27 August 2021

Ida expected to become major hurricane before Gulf coast landfall

Invest 99L in the central Caribbean was upgraded to Tropical Depression 9 on Thursday morning, then again to Tropical Storm Ida on Thursday afternoon.  Ida is the season's ninth named storm, and formed 17 days ahead of the average formation date of the ninth named storm (using the 1991-2020 "climate normal").  Ida will make landfall very close to where Katrina did sixteen years ago, and on the same date: August 29th.

It is currently a mid-range tropical storm located between the Cayman Islands and western Cuba, but is forecast to become the season's fourth hurricane once it enters the Gulf of Mexico on Saturday.

Then, there does not appear to be much of anything in its way... and rapid intensification to a major hurricane is certainly possible prior to landfall on Sunday.  There are some statistical models that use environmental conditions and historical data to create a probability of rapid intensification, and the numbers are exceptional for Ida.  First, "rapid intensification" is conventionally defined to be an intensity increase of at least 35 mph in a day.  That rate is way out on the wings of what storms can do... the top 5% or so.  That model is showing a 28% probability of the intensity increasing by 65 mph in the next two days, which is an incredible 6x more likely than climatology.

NHC is currently forecasting Ida to reach major hurricane status (Category 3+) by midday Sunday, and make landfall shortly thereafter in Louisiana.  Storm surge will be a huge threat to the east of where the center makes landfall, and early surge forecasts indicate the potential for 7-11 feet in eastern Louisiana, but that will be refined with each new forecast cycle.

In addition, heavy rain will affect a much wider area, and much farther inland. Much of Louisiana and Mississippi could see flooding rains, and then up into Tennessee where a major flash flood disaster just occurred last Saturday.

As I've mentioned several times before, "I" storms have a bit of infamy associated with them... that letter happens to be the most frequently retired since storms were first given names in 1953.  Over that time period (1953-2020), the average date of the ninth named storm formation is September 26th, certainly still in the prime of the season.  Curious what those 11 storms were? Check out https://twitter.com/BMcNoldy/status/1430840243066863617

Elsewhere, there's a strong easterly wave tagged as Invest 98L that's located about 1000 miles east of the Lesser Antilles. It's likely to become the season's next named storm, Julian, this weekend.  Model guidance shows it turning to the north well before reaching the islands though, and remaining in the open central Atlantic.

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