Re-thinking Storm Surges

[Downdraft]

What we have to rethink now is that once a system makes CAT 4 or 5 in the open Gulf it may keep that surge even though the storm itself weakens prior to landfall. In short, the surge does not dissipate as quickly as the windfields. Katrina certainly did not have a surge corresponding to her windfield. The same with Ivan last year. Both storms produced a similar Tsunami effect not related to weakening in windfields or pressure.
I think all the surge models will need overhaul after this season. Katrina proved that Ivan's storm surge was not a freak. Not sure how this would play out for a system in the open ocean coming at the east coast from the west but it certainly seems the Gulf needs to be looked at seriously now.

[Forecaster Colby]

I think what we saw with Katrina and Ivan were two very powerful storms with large windfields that had time to build up a mega-surge. All that momentum takes a while to wind down, and exceptionally low pressures helped a bit too.

[Downdraft]

What I am thinking about is how we warn the public. Dr. Steve's graphs on storm surge almost cost Jim Cantore his life. I've seen Cantore do a lot of storms but I never saw the look of fear on his face like he had that day in the Veteran's home parking lot. If a professional met with years of storm experience can get caught I can imagine what the general public went through. We can only wonder how many died because they were told to expect less but they actually got much much more.

[Scorpion]

The shallow waters that help weaken NGOM storms also give them bigger surges.

[mike815]

Yes exactly. Thats why on the east coast of fl u need a cat 4/ 5 to produce a larger type surge.

[wxmann_91]

The shallow waters that help weaken NGOM storms also give them bigger surges.

Exactly.

[Recurve]

Good points Downdraft. Unless there's some other explanation, this has to be taken into account. Since as others said the shallow/cooler waters often reduce the intensity of north gulf landfallers, the public needs to be told that diminshing winds might not reduce surge danger.

I hope the common wisdom that Fla E coast is less vulnerable to surge isn't also proven wrong anytime soon.

[f5]

thats when i notice Katrina weaken froma CAT 5 to a CAT 4 was when she hit that shallow water also she didn't start pulling in dry air until she started going due north 50 miles from landfall .That dry air/sunshine would also later kick off strong tornadoes in georgia.i heard Dr.Greg Forbes on TWC say she spawn some unusually strong tornadoes for a landfalling tropical cyclone so everything pretty much work for her advantage

[MGC]

Both Ivan and Katrina were producing 50+ foot waves on the open GOM only a few hundred miles off the coast. Waves in open water propagate well with little resistance. Once these waves reach the shallower waters they are trapped by land and the water level can only go up. Considering the shape of the coast line is also an important factor in surge potential as the water can not escape by running along the coast. Where have the two largest surges been recorded in America from hurricanes? Mississippi......MGC

[thunderchief]

I think all the surge models will need overhaul after this season.

I disagree. The NHC surge forecast for katrina was 28 feet... which seems to be only a few feet off of the 30+.

[f5]

what does locally 28 feet mean?

[senorpepr]

what does locally 28 feet mean?

I can't find an official definition on "locally" at this time, but essentially it means isolated. Basically, if you hear, "storm surge heights of 18-20 feet, locally as high as 30 feet," that means you'll typically see 18-20 feet, but in some locations where specific microscale features can "boost" surge heights, 30 feet would be expected.

[wxmann_91]

I think all the surge models will need overhaul after this season.

I disagree. The NHC surge forecast for katrina was 28 feet... which seems to be only a few feet off of the 30+.

Maybe, but after Katrina weakened to a Cat 4, the NHC lowered the surge forecast, perhaps giving residents along the MGC a false sense of security.

However, the earlier statement of "all the surge models will need overhaul" is false. In fact, I believe a pro met on here posted a graphic of the SLOSH model forecasting a 30 ft surge for a Katrina-sized Cat 4 hitting at the same angle, at the same place.