Rita Downgraded?

[Stormcenter]

The following is a paragraph from my post Katrina Report detailing surge effects. You can apply it to Rita as well.

On a personal note, I have been 3 times to SW LA and fully agree cat 1-2 winds were the norm. The surge was impressive however.



Katrina made landfall as a category 3 hurricane yet produced a "category 5" storm surge. Why? The Saffir Simpson scale is based on sustained wind speeds with an indirect correlation with respect to storm surge. Storm surge inundation forecast are compiled from a number of factors and each hurricane landfall is unique. The factors that control storm surge include: the size of the wind field, the intensity of the storm, the duration of the event, the angle of approach to the coast, the depth of the offshore coastal shelf, concavity of the coastline, and special conditions such as storm surge run-up into smaller bays and rivers.

Katrina’s large wind field and category 5 intensity while over the open Gulf of Mexico created a large water level bulge that arrived as a catastrophic storm surge when the hurricane made landfall. The rapid sustained wind speed weakening a few hours before landfall and reduction in Saffir Simpson category did not represent the impending storm surge effectively to the public although preparations were made for a category 5 event. The concavity of the coastal area impacted, the size of the wind field, and the built up water level bulge when Katrina was a category 5 over the open Gulf of Mexico is why Katrina produced such an extreme surge.

Education of the public should be addressed to inform persons in the inundation zones of the different factors involved in storm surge forecast and the potential failures of the Saffir Simpson scale to appropriately predict storm surge levels. Prior to a hurricane landfall, residents should base evacuation and preparation decisions on local NWS forecast of storm surge inundation using computer modeling and experience and not the general Saffir Simpson scale values assigned per category. Large rapidly weakening hurricanes are still likely to bring a large and destructive surge ashore. Additionally, residents at the heads of bays, the mouths of rivers and creeks, and small inlets can receive a substantially higher surge due to water level run-up into an increasingly smaller area. Such affects are unique to a certain coastal location and usually depend heavily on the exact track (ie. MRGO and IHNC surge effects due to track).

Great post, thank you!!!

[wxman57]

I hear what you mets are saying but what about some of the other storms that hit the coast.Elena came in as a Cat 3 but only produced 6'-8' surge at Dauphin Island.If the the surge is a by product of the size and strength of the wind field shouldn't some areas seen a larger surge but a smaller area?I am just having a problem with seeing the energy displacement of the once potential higher Cat storm dissappearing so quickly.Where did it go?

The Perfect Storm generated some serious swells quickly in the N.Atlantic were they not wind generated in some manner?

Oh by the way you might have a classroom of idiots here with me at the top of the list

As I mentioned in some of my posts, many factors go into storm surge calculation. One of those factors is "angle of incidence". That is, the angle at which the track intersects the coastline. Greatest surge occurs when this angle is 90 degrees (i.e., Katrina). Elena moved inland at an oblique angle to the WNW. It's right front quadrant winds were blowing offshore. This low angle of incidence significantly reduced Elena's storm surge along the MS/AL coast.

As for the "Perfect Storm", all swells are wind-generated. But I don't understand your point. A hurricane's storm surge is not the same as its wind-generated waves.

[wxman57]

...snip...
Katrina’s large wind field and category 5 intensity while over the open Gulf of Mexico created a large water level bulge that arrived as a catastrophic storm surge when the hurricane made landfall. The rapid sustained wind speed weakening a few hours before landfall and reduction in Saffir Simpson category did not represent the impending storm surge effectively to the public although preparations were made for a category 5 event. The concavity of the coastal area impacted, the size of the wind field, and the built up water level bulge when Katrina was a category 5 over the open Gulf of Mexico is why Katrina produced such an extreme surge. ... snip...

Jeff,

I have to disagree with the paragraph above. I've studied Katrina's wind field quite thoroughly. The radii of 60-100 mph winds did not change significantly when the small area of Cat 5 winds near its center weakened to a Cat 3. So I think that it's incorrect to say that Katrina built up a Cat 5 surge well offshore and that surge lasted all the way to the coast.

