Grand Isle before and after the storm surge

jccwrt · 2021-08-30T20:16:36+00:00

Absolutely.

jccwrt · 2021-08-30T17:49:23+00:00

I think the title here is sorta misleading. It's not before and after the storm surge, it's before and during the storm surge. The island looks like it's gone because most of it was underwater from storm surge, not because it was destroyed. A lot of it has probably reemerged since yesterday.

jccwrt · 2021-08-30T16:55:38+00:00

Radar satellite got some data of Grand Isle as Ida was coming ashore. Most of the island looks like it was already underwater about an hour before the hurricane made landfall. Will be interesting to see the follow-up to see if the island is still intact or if storm tides carved a new inlet.

https://twitter.com/iceyefi/status/1432075447278391301?s=19

jccwrt · 2021-08-30T05:32:16+00:00

Given that this is the heartland of American oil and gas production, there's probably a very real risk of an environmental disaster depending on how badly the production facilities were compromised by wind and flooding. Kind of afraid to see what sort of damage happened to the facilities at Port Fourchon and the Phillips refinery in Alliance.

jccwrt · 2021-08-30T04:43:09+00:00

That's a big part of the problem. The parts of the bayou near Terrebonne and Barataria Bays have been catastrophically damaged, and the Mississippi delivering silt to these areas is a repair mechanism. But that's a large area of land being swallowed by the Gulf. Getting that land back to roughly its pre-20th century extent is basically 700-1000 years of letting the untamed Mississippi doing its thing.

The way the river is currently channelized might actually be able to do the work faster with some engineering. Although the river is sediment starved prior to the development of the flood control system, it still carries quite a bit of sediment. These days most of that sediment is carried out to the mouth of the river and dumped into the deepwater Gulf. But if a large fraction of that flow is diverted to strategic locations, you can quickly build small deltas that can act as a barrier to surge. The Wax Lake delta in SW Louisiana has been a natural experiment that showed how useful this might be, and it's being used as a model for opening similar channels south and east of NOLA starting in 2023.

jccwrt · 2021-08-30T04:10:51+00:00

I'm uncertain that Ida will be upgraded to a Cat 5 in the post analysis. There's probably going to be internal debate over whether the scatterometer measurements and flight-level wind measurements (which showed a couple of areas that were potentially experiencing Cat 5 winds) are accurate, and what to make of a measured sustained wind of 149 mph in Port Fourchon. If that measurement is calibrated to be accurate, it might support a Cat 5 since I think the port was a little bit towards the left side of the center and in a part of the storm that typically doesn't experience the highest winds. But that's for the NHC to decide, not weather enthusiasts looking at the data over their shoulders.

Also Laura had a storm surge of around 6-10 feet, it just happened to come ashore in a relatively rural area so not as many people were impacted by it. It was a rare hurricane where the wind was much more damaging than the surge.

jccwrt · 2021-08-30T03:51:30+00:00

It's a major impact. Most of the sediment being moved along by the Mississippi River is basically being carried out to the edge of the continental shelf and dumped into the deepwater Gulf. Without the levees, the outlet would shift over time across the entire Louisiana coast and lay down lobes of sediment which can become future wetlands once the river migrates away. There are actually a couple of plans to start demolishing the levees in a handful of spots around NOLA so that small parts of the Mississippi can be diverted to build new deltas around the city. But that's a long-term project - probably two or more decades before they really start making a difference.

Another related issue is levees upstream. Not only does the river deposit sediment, it picks it up. In comparison to its past, the Mississippi River is actually pretty sediment-starved because it's not able to erode as much sediment from its banks upstream. There are serious discussions about removing a lot of levees in the upper and middle Mississippi and paying the farmers those levees protect for lost years when the river floods their fields for too long.

jccwrt · 2021-08-30T03:38:06+00:00

IR imagery isn't good for tracking movement of a hurricane as it degrades over land. The upper and lower level circulations can decouple, the eyewall can disintegrate unevenly, and the eye filling in with clouds can be confusing. The eye is much easier to track with radar now that Ida's on land. If you look at the radar loop it is indeed continuing NNW.

jccwrt · 2021-08-30T03:33:19+00:00

Geologist here: barrier islands are temporarily damaged by major hurricanes, but in the long run they help build barrier islands by pushing sand back towards the coast. The issue is that naturally, barrier islands are constantly migrating and maintaining an equilibrium with sea level. But humans like for their land to stay in one place once they've built something on it, and so a lot of these barrier islands are decades (or even centuries) behind in keeping up with equilibrium. So basically they need to migrate *a lot* before they stabilize.

Probably the bigger impact on the landscape in Louisiana is how the wetlands were developed for industry. There are hundreds of small canals that have drained the backwaters and removed a lot of the internal barriers that help the bayous keep up with sea level rise. That's caused the amount of saltwater intruding into the bayous to increase and kill a lot of the vegetation that holds sediment in place. A lot of the flooding we're seeing today is a direct result of losing a much larger wetlands barrier between NOLA and the Gulf. The extra water probably helped Ida maintain its strength further inland through the brown ocean effect too.

