Be Excellent To Each Other

Hurricane Ida
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Author:  NervousPete [ Sat Aug 28, 2021 16:25 ]
Post subject:  Hurricane Ida

Hello all. For sake of blog content and organise my own thoughts as a weather nerd I thought I'd post this ramble about a developing situation in the US. This one is Hurricane Ida, which for a while looked to be a potential Katrina part II. It's a bad hurricane and there's a long way to go before landfall happens around midday tomorrow US time, but thankfully despite its track and strength New Orleans should just about be able to weather it.

However if the hurricane over-performs - and it is possible - the situation could be every bit as bad as Katrina. Here's my thoughts...


So we have a concerning situation for the gulf coast and New Orleans as Hurricane Ida makes her approach to central Louisiana. I've been following this system since it was a tropical depression south of Jamaica way back earlier in the week, and even back then it showed worrying potential. Initially it was predicted to cross the Yucatan, weaken, and then power up again to hit Mexico or southern Texas. Then gradually as it crept more and more in a north-westerly direction towards Cuba the forecast models swung around past Houston to point at the Florida panhandle, before swinging back again to centre on New Orleans - a forecast track from which it has barely shifted over the past few days.

At first the models were mildly concerning, predicting a Category one to two landfall. However the danger quickly became more evident as several factors came into play. One was a ridge forming over Florida that would effectively lock the track in place and prevent it from landing more harmlessly in the underpopulated Florida panhandle. The second was a lack of forecast sheer in the gulf north of Cuba. Sheer is what disrupts hurricane formation, though sometimes in rare cases coming in at the right angle it can ventilate and strengthen development. The atmosphere over Cuba was moister than usual, meaning dry air intrusion wouldn't be too significant, and dry air is the other main factor that helps choke off hurricane development. Then - and this was arguably the most dangerous factor - there was a bath-tub warm eddy of water right on the forecast track, an eddy that ran deep and would provide an ample reservoir to help a hurricane rapidly intensify. Not only that, but unseasonably warm shelf waters south of New Orleans would help any approaching hurricane to maintain and even increase intensity as it climbed out of the gulf and on to land.

The problem was that this threat was rather nebulous and difficult to communicate for the National Hurricane Centre. Though it was still a tropical depression, it was moving fast and as soon as it moved to the warm water just south of Cuba it would explode into a tropical storm, a storm which would weaken only slightly as it passed over land thanks to the largely flat lands and sparse mountains of western Cuba. Once on the other side if still intact it had ample fuel to feed on. The concern was that if Ida could maintain her basic structure as a tropical storm she could power up to be a category three hurricane by the time she landed ashore on Sunday on the gulf coast.

Unfortunately, Ida managed to achieve hurricane status SOUTH of Cuba, and crossed as a minimal category 1.

This was really concerning. The forecast models for intensity were now showing high-end Category 3's and 4's for landfall. One even showed a Category 5, the maximum a hurricane can achieve. Small mercy then that Ida remained a modest in size at only a third the width of Katrina, and that a train of dry air off of the mountains of Cuba wrapped into her core delaying her intensification. The category forecasts swung back in the main to a borderline Category 2 / 3.

However things are changing again now. Ida has begun to hit the eddy. She's mixed out the dry air and has managed to wrap around her core of convection, beginning the formation of an eye and setting the stage for rapid intensification. However she's doing so a tad slower than anticipated As such the National Hurricane Centre are predicting a strong high-end Category 3 hurricane set at 130mph moving towards Houma, about 50 miles South West of New Orleans. Unfortunately the forming eye of Ida appears to be a little right of the track, setting her to possibly brush the west side of the big easy. As the strongest side of a hurricane is the north eastern side, this is very concerning indeed. During Hurricane Katrina New Orleans was hit by the weaker western side, which meant far lower winds - though the storm surge generated by the far larger Katrina was around 19ft for the city - and 33ft for Gulfport and Biloxi to the east of her.

Ida isn't Katrina sized, thankfully. The storm surge currently forecast for her is 10-15ft south of New Orleans, and 7-11ft in Lake Pontchartrain which borders New Orleans. Thanks to the new defences installed in NOLA, she is rated to survive a Category 3 hurricane and though the levees may overtop new ground mats installed at the base of the levee walls mean that they should not erode and breach. However a mandatory evacuation has been sounded for everyone outside the levees, and a voluntary evacuation for everyone within New Orleans. The super-dome once again is taking people in for shelter.

There are a lot of unknowns. Will an Eye Wall Replacement Cycle cause Ida to collapse her newly forming eye in an attempt to create a newer, wider one? That would broaden the size of the hurricane causing a greater push of water but weaken its wind-speed dramatically. Will she rapidly intensify as predicted, hitting the Category Four 140mph windspeed NHC forecast earlier today before the downgrade? The next twelve hours should set things in stone. Although currently not looking Katrina catastrophic, Ida does look to be the meanest and most dangerous hurricane in a long while for Louisiana and once again, all eyes are on New Orleans.

