question about Dennis 2005

[MGC]

Had Ivan made landfall in the same area Katrina did I believe the surge from Ivan would have been similar to Katrina. The slope of the shelf and the lay of the land play a big roll in storm surge. Along the western Florida panhandle, deep water is a short distance off shore combined with a relativity east-west beach which allows water to flow along the coast away from the hurricane. Dennis produce a large surge in Apalachee Bay a considerable distance from landfall. Water was trapped by the concave shape of the coastline and the slope of the shelf also played a roll in the surge there. Since the top two surge events in the USA have occurred along the Mississippi Coast at Pass Christian due to the same factor that allow a large surge to occurred in Apalachee Bay. Water can not flow westward because it is trapped by the Louisiana coast. The shelf slope is very shallow which forces all the water to pile up as it reaches the coast......MGC

[Derek Ortt]

MS is very surge prone, only NYC is more surge prone.

This is not to say that areas that are less surge prone need not prepare. Ivan leveled Pensacola Beach due to the very high waves that crashed onshore. Usually, if an area is not surge prone, it is wave prone (Miami has that problem, as does Hawaii)

[Vlad]

We still have a discrepency between sources attributing 3' to 30' to pressure-doming that I'd like to see examined.

[f5]

It is interesting how you can look at Katrina and Dennis to see the effects of size of windfield on surge. Dennis was a strong CAt 4 just prior to landfall but had a small area of strong winds relative to the area of strong winds that Katrina had. I know I was certainly expecting to see higher surge values with Dennis than what actually occurred. Dennis surge was significantly smaller than Katrina and Ivan also despite high winds.

docjoe

not only was Katrina bigger but she was a strong CAT 5 vs Dennis which was a strong CAT 4

[wxman57]

It is interesting how you can look at Katrina and Dennis to see the effects of size of windfield on surge. Dennis was a strong CAt 4 just prior to landfall but had a small area of strong winds relative to the area of strong winds that Katrina had. I know I was certainly expecting to see higher surge values with Dennis than what actually occurred. Dennis surge was significantly smaller than Katrina and Ivan also despite high winds.

docjoe

The areal extent of the stronger winds (not necessarily Cat 4-5 winds, but cat 1-3 winds) is extremely important in surge production. But as important as wind field size is offshore topography. Coastal areas with shallow water offshore are much more surge-prone than areas with deep water offshore.

In the image below, look where Dennis hit. The shoaling factor was only 0.6 vs. 1.75 where Katrina hit. That means a storm which would produce 10 feet of surge in Pensacola would produce a 30 foot surge on the MS Coast. It's all about wind field size and location of impact. SS rating doesn't matter so much.

[docjoe]

It is interesting how you can look at Katrina and Dennis to see the effects of size of windfield on surge. Dennis was a strong CAt 4 just prior to landfall but had a small area of strong winds relative to the area of strong winds that Katrina had. I know I was certainly expecting to see higher surge values with Dennis than what actually occurred. Dennis surge was significantly smaller than Katrina and Ivan also despite high winds.

docjoe

The areal extent of the stronger winds (not necessarily Cat 4-5 winds, but cat 1-3 winds) is extremely important in surge production. But as important as wind field size is offshore topography. Coastal areas with shallow water offshore are much more surge-prone than areas with deep water offshore.

In the image below, look where Dennis hit. The shoaling factor was only 0.6 vs. 1.75 where Katrina hit. That means a storm which would produce 10 feet of surge in Pensacola would produce a 30 foot surge on the MS Coast. It's all about wind field size and location of impact. SS rating doesn't matter so much.

so does this mean Ivan would have produced 30-45 feet of surge if it had made landfall where Katrina did? Looking at that graph clears up alot. Thanks

docjoe

[wxman57]

so does this mean Ivan would have produced 30-45 feet of surge if it had made landfall where Katrina did? Looking at that graph clears up alot. Thanks

docjoe

Perhaps the 10-15 ft for Ivan was a bit generous (as far as the 15 ft surge). However, Ivan's wind field was quite comparable to Katrina's prior to landfall. Had it made landfall in the same area as Katrina (and on the same track) then it's likely that Ivan would have produced a 25-30 ft surge along the MS Coast.

However, Ivan hit Miami, the surge may have been only 5-8 ft.

That's why you can't use the SS scale as a storm surge reference. A storm that produces a 5 ft surge in one area could produce 5-6 times that surge in another area.

[docjoe]

Ivan reportedly put 15-18 feet of surge in Escambia Bay just north of the I-10 Bridge in Mackey Cove and Floridatown. I have seen video of a house on the bay built 12-14 feet above the water with water coming in the house. Blackwater Bay and East Bay also had areas of surge approaching 15 feet.

I am assuming that the previous graph holds true for the beaches and not the inland bays? If so is there a similar product for inland waters??

docjoe

[Derek Ortt]

I would think Bay's would be higher.

Also, one must remember that areas that are less surge prone, are far more wave prone. The waves on the coast in Ivan may have been 2-3 times those in Katrina. For example, lets take the last cat 4 to hit the USA before Charley, Iniki. The peak surge at Kauai was only 6 feet; however, there were 35 foot waves crashing onshore. Those waves are very destructive and can put water into a house 10-15 feet above sea level quite efficiently. This may explain the high water in Pensacola during Ivan

[MGC]

In theory Ivan should have produced greater wave height on the beach than Katrina did. However, the water damage caused by both static surge and waves imposed on the surge were far greater along the Mississippi coast in Katrina than along the Alabama and Florida coasts with Ivan. Katrina took everything except concrete reinforced steel building that were right on the coast. Along Pensacola Beach and Perdido Key many wood framed structures survived (but were damaged beyond salvage). I can offer no explanation as to the difference in the degree of damage between the two storms. For some reason Katrina knocked down nearly everything in its path and Ivan didn't.......MGC

[docjoe]

I would think Bay's would be higher.