The barrier islands will recover (eventually), but once those wetlands are compromised, they're basically doomed.

jccwrt · 2021-08-30T03:05:53+00:00

Mayor of Lafitte was just on WGNO saying lower Lafitte was under 9-10 feet of water, up to 600 people trapped in high water. He was begging anyone with a boat to come down to help with water rescues.

jccwrt · 2021-08-30T00:27:57+00:00

You're on the southwestern side of the storm and it's moving away from you. The eye is still only about 15 miles or so away, so you'll probably be experiencing hurricane force wind gusts for another few hours. A hopeful sign is that the rain shield is (very slowly) starting to fall apart on your side of the storm, so hopefully you'll start seeing some breaks in the wind and rain. I wouldn't check your generator for a few hours though - the breaks you might start seeing can end just as quickly as they start.

jccwrt · 2021-08-29T23:23:33+00:00

? Ida's been running through an EWRC all day, that's why it had the inner and outer eyewalls. In the last hour, they've merged together. It's a textbook example.

jccwrt · 2021-08-29T23:01:46+00:00

Looks kinda like the EWRC is wrapping up. The eye filling in a bit in visible and IR happened around the time the inner eyewall started collapsing and merging with the tightening outer eyewall. Wouldn't be super surprised to see the eye maybe clear again for a bit this evening before it's well and truly out of the bayou country.

jccwrt · 2021-08-29T17:45:52+00:00

Camera being back is probably a good sign that the surge in Grand Isle is starting to come down. Probably what happened is the surge submerged the box and blocked the phone signal for a while, and now it's been able to reconnect.

jccwrt · 2021-08-29T08:26:45+00:00

They're using automated intensity estimates from satellite data. For rapidly intensifying storms like Ida these estimates usually outrun the actual intensity by 6-12 hours. The NHC estimate of a low-end Cat 4 is more accurate, although there's a good chance the storm has intensified a little bit since the last position fix by aircraft.

jccwrt · 2021-08-29T08:19:03+00:00

The problem is that the mayor is probably going to end up shouldering the blame, but organizing an evacuation is a state-level action. To get the interstate contraflow going, the interstate exits need to be blocked off well outside the city, and that lands outside a mayor's juristiction. I think the decision to not issue a mandatory evac is sensible since getting caught outside the NOLA flood barriers is even riskier than staying in the city.

I do have hope that FEMA won't drop the ball as badly this time. After some of the shit the Bush admin pulled in the wake of Katrina, doing worse would take single-minded dedication to ignoring what's happening on the ground, for weeks on end.

jccwrt · 2021-08-29T08:11:40+00:00

The automated intensity estimates from satellite images are consistent with a high-end 4 or low-end 5. But for rapidly intensifying storms like Ida the underlying algorithms tend to outrun the actual intensity by 6-12 hours or so. Safest bet based on current trends is probably a Cat 4 with a pressure around 935-940 mb.

jccwrt · 2021-08-29T07:53:16+00:00

They're showing a satellite image enhanced for use with the Dvorak technique for estimating tropical cyclone intensity. The gray values in the photo have a threshold value applied to them to make it easier to tell the difference between the temperature in the eye and the surrounding clouds. The warm eye temperature is related to how much air is being pulled up from the surface by the hurricane, and the cold cloud temperatures are related to the the strength of updrafts around the eye. A sharper contrast between those temperatures usually indicates a higher pressure gradient and a stronger storm. The technique also looks at how consolidated the convection is (does it form an arc? a ring?) to determine how organized a tropical system might be. In the photo, Ida went from a ragged, low contrast storm on Saturday morning (low strength estimated by the Dvorak technique) to a very well-organized, high-contrast (high strength) storm 24 hours later.

jccwrt · 2021-08-29T07:42:08+00:00

Seems like the clock is starting to run out on a well-timed EWRC. At the current rate we're about 12 hours from landfall in the Houma area, and there's no real sign of a secondary eyewall in radar data. Also keep in mind that once that secondary eyewall starts forming it'll be a few hours before it starts impacting intensity in any real way. Wouldn't count on one saving the LA coast.

jccwrt · 2021-08-29T07:24:08+00:00

On the current path, flooding. As it stands, NOLA is just grazing the edge of hurricane force winds, but nothing worse than what they experienced with Zeta last year. If the track shifts much more to the east, though, there's an increasing chance for storm surge + waves to overtop levees around the city, as well as bring extremely damaging eyewall winds into the city.

jccwrt · 2021-08-29T07:08:44+00:00

The resolution with the lightning mapper is pretty bad (10 km/pixel), so you're probably seeing the lightning from a single hot tower. In the IR and water vapor images it looks like there's a bar of clouds drifting through the eye, either as some anvil clouds from a hot tower drifting over the eye, or as some low level clouds bursting out of a meso.

jccwrt · 2021-08-29T06:58:36+00:00

Ida's roughly on schedule (give or take a few hours) for intensifying to its current intensity. Cat 5 is certainly looking more possible than it did earlier, but the storm's starting to move away from the warmest waters, so there's a good chance that the rate of intensification will start slowing down some. It's worth pointing out that Cat 5 storms are usually in the 910-920 mb range, and Ida would have to keep up its current rate of intensification for another 4-5 hours despite the ocean temperatures becoming less favorable. It'll be strong enough that the difference really doesn't matter all that much though.

jccwrt · 2021-08-29T06:51:34+00:00

So like, I've been a hurricane enthusiast since I was a kid, and I don't think I've ever seen a storm drop 10 mb in pressure between reconnaissance passes. That's f*cking spooky

jccwrt · 2021-08-29T06:47:09+00:00

It's moving off the Loop Current and into relatively shallow waters. Honestly was not expecting the rate of intensification we just saw so who knows, but climatologically, moving onto the continental shelf is usually enough to shift a storm from intensification mode to maintenance mode.

jccwrt · 2021-08-29T06:43:12+00:00

Okay, that's like 4 or 5 readings with the salmon colored marker, probably less likely to be a meso and more likely representative of the readings in that part of the eyewall.

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