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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Quoting BrazoriaMan:


I see. So the high is building in towards the west, pushing Isaac that way. And I am assuming the high in the plains is kind of blocking Isaac as well. Could this thing really be headed for the TX/LA border like some of the models suggested days ago? It's amazing how fast weather changes! Thanks!


Issac is on rails! someone de-rail him!
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Nope..no rain bands or wind..dead. calm..not even cricket or frog sounds tonight..so far..sabine river here
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Evening all,I see our friendly friends from southeast tx still say its heading in that direction.It is but it'll be inland and probably more weaker than it is now! So just expect rain out of it. Winds won't be that bad! Lol silly guys!!
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966mb at 10:35pm cst the lowest pressure yet by a reccon plane
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Quoting JeffM:


Maybe rain bands isn't the word I'm looking for. The blue area is close to Houston.



Outer cloud canopy bands?
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Quoting LisaNola:


I'm fine thanks!
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Quoting ProgressivePulse:
Notice the AB high flat on the western edge? It wasn't like that three hours ago.



I see. So the high is building in towards the west, pushing Isaac that way. And I am assuming the high in the plains is kind of blocking Isaac as well. Could this thing really be headed for the TX/LA border like some of the models suggested days ago? It's amazing how fast weather changes! Thanks!
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462. JeffM
Quoting lottotexas:
there are NO rain bands reaching Houston at this time. None even in Lake Charles


Maybe rain bands isn't the word I'm looking for. The blue area is close to Houston.

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461. Skyepony (Mod)
Some storm chasers in there. Zoom in & click on which ever one to open up live stream..
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Many of these animations are historical Gifs for our met. collection.... Too bad that it can imply the loss of life and affect the lives of many....

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Quoting Jwd41190:
Weather channel not working....
Ok I thought it was my TV.
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Quoting WetBankGuy:
Anyone seen a post from the person saying they were going out to take a picture from Bayou St. John? If you're still on the board text me 504 579 9672 or I'm thinking of calling 9-1-1. There was a tremendous gust just after she posted that message.

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i have a bad feeling that one of my outer walls might collaps. everytime a strong gust of wind it pushes in and out.
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Quoting ProgressivePulse:
Notice the AB high flat on the western edge? It wasn't like that three hours ago.



Nice obs! :) Thanks.
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GFS model has delayed again at 57 hrs.

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Bourbon Street wind REALLY picked up about 10 minutes ago, but rain still medium'ish. No banners, signs, traffic-lights, or awnings missing. People still strolling & cops still cruising ith blue lights on.
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Weather channel not working....
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I did just check the radial velocity on the Lake Charles radar though and it seems to show 40 knots from the North across a wide swath of the radar.

I selected the lowest elevation available (0.5 deg), but I haven't done the trig to see what altitude that wind would be at.

It sounded too high, so I checked weather at the airport on the east side of town and it shows wind out of the north at 10 knots.

Does that mean radar estimated windspeeds are typically a gross overestimate?
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Quoting BrazoriaMan:


Pearland here! Where are you?


Small world - I'm in Pearland as well.

Back to lurking.
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Blizzard in New Orleans!! And...Isaac has no wind field!!
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Quoting lottotexas:
there are NO rain bands reaching Houston at this time. None even in Lake Charles


I have not seen a drop of rain here.
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Howdy to you too.
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Local news out of beaumont.. one news station said we will start getting some wind and rain tonight..other one basically said we are not going to get anything but a little rain in a few days.(?) As went off my noaa radio went off..under a tropical storm warning now in setx..i'm thinking i wont sleep tonight
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Quoting lottotexas:
there are NO rain bands reaching Houston at this time. None even in Lake Charles


Nor here in Orange either....
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Quoting sgvalenti:
Per Wungerground twitter feed "Hurricane hunter data showing Isaac taking a jog to the southwest. "


That's true, but that was at 10:00 PM CST, an hour ago. Although, there have been hints it is happening again.
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Quoting ProgressivePulse:
Notice the AB high flat on the western edge? It wasn't like that three hours ago.



What does that mean?
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Quoting snow2fire:


From the USACE maps, it looks like flood stage is 17'. The river is a little over 11' now. It was very low at 2-3' ft yesterday. The river can handle a lot of water coming downstream - or as it turns out, upstream as well.

