Hurricane Isaac hits Louisiana, driving dangerous storm surges

By: Dr. Jeff Masters , 2:27 AM GMT on August 29, 2012

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Hurricane Isaac is ashore over Southeast Louisiana, having officially crossed the coast on the Mississippi Delta 90 miles southeast of New Orleans at 7:45 am EDT on August 28. Isaac intensified right up until landfall, striking with 80 mph winds and a central pressure of 970 mb. The storm's large size and large 50 - 60 mile diameter eye kept the intensification rate slow today, but it came quite close to becoming a significantly more dangerous storm. That's because at landfall, Isaac was in the midst of establishing a small inner eyewall within its large 50-mile diameter eye, a very rare feat I've never seen before. Usually, when an eye first forms, it gradually contracts, eventually becoming so small that it becomes unstable. An outer concentric eyewall then forms around the small inner eyewall, eventually becoming the only eyewall when the inner eyewall collapses. But Isaac is a very unusual storm that has continually surprised us, and this inside-out concentric eyewall formation fits the storm's unusual character. The storm isn't in a hurry to move fully inland, and has slowed down to a crawl this evening. This will give the storm the opportunity to keep its center mostly over water a few more hours, and maintain hurricane strength into the early morning on Wednesday.


Figure 1. Radar reflectivity image from New Orleans as Isaac made landfall at 6 pm CDT August 28, 2012.

A dangerous storm surge event underway
Isaac is bringing large and dangerous storm surge to the coast from Central Louisiana to the Panhandle of Florida. At 10 pm EDT, here were some of the storm surge values being recorded at NOAA tide gauges:

6.2' Waveland, MS
9.9' Shell Beach, LA
3.0' Pensacola, FL
4.4' Pascagoula, MS
3.4' Mobile, AL

The 9.9' storm surge at Shell Beach, which is in Lake Borgne 20 miles southeast of New Orleans, exceeds the 9.5' surge recorded there during Category 2 Hurricane Gustav of 2008. Research scientists running a Doppler on Wheels radar located on top of the 16' levees in Plaquemines Parish near Port Sulphur, LA, reported at 8:30 pm EDT that a storm surge of 14' moved up the Mississippi River, and was just 2' below the levees. Waves on top of the surge were cresting over the west side of the levee. Needless to say, they were very nervous. Over the past hour, the surge has retreated some, and waves were no longer lapping over the top of the levee. This is probably due to the fact that we're headed towards low tide. A storm surge of 9.5' has moved up the Mississippi River to the Carrrollton gauge in New Orleans. This is not a concern for the levees in New Orleans, since the storm surge has now brought the river up to 2.5' above its normal water level, which was 7' low due to the 2012 U.S. drought. The highest rise of the water above ground level will occur Wednesday morning over much of Southeast Louisiana, Mississippi, Alabama, and the western Florida Panhandle, when the tide comes back in. It is clear now that this storm surge event will be as dangerous as that of Category 2 Hurricane Gustav of 2008. One piece of good news: NWS New Orleans successfully launched their 00Z balloon. However, their discussion noted the atmosphere is "saturated or nearly saturated" all the way up to 470mb, or 20,000 feet. Precipitable water was 2.76 inches, which will be ripe for extremely heavy rainfall.


Figure 2. Tide gauge data from Shell Beach, located on the south shore of Lake Borgne, just east of New Orleans. The green line shows the storm surge. The red line is the storm tide, the height of the water above Mean Sea Level (MSL.)

Portlight disaster relief charity responds to Issac
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, have mobilized resources in advance of the arrival of Hurricane Isaac. Their crew, including 2 EMTs, is at the Biloxi Special Needs Shelter, and will be caring for shelter dwellers and doing rescues of people who call for help. Another team will be surveying all the shelters in the area to ensure that they are accessible to all people. You can donate to Portlight's disaster relief fund here.

I'll have more in the morning. Hunker down, New Orleans. It's going to be a long night.

Jeff Masters

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MSY reporting 50 mph. Last gust here in Gentilly sounded significantly higher.

