A low-pressure system over the western Caribbean centered near the northeastern tip of Mexico’s Yucatan Peninsula on Saturday afternoon, designated Invest 93L, was bringing heavy thunderstorms to western Cuba and the Yucatan Peninsula. This system was close to tropical depression status, and is likely to develop into a tropical depression later Saturday, becoming a tropical storm by Monday as it moves slowly northward into the Gulf of Mexico. Update (5 p.m. EDT Saturday): 93L has been upgraded to Tropical Depression 10, centered about 65 miles northeast of Cozumel, Mexico, with top sustained winds of 30 mph. TD 9 is now predicted to become Tropical Storm Idalia by midday Sunday and Hurricane Idalia by Tuesday as it moves into the eastern Gulf of Mexico.

Satellite imagery and radar loops from the Cancun radar Saturday afternoon showed 93L was steadily organizing. Low-level spiral bands were forming, heavy thunderstorm activity was increasing, and high cirrus clouds were streaming away from the storm along its north and south sides, indicative of upper-level outflow.

Track forecast for 93L

Steering currents are currently weak for 93L, with a slow northward drift expected through Sunday. On Monday, a trough of low pressure passing to the north will grab the disturbance and pull it more rapidly northward into the southern Gulf of Mexico. By Tuesday, a more northeasterly track may occur, though the models are in disagreement about this. In any case, a large swath of Florida Gulf Coast can expect heavy rains of three to five inches, with a higher core of heavier rains near where the center of the system tracks. Lesser rains of perhaps one to three inches can be expected along the coasts of Alabama, Mississippi, and southeastern Louisiana.

Until 93L forms a well-defined center of circulation, predicting where the storm will make landfall will be difficult. And since 93L is expected to approach Florida at an oblique angle, slight changes in the steering currents will result in large differences in where landfall occurs. The 6Z Saturday hurricane models had landfall locations ranging from southwest Florida near Fort Myers (HWRF model), to the Big Bend area between Cedar Key and St. Marks (HAFS-A and HAFS-B models), to the Panhandle near Apalachicola (HMON model).

93L is expected to take a northeasterly track somewhere near the U.S. Atlantic coast on Wednesday. If 93L does not re-emerge into the Atlantic, heavy rains of two to four inches can be expected along a swath of the coast of northeastern Florida, Georgia, South Carolina, and North Carolina. If the storm takes a more southerly track and crosses the Florida Peninsula, it will emerge over the Atlantic and will likely re-intensify, potentially bringing heavy rains of six inches or more along the Southeast U.S. coast later in the week.

Figure 1. Predicted 7-day precipitation amounts ending at 8 a.m. EDT Thursday August 31, 2023. 93L is expected to bring three to five inches of rain across Florida, Georgia, South Carolina, and North Carolina. (Image credit: NOAA)

Intensity forecast for 93L

Conditions for development over the western Caribbean are favorable, with warm ocean temperatures near 28.5 degrees Celsius (83°F), low wind shear of 5-10 knots, and a reasonably moist atmosphere (a mid-level relative humidity near 65%). The main impediment to development appears to be the potential for the center of circulation to be over land — specifically, the Yucatan Peninsula. However, the peninsula is mostly flat, and much of 93L’s circulation would remain over water. On Monday, when 93L is predicted to enter the Yucatan Channel between Cuba and Mexico and move into the southern Gulf of Mexico, land interference will fade and intensification chances increase.

Two factors will be very favorable for development in the Gulf: waters are record-warm, near 31 degrees Celsius (88°F), and these warm waters extend to an astonishingly great depth – around 40-50 meters or 130-165 feet (see Tweet above). In addition, a strong upper-level outflow channel will be available to the north, where a jet stream maximum will be present in the trough of low-pressure steering 93L. The main limiting factors for development on Monday and Tuesday appear to be dry air in the western Gulf and some moderate wind shear. These negative appear relatively modest, though.

The models generally show 93L peaking as a strong tropical storm or low-end category 1 hurricane before landfall; the 12Z Saturday run of the SHIPS model gave a 0% chance of rapid intensification. But given the lack of any obvious highly detrimental impediments to intensification, the current lack of rapid intensification forecasts should not make us complacent. There have been far too many cases in the past of western Caribbean disturbances entering the Gulf which ended up far exceeding model expectations. This includes Hurricane Michael from 2018, which also originated near the Yucatan Peninsula. Just 72 hours ahead of its Florida Panhandle landfall, while it was still a tropical depression near the Yucatan, Michael was predicted to peak at category 1 strength, whereas it ended up striking as a category 5. While the most probable outcome is that 93L will be a strong tropical storm at landfall, we should not be surprised if it makes landfall as a rapidly intensifying hurricane.

In their 8 a.m. EDT Saturday Tropical Weather Outlook, NHC gave 93L two-day and seven-day odds of development of 70% and 90%, respectively. A hurricane hunter mission is scheduled for Sunday afternoon into the system, along with a mission by the NOAA jet. The next name on the Atlantic list of storms is Idalia.

93L may fan wildfires in Louisiana

A hazard we will likely see more of in coming years because of climate change will be wildfires fanned by the winds of a tropical cyclone making landfall some distance away. A hotter atmosphere holds more water vapor. This forces plants to give up more of their moisture and dry out, increasing wildfire risk. In regions along the periphery of a landfalling hurricane or tropical storm where rain bands do not extend to, the winds of the storm will potentially fan any wildfires that are burning. The difference in pressure between the storm and high pressure some distance away can also drive strong winds far from the storm’s center, contributing to wildfire spread. (It’s unclear how much this may have affected the recent catastrophic wildfire in Maui, since Hurricane Dora was more than 800 miles away from Hawai’i, but sinking air induced by Dora may have indirectly enhanced the strong high-pressure gradient that helped drive the winds at Lahaina, Maui.)

Record heat has gripped much of the Gulf Coast this summer, and severe to exceptional drought has developed over much of Louisiana and coastal Mississippi, with moderate drought over coastal Alabama and the western Florida Panhandle. There is one large wildfire burning in Louisiana, the 31,000-acre Tiger Island Fire near the border with Texas. However, this location is too far from the expected landfall location of 93L in Florida to receive winds from the storm. Should any new wildfires develop closer to the storm, though, its winds could fan those fires. The most recent fire weather forecast from the NOAA Storm Prediction Center called for potentially “critical” fire weather conditions over Louisiana because of strong northerly winds caused by the difference in pressure between 93L and high pressure to the west.

Franklin becomes a hurricane

In the waters a few hundred miles north of Puerto Rico, we now have Hurricane Franklin, which became a category 1 storm with 75 mph winds at 11 a.m. EDT Saturday. Franklin is predicted to intensify into the season’s first major hurricane by Tuesday morning, when it is predicted to make its closest approach to Bermuda, about 250 miles to the west. Franklin will bring high surf and the threat of rip currents to the U.S. East Coast and Canadian Maritime Provinces during much of the coming week, but the hurricane is not expected to make a direct landfall.

Two other systems in the remote central and eastern Atlantic have less-than-50-percent odds of development over the next 7 days, according to the Tropical Weather Outlook issued by the National Hurricane Center at 8 a.m. EDT Saturday.

Bob Henson contributed to this post. Website visitors can comment on “Eye on the Storm” posts (see comments policy below). Sign up to receive notices of new postings here.

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...