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 TampaFLUSA:
Lol
RT @reedtimmerTVN: Hurricane #Isaac may have stalled over open water, and possibly started a wobble back northeast! Could be strengtheni ...


Huh?
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Wobbles smoothed out, it's been tracking almost due west for over 5 hours.
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Looks to be drifting sse
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Quoting moonlightcowboy:



Was just thinking the same. Where is this next trough? SFC MAP We don't usually get deep troughs until later in the season.

This is crazy. He can't poleward, doesn't look that way. Has to east or west.


Not much I can see to pull him east.
Go west young man?
Surprising that few, if any, picked up on this conundrum.
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12:00 AM CDT Wed Aug 29
Location: 29.0°N 90.0°W
Moving: WNW at 7 mph
Min pressure: 969 mb
Max sustained: 80 mph
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Is Isaac going to be able to keep it together tonight?

The last two overnights weren't favorable to him.

He's so big he's like a dry air vacume.


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Quoting HurricaneHunterJoe:


How can these wind speeds be like this? hearing 100-120 is that possible in a 80mph hurricane?? Why is the nhc not reporting that?



It's feasible that you could see a 100 to 120 mph gust in a 80 mph hurricane with pressure this low if a very strong convective cell brings powerful winds down to the surface. I mean, 110 is definitely pushing it in a 80 mph hurricane. But remember 80 mph threshold from the NHC means the maximum possible sustained wind is 80 mph. However we all know that wind, especially sustained winds will decrease as it hits land due to friction. But it doesn't mean that 80 mph sustained winds with gusts up to 100 mph aren't only a bit above the atmosphere and deep convection can bring such wind down.

I too am skeptical for sure until I see proof, however it's not impossible.
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Quoting SETXborda:


So, if we are on the border of LA/TX and Lake Sabine...r we in warning or not?


Currently, hurricane force winds are only making it as far was as a line due south of Baton Rouge, but certainly not in Baton Rouge, I mean near the coast.

NHC won't update the warnings unless this becomes a trend rather than a few wobbles.
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Grand Isle has been getting the absolute worst that Isaac has to offer. How long have they been in the eyewall now?
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The eyewall is slowly shrinking and this may explain the apparent sw or wsw movement. The next hour or two will be important in determining the exact storm path.
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New VDM does indicate a jog southwestward of 10 miles or so...but an average of the last 4 fixes would be due west IMO!
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Quoting SETXborda:
In Bridge City, when should I go stock up? Maybe if it gets as far as Morgan City?

NOW!!!
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Quoting xtremeweathertracker:


My sister lives in bridge city off of Ferry Road!


I know your sis... she told me you were on here.
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Quoting Levi32:
Pressure steady last two recon passes.

They just posted last fix 969
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x
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Quoting AllyBama:


Hi! I work over that way but live in the Sklyline area off hwy 90...weather is stormy at the moment. some of the heaviest bands have been training over the last hour or so..


Know that area well, i was the Asst Golf Pro (when it was still Skyline Country Club) for about 3 years. Keep your head down and stay safe!
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Quoting floridaT:
the good news is the further west the more drought relief when it does go north
Amen brother.
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Quoting sunlinepr:
The Worst of it???



Is he a storm chaser or what? Doesn't seem to know a lot about weather. :) I do notice all the lights are still on. It's apparently not hitting the French Quarter too hard. Looking at the Entergy outage map for Louisiana, there are surprisingly few outages outside the immediate New Orleans area.
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Quoting silverstripes:
Issac seems to be getting punched again with dry air. Cloud tops warming evverywhere.


yes...the overall appearance looks a bit diminished from 2 hours ago. I am sure it will rebuild though.

Eye wall looks to tightening on radar.
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Pressure steady last two recon passes.

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Quoting Stormchaser121:



So, if we are on the border of LA/TX and Lake Sabine...r we in warning or not?
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Quoting RTSplayer:
The eye has moved 5 to 10mph to the SW in the past hour, based on using the "base velocity" tool on NEXRAD radar on wunderground and just watching the inflection points move SW.

The "calm" portion of the eye itself was right up to the land, and then it moved back off shore.



And, that's something else I thought about, too. Can't remember the rule, if there is one: Does the eye just have to touch land? Or 51% of the eye to touch land to be an official landfall? I can never remember this. TIA
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Lol
RT @reedtimmerTVN: Hurricane #Isaac may have stalled over open water, and possibly started a wobble back northeast! Could be strengtheni ...
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Quoting want2lrn:
Used to live in Mobile on Vanderbilt (across from USA) near Old Shell and Hillcrest, anybody around that area, curious what the conditions are?


