the Cat 5 wind field. How big were Andrew's Cat 5 winds? I believe they were only in a part of one quadrant of a relatively small hurricane. But Andrew did have a rather intense ring of 100kt winds in all quadrants that did penetrate a ways inland. 100kt winds with gusts 20-40 kts above that could completely destroy homes, as was evident in the post-storm photos. HEre's a shot of Andrew's wind field at landfall. I think it's a pre Cat-5 upgrade analysis, but any Cat 5 wind area must have been quite small.

http://www.aoml.noaa.gov/hrd/Storm_page ... dfall.html

[HurricaneBill]

The NHC should also put more emphasis on the extreme damage Category One and Category Two sustained winds can do.

Especially the ones that move at very slow speeds. (Frances, Katrina, etc.)

[MGC]

I was in Charleston SC shortly after Hugo with the Navy. Charleston city proper reminded me a lot of the wind damage in Eastern New Orleans. Across the river where the RFQ of Hugo Cat-4 winds were felt I observed considerable wind damage. The area was not leveled though. After observing Hugo's Cat-4 wind damage, I have no trouble conceeding that Katrina was a Cat-3 in Mississippi. Wind damage with Hugo was worst than Katrina, but not by much......MGC

[Derek Ortt]

Hugo was probably a cat 5 at landfall

The flight level winds were 141KT. Normally, this would support 125KT at the surface. However these FL winds were at about 625mb, no 700mb. At 625, the FL reduction is basically non-existant. The surface winds could have been closer to 140KT in Hugo

[brunota2003]

and another thing for me...how do you know that that house got destoryed by surge? just because water was there? has anyone stopped to think that maybe the house was destoryed by the winds first, then covered with water? or the winds severely damaged the house and the water finished it off...

[wxman57]

and another thing for me...how do you know that that house got destoryed by surge? just because water was there? has anyone stopped to think that maybe the house was destoryed by the winds first, then covered with water? or the winds severely damaged the house and the water finished it off...

That's a good question. Just watching the storm chasers' videos of landfall, you could see many buildings/structures had roof damage before the surge came ashore. The homes could have had water coming in from roof damage before the surge hit, then the homes were ripped apart by the surge. Would have been easier for the surge to destroy a house if it already had some structural damage.

[Extremeweatherguy]

There seems to be a sharp edge though between the completely washed away surge structures and the still fairly intact homes. I think the homes behind the surge line can show you what kind of damage the homes in the surge would have recieved.

[Scorpion]

Hugo was probably a cat 5 at landfall

The flight level winds were 141KT. Normally, this would support 125KT at the surface. However these FL winds were at about 625mb, no 700mb. At 625, the FL reduction is basically non-existant. The surface winds could have been closer to 140KT in Hugo

I wouldnt be surprised, but still the pressure was quite high at 934. Also it was a very large hurricane. Maybe other factors caused such high winds?

[Derek Ortt]

Scorpion,

stop looking at the pressure to determine the winds

The storm was moving fast; thus, you have a very strong ridge steering it, plus the translational speed has to be added to the winds.

The 141KT was at about 625mb, the flight level they were using at the time of landfall. The reduction is basically non-existant at this level

[HurricaneBill]

Derek can there be occurances where a hurricane's surface winds are stronger than the flight level winds? I'm just curious.

[Derek Ortt]

if the FL is 625, the surface winds should be at least as strong as the winds at FL

It is not uncommon for the 700mb FL winds also to be weaker than at the surface. When Wilma hit Cozemul... dropsondes were near cat 5, but the FL winds were in the mid cat 3 range. The 700mb range is .6 to 1.2 (.9 is just the mean)

Depends upon the vertical momentum transport

[dhweather]

Also, this goes back to the argument that the NHC needs to have a seperate SURGE WARNING for hurricanes.

A storm coming down from a 5 to a 2/3 will still have the surge of a 4/5 at landfall in the GOM.

A storm coming up to a 2 from a TS or 1 will have the surge of a TS or 1, with small areas of 2 surge.

I disagree with the logic above. As per my previous post, storm surge is more a function of the areal coverage of a hurricane's higher wind speeds (74+ mph, for example) than a function of the peak wind that may cover only a limited number of square miles (based on SS scale). Here's an example.

Let's say we have a hurricane with hurricane-force winds extending out 100 miles in all quadrants. That's an areal coverage of 31, 400 square miles. Now let's assume it's a Cat 4 with 140 mph winds only in the NE Quadrant at a distance of about 20 miles from the center. That's a max areal coverage of Cat 4 winds of probably 200 square miles at the most (that may be generous). Now, drop the wind speed in that 200 sq. mile area from 140 to 120 mph and you're not going to significantly affect the storm surge that extends out to 100 miles or more from the center. Such a reduction of ONLY the peak wind speed (all else being equal) may have an insignificant effect on the storm surge of a landfalling hurricane.

That's basically what happened last season with Katrina and Rita. The 74+ mph wind field remained constant but the small area of peak winds northeast of the core weakened a few categories. Therefore, Katrina and Rita produced a storm surge that was greater than what the SS scale would indicate for an average-sized Category 2 or 3 hurricane.

But you can't say that Rita or Katrina produced a Cat 4 or Cat 5 surge, as there's no such thing. Katrina was a Cat 3, so it produced a surge reprensentative of what a much larger-than-average Category 3 hurricane would normally produce. It's just that the SS scale doesn't consider such abnormalities.

So, in the same line of thinking, you can't arbitrarily state that a Cat 2 which weakens to a strong TS at landfall would produce a "Cat 1 surge" or that a Cat 5 that weakens to a Cat 2/3 will have a "Cat 4/5 surge". That's simply not true under all circumstances. It MAY be true that if only the peak wind is reduced (not the areal coverage of 74+ mph winds) then a Cat 5 that weakens to a Cat 2/3 at landfall would produce a storm surge that would be much larger than that expected from a typical Cat 2/3.

On the other hand, if Katrina had not only weakened from a Cat 5 to a Cat 3, but had its hurricane-force and greater wind field areal coverage decreased by half or more, then Katrina would not have produced a storm surge nearly as extensive as it did. A further reduction of the area of Katrina's higher wind radii prior to landfall may have resulted in a storm surge more typically associated with a landfalling Category 3 hurricane.

I still have to look at what history tells me:

Here's a list of storms that were cat 5's in the Gulf of Mexico, and their surge at landfall:

2005: Rita "At least 15 feet" (Cameron Parish, LA)
2005: Katrina 30+ Feet (Bay St. Louis, MS)
2004: Ivan 12 feet (Navarre Beach, FL)
1969: Camille 27 Feet (Bay St. Louis, MS)
1967: Beulah "At least 18 feet" (30MI S Corpus Christi)
1961: Carla 18.5 feet (Port Lavaca, TX)

In the last 45 years, hurricanes that have been category 5 strength
in the Gulf of Mexico, all but Ivan produced category 5 surge.


There are many things that contribute to the storm surge, however, I
believe that once a hurricane reaches category 5 strength in the
GOM, the overwhelming odds are that it will produce category 5
storm surge at landfall, regardless of its SS intensity.

The SS scale has a category 5 hurricane storm surge listed as:

Storm surge generally greater than 18 ft above normal.

[MiamiensisWx]

Also, Ivan reached Category Five status in the Caribbean, not the Gulf of Mexico.

[Scorpion]

Ivan was briefly of Category 5 intensity in the GOM.