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 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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Now 220,000 without power in LA.
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Big Un coming in from the SE, heads up. Odd central structure, contracting maybe.

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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?
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Quoting DavidHOUTX:
Want to know why the Northward movement has stalled? Based off of the latest steering currents, seems like it has no where to go but westward. However, that is not a guarantee and these things have a mind of their own.



see the shortwave?
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NEW INFO from pstl1:

When Isaac did that little loop-de-loop the pressure in that sand bar where he made "landfall" 3 hours ago 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 millibars, 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
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Mike Seidel live at Orange Beach, AL getting sand blasted.
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It comes to show how this Size of systems work:

Hurricane Gordon-
Size Type:Small Hurricane.

110 Mph
965 MB --- Notice Pressure


Hurricane Isaac-
Size Type: Large Hurricane.
80 Mph
968 MB --- Notice Pressure
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HURRICANE ISAAC DISCUSSION NUMBER 32
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
1000 PM CDT TUE AUG 28 2012

ISAAC PASSED OVER THE MISSISSIPPI RIVER DELTA IN EXTREME SOUTHEAST LOUISIANA AROUND 2345 UTC. SINCE THAT TIME...THE CENTER HAS WOBBLED WESTWARD AND HAS MOVED BACK OVER WATER. A NORTHWESTWARD MOTION HAS RECENTLY RESUMED AND A SECOND LANDFALL SHOULD OCCUR
LATER TONIGHT NEAR GRAND ISLE. DATA FROM NOAA AND AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE THAT THE MINIMUM PRESSURE HAS CONTINUED TO GRADUALLY FALL AND IS NOW 968 MB. THE AIR FORCE RESERVE AIRCRAFT HAS MEASURED A PEAK 850-MB FLIGHT-LEVEL WIND
OF 96 KT AND BIAS-CORRECTED SFMR WINDS OF AROUND 65 KT. BASED ON THESE DATA...THE INITIAL INTENSITY REMAINS 70 KT. LITTLE CHANGE IN STRENGTH IS EXPECTED DURING THE NEXT 12 HOURS OR SO AS THE CENTER
MOVES ACROSS THE BAYOUS OF SOUTHEASTERN LOUISIANA. STEADY WEAKENING SHOULD COMMENCE LATER WEDNESDAY AS THE CENTER MOVES FARTHER INLAND.
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Mike Sidel just said Snow is sexy!
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Quoting HoustonTxGal:
Isaac is going to be one heck of a rainmaker!!

click Link image to animate



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.
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Quoting DavidHOUTX:
Want to know why the Northward movement has stalled? Based off of the latest steering currents, seems like it has no where to go but westward. However, that is not a guarantee and these things have a mind of their own.



Yes notice the high to the north kinda pressing on his northern side...imo he has to go west or west-northwest!
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Quoting kidd5433:
I give Al Roker about 30 more min before he calls it quits out there on Canal Street.
Om my, it looked lke he almost fell down in that last shot.
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Rainfall is starting to add up in places.

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I give Al Roker about 30 more min before he calls it quits out there on Canal Street.
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its howling out right now...
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Want to know why the Northward movement has stalled? Based off of the latest steering currents, seems like it has no where to go but westward. However, that is not a guarantee and these things have a mind of their own.

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According to NBC news Isaac has returned to being over the water and no longer on land.
Link
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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 does look semi-studley now.
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27 have parished so far in the wake of the Wrath of Isaac.
Mostly in Haiti, at this time, due to structures collapsing.
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Quoting NGCSTSKS:
There are people still walking up and down Bourbon street in NO after dark according to the live cam
I noticed that too.. crazy
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Quoting NGCSTSKS:
There are people still walking up and down Bourbon street in NO after dark according to the live cam


Does that surprise you I mean it is Bourbon street
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Quoting HurricaneHunterJoe:
Just saw Billy Nungesser,President Plaquemines Parish saying he lost part of the roof off his home and a partially caved in wall also,and a lot of water in southern Plaquemines Parish...Levee's holding for now. Mississippi River 2 feet from the top!


After the storm, make that levee higher.
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just call it a mudfall

its not land its mush now...
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Al Rocker Just said "we have lost Mike Sidel." That's a tough way to tell his loved-ones that he is gone, isn't it? Don't you think they are watching?
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There are people still walking up and down Bourbon street in NO after dark according to the live cam
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Quoting Joanie38:
Good evening all....Wow, I am sitting here in Central LA...(Alexandria) waiting on this thing to come through and come to find out it might be stalling???? Oh boy, this is not good...What does this mean for the rest of Louisiana???


The NHC said it had resumed moving NW. I don't know how fast though.
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Quoting angiest:
The eye appears to be getting less well-defined on radar.


geee i cant imagine why... not
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Quoting HurricaneHunterJoe:
Does the NHC get credit for location of 1st land fall, since the eye hit that tip that sticks out? Or since he is back over water..it don't count?


HERE'S MY TAKE ON THIS:

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The eye appears to be getting less well-defined on radar.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
How big is Isaac now? anyone know?
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Hard to get used to avg long term motion and direction
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Sorry for the repost...didn't see it.
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Quoting HoustonTxGal:
Isaac is going to be one heck of a rainmaker!!

click on image to animate



Doesn't work for me. :(
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lol power is going on and thn off

just watching al do this stuff is funny
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Quoting Bradenton:
Are you and the WunderDogs okay?


Okay here..this Block has been here for well..a Long time.


Uploading some imagery
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Breezy and stars out in Hattiesburg, with a few clouds that aren't moving like a tropical band. Weird.
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Isaac is going to be one heck of a rainmaker!!

Link

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Just saw Billy Nungesser,President Plaquemines Parish saying he lost part of the roof off his home and a partially caved in wall also,and a lot of water in southern Plaquemines Parish...Levee's holding for now. Mississippi River 2 feet from the top!
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Quoting CybrTeddy:
Via Grand Isle Mayor: Island has significant damage and water on the island is waist deep


Unfortunately, Grand Isle has been repeatedly flooded by every tropical system that has come near LA. It was nearly totally destroyed several times, the first being in 1860. On average, every three years the island is affected by a tropical system. The highest spot on the island is only seven feet, and there are no levees to protect it. The 1,300 residents seem to always pick things up and put them back together again. I know it wouldn't be the spot I'd pick for a summer home.
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So I just read that the name Isaac means "He will laugh." That's definately fitting!
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I hope people in LA are inside and safe. But this is just amazing. Goes to show how unpredictable mother nature can be.

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Quoting Patrap:
Post 169. Looks like it's going to make a 2nd landfall in Grand Isle.
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Quoting Patrap:
Are you and the WunderDogs okay?
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TWC Saying its sorta wobbling west and a ferocious band is on its way in to NO right now
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Quoting aislinnpaps:




I really don't like seeing this. I prefer to be on the west side, not the east, especially as it's about 12 miles from me. As long as it's just a model saying so...


I wonder if Sabin Pass will get snow also...LOL!!
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Quoting canehater1:


ok its been along day..I give up...


OH ..got it now..duh !
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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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