Re: Remembering Katrina / Monitoring Ida
Posted by Chris in Tampa on 8/28/2021, 7:51 pm
Ida is strengthening, but also growing in size. Both are very important.

From 4pm CDT Saturday NHC discussion:

"Earlier aircraft and satellite wind data indicate that the
tropical-storm-force wind field has continued to expand over the
eastern semicircle and the initial wind radii have been adjusted
outward."

A larger storm can generate more surge than a smaller storm. Katrina was stronger earlier on and was large. When it weakened, in terms of wind, it already had pushed a lot of water toward the coast. The surge in that storm was higher than it would have been had the storm not been as strong earlier. And again, it was large.

Hurricane Ike in 2008 ( https://en.wikipedia.org/wiki/Hurricane_Ike ) was a 110mph hurricane when it made landfall in Texas. It was not a major hurricane landfall at landfall in terms of wind, but it did this:


Bolivar Peninsula after Ike. Other pictures: https://www.weather.gov/hgx/projects_ike08_bolivar2

It did it because the wind field was so large. It was stronger earlier on during its life and was not stronger than a category two while over the Gulf. Track:
http://hurricanecity.com/models/models.cgi?basin=al&year=2008&storm=09

With Ida, some of the energy is going toward growing the size of the storm rather than the intensity going up even higher faster, but that isn't a good thing either.

I don't know what Ida will be at landfall, but the storm continues to grow increasingly more and more dangerous because of its increasing wind and size.





The next part is more technical and I kind of hate discussing this part while a storm is so close to landfall because I don't want people to consider raw and/or early information to previous storms as Ida isn't done strengthening. It's going to continue up until landfall. Also, coastline is really different in terms of surge. A lot of complex factors.

While Ida is currently strengthening, because the pressure continues to drop, it can take longer for the surface wind speed to increase in a larger storm than a smaller storm. But again, a larger storm can product a higher surge than a smaller one, so it's still bad.

It's hard to talk about a storm so close to landfall and what's impacts might be. People needed to evacuate for this storm and the time to do that is wrapping up.

One method I like to look at in a storm is the Surge Destructive Potential (SDP). The HurricaneCity best track page for a storm makes a rough calculation based on the simple wind field details. Better analysis would attempt to determine the size of the wind field much more closely using various data sources. That used to be available free but is now done by a private company. Some data from earlier today is on their Twitter feed but it doesn't include the SDP on it:

https://twitter.com/hwind

In the best track system for Ike in 2008:
http://hurricanecity.com/models/models.cgi?basin=al&year=2008&storm=09&display=best_track
The SDP was around 5.1 at landfall. Again, a rough estimate of SDP.

For Ida the value was 2.5 as of 2pm EDT:
http://hurricanecity.com/models/models.cgi?basin=al&year=2021&storm=09&display=best_track
Again, a rough estimate.

To compare some of that data to the RMS HWind images on their Twitter feed, you can do so by turning on "Integrated Kinetic Energy" option in the best track system for a storm at the top of the page. Direct link for Ida:
http://hurricanecity.com/models/models.cgi?basin=al&year=2021&storm=09&display=best_track&simple=&city_distances=&ike=1

You can then compare the terajoule (TJ) rating to other storms. Again, this too is a rough calculation. RMW HWind calculates the data based on recon for example and gets a much better idea of the wind field. The calculations I use assume a uniform wind field in a quadrant. (It's from an equation on NOAA's site, but still a rough estimate: https://www.aoml.noaa.gov/hrd/ike/Calculator_AllQuad.php)

Again, I hate making this kind of comparison now because the storm is still strengthening. I definitely don't want to give people the wrong idea. But we discuss the technical stuff on this board sometimes. I don't understand the details, but I do like comparing things sometimes. And the SDP scale is 0 to 6 so it's kind of like the wind scale in a way.

Also, the storm continues to grow, so best track information is already about 6 hours old. (as is the calculations)

Form the NOAA page:

"The SDP box is the Surge Destructive Potential Rating based on a scale of zero to 6. SDP is interpreted similar to the Saffir SImpson scale in that the most destructive storms are rated above 5 and the least destructive storms < 1."

Again, no one needing to evacuate should pay attention to this part. Don't make any decisions based on this raw data as the storm will be stronger and larger tomorrow.

Also, in terms of the eyewall replacement cycles. (ERC) Those can be bad too. While it might temporarily pause strengthening, even weaken some, the intense winds expand. You get a double eyewall and then inner one collapses. The second eyewall will be at a greater radius. So that means the intense winds will be over a larger diameter. So that too is mixed, strengthening might take a pause, but more people will feel the intense winds. And then once an ERC is complete it can then continue to strengthen more rapidly. I can't recall if some storms strengthen while they undergo an ERC. I forget. So don't always assume that the strengthening will pause or even weaken during that time.

This all gets back to every storm is different. Sometimes you don't necessarily know how bad a storm actually is until it's over. Then it's too late. You have to go with the information you have and the NHC is doing a good job on that with Ida.
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Remembering Katrina / Monitoring Ida - 1969, 8/28/2021, 11:06 am
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