Also, one must remember that areas that are less surge prone, are far more wave prone. The waves on the coast in Ivan may have been 2-3 times those in Katrina. For example, lets take the last cat 4 to hit the USA before Charley, Iniki. The peak surge at Kauai was only 6 feet; however, there were 35 foot waves crashing onshore. Those waves are very destructive and can put water into a house 10-15 feet above sea level quite efficiently. This may explain the high water in Pensacola during Ivan

There were certainly waves but this appeared to be actual water level. There was a foot or two of water in his house. As waved passed under the house you could actually see the floor lift up and even the furniture would lift up. Small pieces of debris were floating in the house. This occurred in Floridatown....a mile or two north of the I-10 bridge..I wish I could provide more details on it but it did seem that water level there was approaching 15 feet without the waves. There was debris in the trees across the street 15-20 feet above the ground. I suspect the extra high water in this area was due to the bay coming to a "point" and probably discharge from numerous rivers in the bay in this same area

docjoe

[docjoe]

In theory Ivan should have produced greater wave height on the beach than Katrina did. However, the water damage caused by both static surge and waves imposed on the surge were far greater along the Mississippi coast in Katrina than along the Alabama and Florida coasts with Ivan. Katrina took everything except concrete reinforced steel building that were right on the coast. Along Pensacola Beach and Perdido Key many wood framed structures survived (but were damaged beyond salvage). I can offer no explanation as to the difference in the degree of damage between the two storms. For some reason Katrina knocked down nearly everything in its path and Ivan didn't.......MGC

Could the difference in the speed of the rise of the water account for the difference. Slowly rising flood waters around a river can destroy houses but often leave the main structure intact. However a flash flood levels everything in the path. Maybe the surge with Katrina was much "faster" than Ivan....just a guess

docjoe

[f5]

Ivan nver had 175 mph sustained winds in the gulf like Katrina had even though they were the same size .Ivan was mostly a CAT 4 in the gulf much like Dennis was

[wxman57]

Ivan nver had 175 mph sustained winds in the gulf like Katrina had even though they were the same size .Ivan was mostly a CAT 4 in the gulf much like Dennis was

A wind of 175 mph over a very tiny area makes just about zero difference as far as storm surge. It's the areal coverage of the higher winds that generates the volume of water moving into the coast. Ivan's wind field, in fact, was a little larger than Katrina's prior to landfall. The main difference was where Ivan hit - an area much less prone to storm surge due to deep water offshore.

[docjoe]

the fact that it is the intense hurricanes that struggle in the NGOM makes me wonder if there is a very low true MPI for that area. Katrina should not have went into full collapse mode in the final hours before landfall as unlike Lili, it was a well-organized and large storm, that would have resisted some of the atmospheric impacts.

Fortunately, it did weaken as much as it did (probably saved the center of New Orleans, as had it not weakened, cat 2 or cat 3 winds would have been likely in NO instead of the cat 1 that occurred). Unfortunately, it was so intense in the GOM

Two storms that spared NO that probably would not have weakened were Andrew and Ivan., Those two almost certainl;y would have struck as cat 4's

Bump..just wanted to see if there was any additional info regarding tendency of storms to weaken in the NGOM since this thread was active. Also just curious about the above statement that Ivan and Andrew almost certainly would have struck as Cat 4 in the NOLA area....i am confused as to why?? (course my complete lack of meteorological training may explain that)

docjoe

[miamicanes177]

I would like to know how in the heck katrina went from 175mph to a cat 3 like it did when it was in a perfect environment.

[harmclan]

I believe it went through an eye wall replacement cycle prior to landfall. Also if you remember a large cluster of storms were located over Texas prior to landfall, which created dry air in front that was entrained into katrina.

[f5]

I believe it went through an eye wall replacement cycle prior to landfall. Also if you remember a large cluster of storms were located over Texas prior to landfall, which created dry air in front that was entrained into katrina.

what if those Thunderstorms haven't formed would Katrina came in as a 5 instead of as a 3?

[CrazyC83]

It's interesting about all those close calls. But what about the ones that strengthen before landfall?

I know it's the weaker storms that tend to strengthen rather then fall apart before landfall in the Gulf.

The best example in recent years would probably be Claudette in 2003. Claudette was badly sheared in the Gulf, but once conditions became favorable, she suddenly came together. She looked very impressive at landfall.

I read somewhere that no evacuations were ordered prior to Claudette and this resulted in many people having to ride out the storm in their homes.

I can imagine a Claudette-like scenario is one meteorologists dread.

How about a scenario of explosive intensification like Wilma in the Caribbean - just before landfall?

[Ptarmigan]

I believe it went through an eye wall replacement cycle prior to landfall. Also if you remember a large cluster of storms were located over Texas prior to landfall, which created dry air in front that was entrained into katrina.

I remember those storms. It rained all night that time. It came a from a trough northeast of Houston. These storms came before Katrina became a Cat. 5.