Author:  NervousPete [ Sun Aug 29, 2021 13:27 ]
Post subject:  Re: Hurricane Ida

ida approach.jpg

This is bad.

This is probably the worst news I could have woken to.

Yesterday late afternoon and evening Ida seemed to be struggling a little. She was a strong hurricane but she was still a Category 2 and she seemed to be having trouble getting her eyewall together. She was still fighting off the effects of dry air. Her wind-field wasn't so wide. Her pressure was slowly deepening but not at the rate I feared, despite the warm water. I went to bed as hit 970mb, barely a Category 3 storm in pressure and at 105mph winds still a category two in windspeed. I was concerned still, but feeling slightly relieved it wasn't worse as my head hit the pillow. With only eighteen hours left before land I figured she couldn't be more than a strong Category 3 when she hit land. A bad hurricane to be sure, and one that would cause a lot of localised damage, but with a track taking her far enough east of New Orleans to spare her the worst.

This morning I awoke to bad news. Ida's pressure overnight had begun to steadily and speedily drop. The eyewall wrapped up to a pinhole. Her wind-field expanded. She was now a 130mph hurricane, a borderline Category 4. This was very bad. Surge for the immediate area was now forecast at 12-16ft. Surge for New Orleans had increased to 8-12ft. The track had shifted a little east, potentially meaning that Ida's eastern eyewall where the strongest winds are located are going to brush up against the western side of New Orleans. Feeling very concerned, but brushing off the bad news I headed out to have breakfast with a chum.

Two hours later I returned to pretty apocalyptic news.

Hurricane Ida is now a 150mph hurricane. That's just 7mph shy of a Category 5. In history no Category 5 has ever hit Louisiana. Katrina was a Category 5 out to sea pushing a tremendous amount of surge but by the time she made landfall she'd weakened owing to dry air and dropped to a moderate Category 3. She'd also shifted east to place New Orleans on her weaker side. Hurricane Ida has New Orleans on her strong side. She's about to become a Category 5 and she is still strengthening with no weakening forecast until she hits land around six hours from now. The strengthening of Ida is almost unprecedented. She's dropped a full 10mb in one hour. For contrast, the usual hurricane considered to be undergoing rapid intensification drops her pressure at around 3mb an hour. The only saving grace is that her wind-field still hasn't expanded to Katrina size, but is about two-thirds the breadth. However a hurricane hunter recon flight just a few minutes ago has just measured 171mph flight level winds. These winds may have time to mix down to the surface.

Worse yet New Orleans has not been able to undergo a mandatory evacuation. They had three days warning with Katrina but the severity of Ida wasn't really truly hinted at outside of outlying experimental forecast models until after she had passed Cuba - by which point it was too late. Rather than risk an evacuation that could see major traffic jams caused by gas stations running out of fuel, with the potential of many thousands of people being hit by the storm while still in their cars on the evac routes, the mayor instead called for a voluntary evacuation, opened up the Superdome for shelter and hoped for the newly upgraded defences to whether the strong Category 3 Ida looked to be. In my opinion this was a reasonable decision. You cannot evacuate a major city in less than 48 hours, the window of time just was not there. The new levees are stronger, rated for a Category 3, and they are designed in such a way as that although they may be overtopped they should breach. Therefore at the worst flooding was anticipated to be localised in the city and no more than five to six feet. Yesterday a lot of people left New Orleans, but there are currently now more people left in New Orleans than at the time Katrina hit. There have also been reports of residents ignoring evacuation orders south of New Orleans, including sixteen people electing to stay on Grand Isle.

Grand Isle will shortly cease to exist and the window for evacuation is over.

The NHC have released a statement that they anticipate the surge may be higher than forecast if the storm continues to strengthen. They also state that the hurricane will likely increase to over 155mph - possibly higher - making it a Category 5. The current pressure has fallen to 928mb. Ida shows no inclination to weaken, only strengthen and there's no sheer and no dry air in her path. Nothing but fuel. The small cities of Houma and Thibodaux south of New Orleans may be wiped out despite their levees. Whether the New Orleans levees hold is an open question, the only good news being that with Katrina the hurricane approached at such an angle that though she spared New Orleans from the wind her path rammed the waters of the lake north of her in the city. The winds from Ida should push the water from the lake away from New Orleans, with the surge coming from the south this time. However unlike Katrina this time, sadly, the city may see significant wind damage depending on how close the eyewall gets. A 150mph-plus eyewall brushing a major city is not something I want to think about.

In the history of the United States only four hurricanes have ever made landfall as a Category 5. It is incredibly rare. I don't know how this is going to pan out, and thank Christ the track for Ida isn't directly over the city proper, but I think this is going to be very, very bad for New Orleans and central Louisiana. This is historic.

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