More "I" named storms have been retired than any other. Will Idalia be next?

The hurricane season typically peaks right around Labor Day in the Atlantic basin, and this year appears to be following that trend. As of Monday night, Hurricane Franklin was a powerful Category 4 storm, with winds sustained at 150mph at its center. Thankfully, the National Hurricane Center predicts that the strongest winds and heaviest rain from Franklin will remain over the Atlantic Ocean. That said, all eyes in the continental United States are trained on Tropical Storm Idalia, which is also forecast to become a major hurricane before making landfall in the Gulf Coast. The current forecast track of Idalia takes it into the "Big Bite" on Florida's west coast, an area with a lower population density than the major city of Tampa to the south. But given the recent history of storms beginning with the letter "I", forecasters are not letting their guard down.

Forecast track of Idalia as of 7PM CDT on Monday, August 28th.

(NOAA National Hurricane Center)

As I mentioned in my hurricane season preview blog post back in May, more "I"- named storms, the ninth position in the naming order, have been retired than any other letter. Thirteen "I"- named storms have been retired since name retiring began in 1954 (I'm not counting Iota, since that is a Greek letter, not the ninth named storm of the season). Though it makes sense that more intense and destructive storms would happen later in the year as the ocean heats up, "I" still stands out. The chart below shows all the retired names, broken down by letter. The next highest, "F", has been retired 10 times. "I" truly is an oddity.

However, the timing of the retired storms beginning with the letter "I" is much more intuitive. The Atlantic tropical season peaks on or around September 10th, statistically speaking. Six of the retired "I" storms' lifespans overlap with September 10th, and all but two of the retired storms, Iris (2001) and Irene (2011), were active during the month of September, which by far the most active month of the year. 

Hurricane and Tropical Storm climatology, from the National Hurricane Center

But, to an extent, it's a coincidence; just because Idalia begins with an "I" doesn't mean it will be more dangerous than an "H" or "J"-named storm. The more important factor is Idalia's environment- and unfortunately, that environment is primed and ready for strengthening. 

You might have seen headlines earlier this summer about hot tub-temperature water in the Gulf of Mexico. Those supercharged sea surface temperatures (SSTs) are still in place, setting the stage for a process called rapid intensification, or RI. A tropical system is deemed to have undergone RI when the central pressure drops by at least 24 millibars in 24 hours or less. Why is that important? The central pressure of a storm is almost directly linked to its maximum wind speed. The lower the pressure is inside the storm, the faster the winds tend to be. Super-warm water is the key ingredient for RI. The other elements, wind shear and dry air, are actually detrimental to storm development if they're present. 

ECMWF forecast model depiction of 500mb winds, about 20,000ft above ground level.

Winds in the Gulf of Mexico are weak at this level, except surrounding the area forecasted to include Idalia. 

(College of DuPage Weather)

Wind shear is any change in wind speed or direction with height. If winds are getting stronger through the vertical column of the atmosphere, they disrupt a tropical storm's intensification process. (The opposite is true for midlatitude cyclones, such as Nor'Easters. We'll talk about that after hurricane season!) The subtropical jet stream is the most common source of wind shear during hurricane season.

Forecast animation of near-ground aerosols, such as salt and Saharan dust.

The dust, in red/orange, is not reaching the Gulf of Mexico.

(NASA's Scientific Visualization Studio/NASA Center for Climate Simulation)

Dry air, however, most often comes all the way from the Sahara Desert to impact the Atlantic basin. If the trade winds blow in the right speed and direction, they can inject desert dust into the tropical Atlantic. In the late Spring, Saharan dust was observed as far west as the Florida Keys! But today, neither wind shear nor Saharan air are present over the Gulf of Mexico. And that's why forecasters think that nothing is stopping Idalia from reaching major hurricane status. 

You can get the latest information on Idalia and its forecast path from the National Hurricane Center. For detailed information about impacts to your neighborhood, I recommend finding your local trusted information source, like a local TV station. Most TV meteorologists will answer your questions directly on social media, too. And finally, get tips on hurricane preparedness from start to finish from NOAA's Weather Ready Nation site.