I propose that had Katrina never been a Cat 5 but that its wind radii had been the same at landfall as 500 miles offshore (with a Cat 3 core), that its storm surge would still have topped 30 feet. It's the great expanse of 60-100 mph winds that produced the surge at landfall, and that expanse of higher winds did not change from when Katrina was a Cat 5. The areal coverage of Katrina's 75 and 100 mph winds at landfall were in the top 10 percentile of all hurricanes from 1988 to present. Combine that with a 90-degree angle hit at about the worst place along the U.S. coast and you have the recipe for a major disaster.

Bottom line, it makes no difference that Katrina weakened from a 5 to a 3 prior to landfall with respect to the size of its storm surge, as the general wind field remained the same minus that small area of higher Cat 5 winds.

Here, take a look at the HRD analysis of Katrina's winds from Sunday-Monday. Note that the areal coverage of 70+ kt winds actually expanded as Katrina weakened. That would have the effect of increasing the expanse of the storm surge.

http://www.aoml.noaa.gov/hrd/Storm_page ... all_kt.pdf

[MiamiensisWx]

Good points, wxman57.

[Javlin]

Wxman 57 I was thinking that all the energy derived by the storm would in some way be imparted on the surrounding environment in other means.Whether it becomes truly a function of wind or is imparted in wave action.Thus the stronger the intensity of the system the more energy is dispersed into these other factors.I am just thinking more in the line of the physics involved.

[wxman57]

Wxman 57 I was thinking that all the energy derived by the storm would in some way be imparted on the surrounding environment in other means.Whether it becomes truly a function of wind or is imparted in wave action.Thus the stronger the intensity of the system the more energy is dispersed into these other factors.I am just thinking more in the line of the physics involved.

Javlin,

I see where you stated "I am just having a problem with seeing the energy displacement of the once potential higher Cat storm dissappearing so quickly.Where did it go?"

My contention is that there wasn't a significant drop-off in energy when Katrina went from a 5 to a 3. The fact that Katrina's core wind speeds in a small part of one quadrant dropped by 40-50 mph probably did not represent a significant
reduction in total energy being produced by the hurricane. It was countered by a slightly expanded wind field as the core weakened. So I don't really think that Katrina's surge would have been much higher if it had remained a Cat 5 up to landfall.

[Javlin]

I see what you are saying and understand that much and I hope a surge could not get any higher.Lets just say what if Katrina never in her life got above a Cat 2 with roughly the same wind for the most part.Then would of the surge been different?Energy level is not increasing exponentially viva the force of the wind.There would not be as much transfer of energy to the surface of the ocean in wave action because it was not there to begin with.I understand most of the parameters you have set up and I agree.It's the energy thing there is some kind of correlation in some manner I would think.

[f5]

Katrina's storm surge wiped Waveland,ms off the map.that goes to show you what storm surge can do if it hits just right

[Ixolib]

Bottom line, it makes no difference that Katrina weakened from a 5 to a 3 prior to landfall with respect to the size of its storm surge, as the general wind field remained the same minus that small area of higher Cat 5 winds.

Is it safe to say, then:

A. ...that a cat 4/5 storm with a very small radius/field of hurricane force winds (like Charley) will consistently produce a relatively small surge.

AND

B. ...that a cat 2/3 storm with a very large radius/field of hurricane force winds (like Katrina) will consistently produce a relatively large surge?

Your point - if I'm reading it right - is that it's not the pressure and the wind speed, or the size a storm was 24 hours prior to landfall, but rather the extension or expanse (or lack thereof) of the wind field that tells the story on storm surge???

