Here’s why Ida wasn’t upgraded to a Category 5 hurricane
BATON ROUGE, La. (WAFB) - The National Hurricane Center (NHC) released its final report on Hurricane Ida on Tuesday morning. The headline for most in south Louisiana was that NHC maintained landfall at Category 4 intensity, with maximum sustained winds estimated at 150 mph.
Ida’s intensity at landfall has been a source of controversy, particularly for those in south Louisiana who suffered severe damage, since reports of potentially stronger winds emerged from Port Fourchon in the hours and days after the storm made landfall. One such report was even shared by our local National Weather Service office.
Some quick investigative work by Philippe Papin, a Hurricane Specialist at NHC, found that both the 149 mph and 173 mph numbers shown in that image were likely instantaneous gusts. That’s an important point since sustained winds are used to categorize a hurricane’s intensity with the Saffir-Simpson scale. Sustained winds are calculated by averaging wind speeds over the course of 2 minutes. In other words, instantaneous gusts are typically going to be much higher than sustained winds.
Additional internet sleuthing found that the ship observation was likely measured at an elevation higher than the standard 10 meters (~33 feet). Any observations above 10 meters require some measure of reduction since wind speed is known to increase with height.
Additional intrigue appeared when social media accounts representing Port Fourchon shared a graphic claiming a wind gust of 228 mph.
So why is Ida still classified as a Category 4 hurricane with winds that reportedly topped 200 mph? Even in gusts, a reading that high would be supportive of Category 5 intensity. Let’s turn to the official report from NHC for some answers. First, there is this important note in relation to sustained winds:
“The highest reported sustained wind close to the landfall area was 101 kt [116 mph] at 1630 UTC 29 August from a United States Geological Survey (USGS) station located 11 n mi north-northeast of Grand Isle, Louisiana (anemometer elevation unknown).”
A maximum wind speed of 101 knots [116 mph] is actually only supportive of Category 3 intensity, but data from Hurricane Hunter aircraft near landfall, along with the understanding that rarely, if ever, are maximum winds in a hurricane actually sampled, still support the Category 4 intensity. But the lack of anything stronger makes it difficult to support a Category 5 upgrade.
Additionally, NHC directly addresses both of the ship reports from Port Fourchon:
“It should be noted that two ships moored at Port Fourchon reported instantaneous wind gusts of 194 kt [223 mph] and 150 kt [173 mph] as Ida passed over. The details of these reports, including the elevation and exposure of the instruments, are unavailable at this time, and thus they are not considered in the landfall intensity determination or included in the data tables. This report will be updated if detailed data from these ships becomes available.”
So ultimately, NHC has been unable to verify the accuracy of those ship reports in the months since Ida’s landfall. With every storm, there is a concerted effort to only use data that can be verified and considered accurate. But also note that NHC doesn’t close the door on a future upgrade should they eventually obtain more information on those ship reports.
In fact, the door is never completely closed when it comes to assessing a storm’s intensity. Look no further than 1992′s Hurricane Andrew for a prominent example of the never-ending reanalysis process. Originally classified as a Category 4 hurricane at landfall in south Florida, it was a full 10 years later that it was upgraded to Category 5 intensity using new data and better analysis techniques (https://www.aoml.noaa.gov/hrd/hurdat/andrew.html).
In the end, Ida was an historic hurricane for Louisiana and the United States, regardless of exact landfall intensity. The current estimate of 150 mph winds at landfall ties it with Laura (2020) and the ‘Last Island’ Hurricane (1856) for strongest winds in a landfalling storm in Louisiana. Economic losses currently are estimated around $75 billion, making it among the 5 costliest hurricanes on record for the United States.
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