A tropical wave in the central Caribbean designated 99L by the National Hurricane Center (NHC) is expected to develop into a tropical depression by this weekend and track into the Gulf of Mexico by Sunday. This system has the potential to intensify into a tropical storm or hurricane that will hit the U.S. Gulf Coast next week.

Satellite images on Wednesday showed that 99L had a modest area of heavy thunderstorms that were slowly growing more organized. Conditions were favorable for development, with warm waters of 28.5 degrees Celsius (83°F), moderate wind shear of 10-20 knots, and a moist atmosphere with a mid-level relative humidity of 70%.

99L had an elongated area of heavy thunderstorms, from the southwest near the coast of South America to the northeast over the central Caribbean. The future track and intensity of 99L are difficult to forecast until it becomes clear where along this axis 99L consolidates. Preliminary indications early Wednesday afternoon pointed to the northeastern area, which implies more long-range threat toward the east (Louisiana) and less toward the west (Texas). The first hurricane hunter mission into 99L is scheduled for Thursday afternoon.

Figure 1. Track forecasts out to seven days for 99L from the 6Z (2 a.m. EDT) Wednesday, August 25, 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 99L. There was high uncertainty in the landfall location, but most of the strong members predicted a U.S. Gulf of Mexico landfall. (Image credit: Tropical Tidbits)

Forecast for 99L

The outlook calls for 99L to likely bring rains of 1-3 inches to Jamaica, the Cayman Islands, and northeastern Honduras on Thursday and Friday as the system moves west-northwest to northwest at 10-15 mph. Heavier rains will affect Mexico’s Yucatan Peninsula and western Cuba on Friday afternoon through Saturday. One of the top three models for predicting tropical cyclone genesis, the GFS, predicted with its Wednesday morning run that 99L would become a tropical depression by Saturday. The other two, the UKMET and European models, predicted that 99L would develop into a tropical depression by Monday in the Gulf of Mexico.

Passage over the Yucatan Peninsula or western Cuba is likely to cause some disruption of 99L, slowing the system’s development once it enters the Gulf of Mexico. However, Hurricane Grace last week managed to intensify into a category 3 hurricane after emerging from the Yucatan Peninsula as a tropical storm, and 2017’s catastrophic category 4 Hurricane Harvey was not even a tropical cyclone when it moved off the Yucatan. 99L may have similar conditions favoring intensification, as sea surface temperatures are very warm in the Gulf, 29-31 degrees Celsius (84-88°F), and warm waters extend to great depth, giving the system plenty of ocean heat to power it (Figure 2). The likely very moist atmosphere will also favor intensification.

The main question is how much wind shear 99L will encounter. The 12Z Wednesday run of the SHIPS model was predicting high wind shear of 15-25 knots over the Gulf of Mexico Sunday-Tuesday, which would keep any development slow. However, some of this predicted shear may be from strong upper-level winds flowing out from an Eastern Pacific tropical disturbance (95E): If this system develops less than expected, wind shear over the Gulf of Mexico could be significantly less, favoring a stronger 99L.

The bottom line is that residents of the U.S. Gulf Coast should anticipate the potential for 99L to develop into a tropical storm or hurricane that will make landfall by Tuesday. Some long-range model runs show steering currents for 99L collapsing after the storm makes landfall, resulting in an extended period of heavy rains over the Gulf Coast states. This will be capable of causing major flooding, since soil moisture is in the 90th percentile or higher over most of this region.

In its 8 a.m. EDT Wednesday Tropical Weather Outlook, the NHC gave 99L 2-day and 5-day odds of development of 40% and 80%, respectively. The next name on the Atlantic list of storms is Ida, although there is a chance that name may get taken first by a system further east in the Atlantic (see below).

Figure 2. Ocean Heat Content (OHC) levels on August 25, 2021. Much of the western Caribbean and Gulf of Mexico had OHC values in excess of 90 kilojoules per square centimeter (yellow colors), which are highly favorable for rapid intensification of tropical cyclones. Near-shore waters along the continental shelf are too shallow to have a relevant number for OHC, and are left white in this image. The highest heat content waters were in an eddy that had broken free from the Loop Current. (Image credit: University of Miami RSMAS)

Busy peak portion of Atlantic hurricane season expected

The peak portion of the Atlantic hurricane season is at hand, and indications are that the next two weeks leading up to the climatological peak date of September 10 will be busier than usual. The Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the equator that moves around the globe in 30 – 60 days, is expected to be in a position that favors tropical cyclone formation in the Atlantic over the next week or two (see Tweet by Eric Webb below).

For the next six weeks, look for a host of tropical disturbances capable of developing into tropical depressions to roll off the coast of Africa or form in the subtropics (like Henri did). There are two other tropical disturbances being tracked by the NHC that warrant attention today, and long-range model runs show several new tropical systems with the potential to develop will likely come off the coast of Africa next week.

Disturbance 97L in the central Atlantic expected to develop

In the central Atlantic, about 800 miles southeast of Bermuda, lies a poorly-organized tropical disturbance designated 97L. Conditions are favorable for 97L to develop this week, with sea surface temperatures of 29 degrees Celsius (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%, slowing development. The system has good model support for development by this weekend, with a predicted track that will turn 97L more to the east, potentially bringing it near the Azores Islands on Monday or Tuesday. NHC, in an 8 a.m. EDT Wednesday Tropical Weather Outlook, gave 97L 2-day and 5-day odds of development of 30% and 80%, respectively. The next name on the Atlantic list of storms after Ida is Julian.

Disturbance 98L in the eastern Atlantic not a threat to land

In the far eastern Atlantic, a few hundred miles southwest of the Cabo Verde Islands, lies a tropical wave designated 98L. Over the next few days, 98L will be moving west-northwest at 10-15 mph over warm waters with sea surface temperatures (SSTs) of 27-28 degrees Celsius (81-82°F), and some modest development of this system may occur until wind shear grows prohibitively high this weekend. NHC, in an 8 a.m. EDT Tropical Weather Outlook, gave 98L 2-day and 5-day odds of development of 20% and 30%, respectively. 98L is likely to be a concern only to marine interests.

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...