Eye does look funky on radar. Is a new, smaller eye trying to form?

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Quoting Patrap:
Nola Roux with first two..


They picked a heck of a night to come into the world. Very cute...

I can't believe you still have power.
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Quoting tennisgirl08:


Local mets in Gulf Shores are really getting pounded right now. Surprised at the strong conditions there. Not real bad in Mobile, right now.


24 mph now in Mobile and 26 mph in Gulf Shores, but they are getting 44 mph gusts from a rainband that looks like it will be in Mobile soon.
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Quoting floridaT:
its so silly we still string power lines from poles


It's cheaper and shareholders dividends would be lower if they buried them. Can't have that.
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Pat - what did you name the female?
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Quoting HrDelta:


Well, something ain't right. That means either the instrument has gone bad, or somebody is underestimating the storm. Would gusts of that speed indicate higher sustained?


i heard him say it on the news....
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So based on the current recon mission Isaac's center wobbled SSW 18 miles since last center fix?

29.N 89.55W @23:57
28.75N 89.6167W @01:26

They are heading in for another fix now.
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Quoting floridaT:
to impress pretty ladies like you,
It totally works!
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Quoting avthunder:
Oh that's sweet . Nice to have some good
news on a tough night for the Gulf Coast. Just promise you won't name one Al Roker.


O man I'm gonna use dat.

..er, the Line, not da name. : )
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Quoting EvilGN:
looks like a good portion of the west bank of NOLA proper is out of power, my neighborhood still has power according to the entergy website [...] But I hope everyone else in the area stays safe!


still have power: Orleans and Bayou St. John.
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Uh, this is going to farther west than what NHC thinks, that ridge is too strong over the Midwest.
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Quoting floridaT:
to impress pretty ladies like you,


LOL!!
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Quoting HurricaneHunterJoe:
Parish prez saying the mayor of Grand Isle reports wind gusts to 120mph


Well, something ain't right. That means either the instrument has gone bad, or somebody is underestimating the storm. Would gusts of that speed indicate higher sustained?
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every time theres a jog west the texans pull out those fancy maps

isaac pulls out his muddle finger and goes north
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Quoting ProgressivePulse:
Heads up Al, this is a big un.
Hey you were right on with that eye in the eye thing!
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Quoting hurricanehanna:

awe Pat...how precious! She is a proud Mama


Yeah..she iz a good Dog and a Good Mamma fo sho'
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looks like a good portion of the west bank of NOLA proper is out of power, my neighborhood still has power according to the entergy website, I however am a distant observer of all of this, my family is stuck in nebraska, we were suppose to fly back tonite, but that was canceled two days ago. both a blessing and curse, we are safe of course, but we never were able to prep the house for any of this. Its frustrating not knowing the status of my home through tonite. But I hope everyone else in the area stays safe!
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Quoting allancalderini:
Isaac may be gone this year,and imo Kirk will be other Florence instead of other Joyce I can`t degrade the system much LOL.

Leslie maybe will be our first major Hurricane........Specially he maybe recurve to open the waters of the atlantic
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Heads up Al, this is a big un, lol.
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Quoting Patrap:
Nola Roux with first two..
Oh that's sweet . Nice to have some good
news on a tough night for the Gulf Coast. Just promise you won't name one Al Roker.
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Quoting Patrap:
Nola Roux with first two..


Super! Good to see some good news on this "ruff" night....
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Quoting Patrap:
Nola Roux with first two..

awe Pat...how precious! She is a proud Mama
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Quoting floridaT:
its so silly we still string power lines from poles


Not only silly, Ugly, inneficient, less cost effective in the long run, and harder to maintain.
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Quoting wxchaser97:
Now 220,000 without power in LA.


I got 99 problems but that aint one...
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Quoting Patrap:
Nola Roux with first two..
Awwww God bless her and her pups!
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Quoting HoustonTxGal:
why do weather people stand out in the rain and howling wind? do they think we will not believe it is happening if we don't see them get wet?
to impress pretty ladies like you,
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Quoting Patrap:
Nola Roux with first two..
The Roux Deux!
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240. txjac
Quoting Patrap:
Nola Roux with first two..