Yeah I was looking at it earlier but the data points I was looking at are offline now so I didn't remember the number for the floodstage.
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HRRR still shows a coastal hugger for the duration of its run at 01Z:

Link
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pressure unchanged at 968. Looks it going mainly west with jogs above and below that.
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Quoting IFuSAYso:


New roofs take a week at 70+ deg for the seal-tabs to adhere. Seal-tabs are the black tar dots on the shingles.
I just found that out today from someone at work.
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Quoting WetBankGuy:
Anyone seen a post from the person saying they were going out to take a picture from Bayou St. John? If you're still on the board text me 504 579 9672 or I'm thinking of calling 9-1-1. There was a tremendous gust just after she posted that message.



im safe. Thanks.

Just got in a while back.
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438. FOREX
Quoting HurricaneHunterJoe:
Looks like Isaac will stay over water for some time?
Quoting HurricaneHunterJoe:
Looks like Isaac will stay over water for some time?
Quoting HurricaneHunterJoe:
Looks like Isaac will stay over water for some time?


Been away all day. Any info on potential track for wave off of Africa. Not kirk, the other wave.
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Quoting zillaTX:


high steppin for survival!


lol that is the truth. It is not overly rare to see storms get close to land and barely make "landfall" just to bounce off land and move elsewhere. A big loop is not out of the question with this storm at all. Just my opinion though.
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Quoting AllBoardedUp:
Hitchcock


Well then howdy neighbor.
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Notice the AB high flat on the western edge? It wasn't like that three hours ago.

Member Since: August 19, 2005 Posts: 5 Comments: 5450
Quoting AllBoardedUp:
Yes, but this job is an insurance claim from hail damage! I might as well pay this roof contractor on retainer!


New roofs take a week at 70+ deg for the seal-tabs to adhere. Seal-tabs are the black tar dots on the shingles.
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SUMMARY OF 1100 PM CDT...0400 UTC...INFORMATION
------------------------------------------------- -
LOCATION...29.1N 89.9W
ABOUT 65 MI...100 KM S OF NEW ORLEANS LOUISIANA
ABOUT 60 MI... 95 KM SE OF HOUMA LOUISIANA
MAXIMUM SUSTAINED WINDS...80 MPH...130 KM/H
PRESENT MOVEMENT...NW OR 310 DEGREES AT 8 MPH...13 KM/H
MINIMUM CENTRAL PRESSURE...968 MB...28.58 INCHES

$$
FORECASTER PASCH/ROBERTS
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For Pete's sake my HH plane is hung up on Google Earth...hmmmm maybe they caught a westcaster on board and had to chunk him overboard! LOL
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431. JeffM
11:00 PM CDT Tue Aug 28
Location: 29.1°N 89.9°W
Moving: NW at 8 mph
Min pressure: 968 mb
Max sustained: 80 mph


This thing is a fighter!
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Anyone seen a post from the person saying they were going out to take a picture from Bayou St. John? If you're still on the board text me 504 579 9672 or I'm thinking of calling 9-1-1. There was a tremendous gust just after she posted that message.

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Quoting ProgressivePulse:


Yes it is and I don't think the US ridge is doing the pushing...


Really? What is then?
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this is crazy
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Quoting BrazoriaMan:


Pearland here! Where are you?
Hitchcock
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Quoting HurricaneHunterJoe:
Channel 4 New Orleans talking to Grand Isle mayor again.....I swear I heard him say 100mph -140mph?


That's quite a range, which tells me they aren't measured wind speeds. 140 mph gusts should have pretty much leveled everything in island. That's equal to a strong F2 tornado.
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Quoting sunlinepr:


Convection on NW side flattening out on this loop.
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Quoting DavidHOUTX:


"Stair stepping" away from land rather than towards land ;)


high steppin for survival!
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Quoting JeffM:
Outer rain bands almost reaching Houston! This is a wide storm.
there are NO rain bands reaching Houston at this time. None even in Lake Charles
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in mandeville la. we are getting some nice wind gust but not too much rain. no too much of anything at the moment. can anyone tell me when and if we will see anything stronger than what we are seeing now? thanks oh and fyi i live less than a mile off the lake. tia!
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the center of the eye is now south of the southern most point of LA so it now has to have some north motion involved or its going to texas
Member Since: August 1, 2006 Posts: 28 Comments: 3716
We may have to keep an eye on the storm here in SeTx... hope it doesn't come this way but looking at the loop it has been moving almost due west for the past 3 hours.
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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.