Hurricane Isaac hits Louisiana, driving dangerous storm surges

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

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


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Thanks for the late night update Dr. Masters! I guess the morning light will be the tell-all for what has happened and what is yet to come as high tide hits the coast!
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Checking the various satellite and radar presentations, it looks as if some dry air has made some quick integration into the center (or near it.)
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thanks doc
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Thanks Dr. Masters.

Dang...somebody's slacking tonight.

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"Hurricane Isaac is ashore over Southeast Louisiana..."

*was ashore.
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Isaac has performed a cyclonic loop. Likely heading back NW now.

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Thanks for the update
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Hey Dr. Masters do you see any chance of upwelling from Isaac if he stalls out long enough?
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" 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." --- crazy
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Dr m. The nhc said it moved back overe water so most likey we will see a 2nd landfall. So you need too fix a few things in your blog
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thanks Dr. Masters.

And Orca, I think he's stalled out. Looks like he's getting a complete eyewall too. Fascinating
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Oh, heh. New blog. I stumbled in at the right time. Thanks Dr. Masters. :)
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TY
Did Isaac stall or just slow down?
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Thanks
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Latest Rapid Scan Loop over Isaac shows that cloud tops have warmed rather significantly over the last hour (although there is about a 45 minute gap in coverage). The latest recon pass showed that Isaac's intensity has leveled off and since then, Isaac may have weakened slightly (although we'd need recon to confirm this) based off satellite imagery showing the cloud tops continue to warm over the center.

Latest SRSO GOES-14 Image (click for loop!)

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thanks
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Cat 6 lead authors: WU cofounder Dr. Jeff Masters (right), who flew w/NOAA Hurricane Hunters 1986-1990, & WU meteorologist Bob Henson, @bhensonweather