So cute ...I wanna hold one!
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Quoting floridaT:
its so silly we still string power lines from poles


And we still listen to music on Radios, who knew?
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Parish prez saying the mayor of Grand Isle reports wind gusts to 120mph
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as my louisianian friends lose power the texans are taking over and i dont have the links to combat...

ehh just watch the weather channel westcasters..


TREE JUST FELL
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Quoting WxNerdVA:
Isaac did it. He finally got the dry air out of his system. Good thing he didn't do it yesterday morning because otherwise....



He's going to suck a lot of dry air to his north once he starts to move though. This is probably about as good as he will ever look.
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Quoting SrChiefFan1:


I'm sure that he is wishing that he was in New Orleans and Cantore in Orange Beach getting buffed by the sand...


Local mets in Gulf Shores are really getting pounded right now. Surprised at the strong conditions there. Not real bad in Mobile, right now.
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Quoting HoustonTxGal:
why do weather people stand out in the rain and howling wind? do they think we will not believe it is happening if we don't see them get wet?
I guess originality, I believe they all think they are the first ones to ever stand out in a hurricane. Dan Rather reported standing outside when Hurricane Carla hit the Texas coast in 1961.
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Nola Roux with first two..
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Quoting hahaguy:


I remember during Wilma he fell down twice trying to stand up on a balcony before his camera guy had to hold him down.
Haha I remember that too! he held his legs!! So funny!
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Quoting HoustonTxGal:
why do weather people stand out in the rain and howling wind? do they think we will not believe it is happening if we don't see them get wet?



That's the action shot for ratings...lol
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Quoting wxchaser97:
Now 220,000 without power in LA.
its so silly we still string power lines from poles
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Quoting TreasureCoastFl:
Om my, it looked lke he almost fell down in that last shot.


I remember during Wilma he fell down twice trying to stand up on a balcony before his camera guy had to hold him down.
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Quoting HoustonTxGal:
why do weather people stand out in the rain and howling wind? do they think we will not believe it is happening if we don't see them get wet?


honestly... ITS FUN
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Isaac may be gone this year,and imo Kirk will be other Florence instead of other Joyce I can`t degrade the system much LOL.
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Quoting leftlink:


HERE'S MY TAKE ON THIS:



I think they did it(call landfall) to cover their a$$. They don't want to have no landfall in LA,and then have it show up in Texas. JMO
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Quoting GTcooliebai:
Mike Seidel live at Orange Beach, AL getting sand blasted.


I'm sure that he is wishing that he was in New Orleans and Cantore in Orange Beach getting buffed by the sand...
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Quoting HoustonTxGal:
why do weather people stand out in the rain and howling wind? do they think we will not believe it is happening if we don't see them get wet?


Because it pays more than sitting at the desk
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Does anyone have an idea of the lowest pressure of any landfalling Category 1 in the Atlantic basin? I'm guessing Isaac should be up there within the top 10?
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Quoting HoustonTxGal:
why do weather people stand out in the rain and howling wind? do they think we will not believe it is happening if we don't see them get wet?
Right? You'd think it would be dangerous maybe?
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Quoting leftlink:
NEW INFO: When Isaac did that little loop-de-loop the pressure in an area he already passed went way DOWN.

from 28.70 (with light 9.9kt wind)
to 28.64 (with wind at 44.1 kts)

It is going to be approaching category 2 with this kind of intensification continuing...

in millabars, this is 971.9mb at 6:48pm and 969.86mb at 9:18pm. Someone do the calcs for me... is the second reading indicative of a 965.5mb center?

LINK


ya it would be 265
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Quoting sar2401:


Wait...that's the precipitable water data forecast. That's just an indication of how much moisture is availabe for the atmosphere to work with, but it doesn't neccesarily correlate with the actual amount of rain they will get. As an example, the preciptable water value over central AL this afternoon was about 2.75 inches. We actually got .16 inches.


This may be a better representation...wow a lotta water coming for a wide area!!!

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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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