[MGC]

Katrina pushed a great quanity of water ahead of her due to her large wind field. The water was trapped along the Al/Ms/La coasts due to the topography and shallow shelf. All these factors combined resulted in her surge. Katrina could have weakened to a Cat-1 yet still produced a considerable surge. Tropical Storm Isidore here a few years ago produced a 7 foot surge near my house in Diamondhead. The surge covered the road and I was trapped. Isidore also had a large circulation. Key to large surge IMO is wind field size and intensity, angle of attack and coastal topography......MGC

[f5]

Katrina was the perfect storm in terms of storm surge.thankfully super typhoon size Storms of that size and intenisty are rare in the Atlantic basin

[Derek Ortt]

that would be correct... the size of the wind field plays a larger role on surge than does the intensity.

Example is Gabrielle vs Charley. Gabrielle produced at least as high a aurge as did Charley, and Gabrielle was a borderline TS/cat 1, while Charley was a borderline 4/5

[Pearl River]

IMO storm surge should be removed from the S.S.scale so that is doesn't cause all this mass confusion.

From the NHC report on Katrina.

Overall, Katrina’s very high water levels are attributable to a large Category 3 hurricane’s storm surge being enhanced by waves generated not long before by a Category 5 strength storm.

[f5]

what would the surge had been if everything was CAT 5

[wxman57]

IMO storm surge should be removed from the S.S.scale so that is doesn't cause all this mass confusion.

From the NHC report on Katrina.

Overall, Katrina’s very high water levels are attributable to a large Category 3 hurricane’s storm surge being enhanced by waves generated not long before by a Category 5 strength storm.

That's not too different from what I was saying - a LARGE Cat 3. However, the NHC statement is incorrect concerning the "Cat 5 waves". I've been a marine meteorologist since 1980, so I do know about wave generation. That Cat 5 wind was over such a small area of the ocean that it would not have generated much of an increase in wave heights. Wave height is a function of wind speed, duration, and fetch. Wind speed with no duration or fetch won't build big waves. On the other hand, the large area of 60-100 mph winds had a much longer fetch and duration - so that's where the big waves came from. And such waves were still being generated up to landfall.

[wxman57]

what would the surge had been if everything was CAT 5

f5, you would have to define the scope of Cat 5 winds. As we learned in the last 2 years, no two hurricanes are alike, even with identical wnd speeds. If Katrina had been a Cat 5 but with 160-170 mph winds confined to maybe 10-20 square miles, then the surge wouldn't have been significantly different from what was observed - all other wind radii remaining equal.

BUT, if Katrina had a solid core of 160-170 mph winds out to 30 miles from the center and completely surrounding the eye in all quadrants, it could have produced a higher peak surge near the point of landfall, perhaps another 5-8 feet higher or so. Not to mention, little that surived the surge would be left standing due to the much higher winds.

[f5]

IMO storm surge should be removed from the S.S.scale so that is doesn't cause all this mass confusion.

From the NHC report on Katrina.

Overall, Katrina’s very high water levels are attributable to a large Category 3 hurricane’s storm surge being enhanced by waves generated not long before by a Category 5 strength storm.

That's not too different from what I was saying - a LARGE Cat 3. However, the NHC statement is incorrect concerning the "Cat 5 waves". I've been a marine meteorologist since 1980, so I do know about wave generation. That Cat 5 wind was over such a small area of the ocean that it would not have generated much of an increase in wave heights. Wave height is a function of wind speed, duration, and fetch. Wind speed with no duration or fetch won't build big waves. On the other hand, the large area of 60-100 mph winds had a much longer fetch and duration - so that's where the big waves came from. And such waves were still being generated up to landfall.

the CAT 5 winds were in a small area in the NE eyewall

[Ivanhater]

wasnt ivan's waves the largest waves ever recorded during a hurricane? was it the storm itself or was it the shape of the coastline that caused it?

[Derek Ortt]

those waves from Ivan were in deep water, not near the shelf

[f5]

wasnt ivan's waves the largest waves ever recorded during a hurricane? was it the storm itself or was it the shape of the coastline that caused it?

the same bouy that recorded the 55 ft wave in Katrina was alos the same for Ivan.as far as Katrina remaining a 5 at landfall it she would of she would of went slightly west of NO leveling the city for generations possibly to never be rebuilt again