Hi! I work over that way but live in the Sklyline area off hwy 90...weather is stormy at the moment. some of the heaviest bands have been training over the last hour or so..
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Quoting SETXborda:
In Bridge City, when should I go stock up? Maybe if it gets as far as Morgan City?


My sister lives in bridge city off of Ferry Road!
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Quoting RTSplayer:
This is no joke. You can see the South West movement on the regional composite radar.

the good news is the further west the more drought relief when it does go north
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In Bridge City, when should I go stock up? Maybe if it gets as far as Morgan City?
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This is no joke. You can see the South West movement on the regional composite radar.

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Quoting tennisgirl08:


Not in that area, but in Mobile County. Heavy rain, gusty winds, constant downpours now. 15-20 sustained...gusting 30 maybe. Still have power.


Thanks, that area is pretty low. There is a ditch behind the old house that swelled up pretty good, so i was wondering. Stay safe!
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Quoting HurricaneHunterJoe:


How can these wind speeds be like this? hearing 100-120 is that possible in a 80mph hurricane?? Why is the nhc not reporting that?


Great question!!
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In Springfield I'm getting radar estimated sustained at the boundary of 50kts threshold.

I doubt 50kts is making it to the ground, maybe the tree tops, though there was one particularly persistent gust about 15 minutes ago.

The power has not gone out again since Entergy made some quick repairs several hours ago.


If this is 40 to 50kts sustained, then it's nothing compared to what we got in Katrina. The trees outside still have most of their leaves. I didn't see anything out of the ordinary when I was outside a few minutes ago with the lights.

Katrina partially defoliated all the trees, broke limbs and uprooted stuff, etc.. So that's a big difference.


It's hardly even sprinkling here at the moment, though the radar insists that's going to change.
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Quoting RTSplayer:


It's not just you, they are talking about it on the weather channel.

The new specialist guy (forget his name) said about the same thing I did. The ridge on the west is jamming Isaac and causing it to stall out.


It's probably going to sit around raining for days until the next trough comes along...unless it gets pushed out to sea, and then we'll start this whole process over again....



Was just thinking the same. Where is this next trough? SFC MAP We don't usually get deep troughs until later in the season.

This is crazy. He can't poleward, doesn't look that way. Has to be east or west.
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Live stream from WWLTV-New Orleans :)

http://www.wwltv.com/live-stream/exempt
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Quoting Hurrihistory:
Ok guys, it's offical. The name Isaac will be retired and never used again after this is all said and done!

lol. wait till the WMO meeting.
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Issac seems to be getting punched again with dry air. Cloud tops warming evverywhere.
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Quoting SomeRandomTexan:
Wondering when or if NWS will call for Tropical Storm warnings in Orange and Jefferson counties.


Been under watch in Orange all day...before the channel went down earlier, it flashed the warning....
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Quoting want2lrn:
Used to live in Mobile on Vanderbilt (across from USA) near Old Shell and Hillcrest, anybody around that area, curious what the conditions are?


Not in that area, but in Mobile County. Heavy rain, gusty winds, constant downpours now. 15-20 sustained...gusting 30 maybe. Still have power.
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im watching on channel 349 direct tv.they showing channel 4 new orleans weatherman showing their radar and it shows the top third of the eye over land.
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at least 265,000 without power in NOLA area right now, according to this:

http://viewoutage.entergy.com/la.aspx
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Quoting MercForHire:


Well, no, you're right.

But I went with my answer not for that reason, but because I attended a couple of hurricane parties in Florida during my college years & got purty drunk. So kinda personal experience there.

P.S. Beginning my Masters next Spring, so it'll most likely happen at least once more. :( :(
outstanding a degree in?
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wow the rain is ridiculous now....
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If it sits there much longer it'll be raining gators in NO.
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Big wall in the N for Isaac, it wont be able to cross or move into Conus for a while.... Where will he go?

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Quoting lopaka001:

That's what I thought too but that last frame it looks like Isaac might have close it off. Maybe..


Not dry air. He's hit a brick wall...the plains ridge.
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969.6mb is it weaking now???
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Used to live in Mobile on Vanderbilt (across from USA) near Old Shell and Hillcrest, anybody around that area, curious what the conditions are?
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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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