Tropical Depression Nine (TD 9) formed in the central Caribbean on Thursday morning and is expected to intensify into a hurricane after it passes western Cuba and enters the Gulf of Mexico Friday night. The National Hurricane Center is predicting that TD 9 will rapidly intensify Saturday and Sunday and be near major hurricane strength as it approaches landfall along the central U.S. Gulf Coast on Sunday.

This is an unusually dangerous situation: There is limited time for preparation, the storm may be rapidly intensifying right up until landfall, and there is high uncertainty on how strong the storm may get and exactly where it will track. The next name of the Atlantic list of storms is Ida.

On Thursday afternoon, satellite images and radar from Grand Cayman showed that TD 9 had a vigorous area of heavy thunderstorms that were steadily growing more organized and more intense. There was uncertainty about where the surface center of TD 9 lay, and the first hurricane hunter mission into TD 9, scheduled for Thursday afternoon, should help determine the location of the system. (The mission was delayed for over an hour by heavy thunderstorms at their base in Mississippi, but is now en route.)

Conditions were favorable for development, with warm waters of 29 degrees Celsius (84°F), moderate wind shear of 10-15 knots, and a moist atmosphere with a mid-level relative humidity of 70%.

Figure 1. Track forecasts out to 10 days for TD 9 from the 0Z Thursday, August 26, run of the European ensemble model (EPS). The 51 individual ensemble member forecasts are color-coded by the central pressure they predict for TD 9; a central pressure of 960-979 mb (red colors) is a common one for a category 2 hurricane. Many of the members predicted a landfall in Texas or Louisiana as a hurricane. (Image credit: weathermodels.com)
Figure 2. Track forecasts out to seven days for TD 9 from the 6Z (2 a.m. EDT) Thursday, August 26, run of the GFS ensemble model (GEFS). The black line is the mean of the 31 ensemble members; individual ensemble member forecasts are the thin lines, color-coded by the central pressure they predict for TD 9. Most of the members predicted a Louisiana landfall as a hurricane. (Image credit: Tropical Tidbits)

Track forecast for Tropical Depression Nine 

A ridge of high pressure centered near the Southeast U.S. coast will keep TD 9 moving on a path to the northwest at 10-15 mph for the next three days. The main uncertainty in the track at present lies in the initial position of the storm; once the Hurricane Hunters locate the exact center on Thursday afternoon, the future track of the system will grow more certain. Though Louisiana is at highest risk of a direct strike, a landfall on the Mississippi or northeast Texas coast cannot be ruled out.

Figure 3. Predicted precipitation from NOAA’s HAFS-B model for the 6-day period ending at 2 a.m. EDT Wednesday, September 1. (Image credit: NOAA/AOML)

Tropical Depression Nine expected to dump dangerous flooding rains

TD 9 is embedded in a moist airmass, and its modest forward speed of 10-15 mph over the next three days will make it a prolific rainmaker. The National Hurricane Center is predicting dangerous flooding rains of 6-10 inches for Jamaica; 8-12 inches are expected for the Cayman Islands, western Cuba, and the northeast Yucatan Peninsula. On Thursday afternoon, TD 9 was bringing heavy rains to Jamaica; Kingston recorded 5.65 inches of rain in the 24 hours ending at 8 a.m. EDT Thursday. Winds in Kingston were sustained at 29 mph, gusting to 40 mph, at 10 a.m. EDT Thursday.

Steering currents are predicted to weaken on Monday after TD 9 makes landfall on the U.S. Gulf Coast. The storm is likely to slow down to a forward speed of just 5-10 mph Monday through Wednesday, resulting in an extended period of downpours along the U.S. Gulf Coast. NOAA’s experimental HAFS-B model, which has been successful in making rainfall forecasts for hurricanes over the past two years, predicted with its 6Z Thursday run that TD 9 would dump rainfall amounts in excess of 10 inches along about a 150 mile-wide swath of the U.S. Gulf Coast, with a few isolated areas of 20 or more inches (Figure 3). This precipitation will be capable of causing major flooding, because soil moisture is in the 80th percentile or higher over most of this region (Figure 4).

Figure 4. Soil moisture expressed as a percent of normal for August 25, 2021. Soils were above the 80th percentile for moisture along much of the U.S. Gulf Coast. (Image credit: NOAA)

Intensity forecast for Tropical Depression Nine

TD 9 will have very favorable conditions for intensification through Friday night, when passage over the western tip of Cuba may disrupt it. Once TD 9 enters the Gulf of Mexico, conditions will remain very favorable for intensification: Wind shear will be light to moderate, the atmosphere will be moist, and sea surface temperatures will be very warm, 29-31 degrees Celsius (84-88°F). These warm waters extend to great depth, giving the system plenty of ocean heat to power it. TD 9 is predicted to pass near or over a warm eddy in the Gulf of Mexico with unusually high heat content on Sunday, and the heat from this eddy will be capable of fueling rapid intensification of TD 9 (see Tweet by Bob Henson below).

One potential impediment to intensification may be wind shear caused by upper-level outflow from the Eastern Pacific’s Tropical Storm Nora. Nora is predicted to be near hurricane strength Saturday through Monday as it moves along the coast of southwestern Mexico towards the Baja Peninsula, and if the storm is strong enough, its upper-level outflow could bring higher wind shear over the Gulf of Mexico. This wind shear is more likely to affect TD 9 if it is farther to the west, near Texas.

The Thursday morning runs of two of the top intensity models – the HWRF and COAMPS – predicted that TD 9 would rapidly intensify into a major hurricane by Sunday as it approached landfall on the U.S. Gulf Coast. The National Hurricane Center’s 11 a.m. EDT Thursday intensity forecast was an unusually aggressive one (see Tweet by Sam Lillo below), explicitly predicting rapid intensification. The Hurricane Center predicted that TD 9 would top out with 110 mph winds, just below category 3 strength, on Sunday morning. This was still about 12 hours before the predicted landfall time, and TD 9 potentially has enough time over the Gulf to intensify into a category 4 hurricane.

Disturbance 97L in the central Atlantic expected to develop

In the central Atlantic, about 600 miles east of Bermuda, lies a poorly organized tropical disturbance designated 97L. Conditions are favorable for 97L to develop this week, with sea surface temperatures of 28-29 degrees Celsius (82-84°F) and light wind shear of 5-10 knots. The atmosphere surrounding 97L is dry, though, with a mid-level relative humidity of 50%, which will slow development. The system has good model support for development by this weekend, with a predicted track that will take 97L to the east at 5-10 mph, potentially bringing it near the Azores Islands on Tuesday. The National Hurricane Center, in a 2 p.m. EDT Thursday Tropical Weather Outlook, gave 97L 2-day and 5-day odds of development of 50% and 70%, respectively. The next name on the Atlantic list of storms after Ida is Julian.

Disturbance 98L in the eastern Atlantic may also develop

In the eastern Atlantic, about 1,000 miles west of the Cabo Verde Islands, lies a tropical wave designated 98L. Satellite imagery showed the disturbance was more organized on Thursday than on Wednesday, with a modest area of heavy thunderstorms that was increasing in areal coverage and intensity. Over the next few days, 98L will be moving west-northwest at 10-15 mph over warm waters with sea surface temperatures of 27 degrees Celsius (81°F), and some modest development of this system may occur until wind shear grows prohibitive on Tuesday. The National Hurricane Center, in a 2 p.m. EDT Tropical Weather Outlook, upped their 2-day and 5-day odds of development to 60% and 70%, respectively. The next name on the Atlantic list of storms after Julian is Kate.

Bob Henson contributed to this post.

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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...