Tropical Depression Thirteen (TD 13), which formed in the central Atlantic on the evening of Wednesday, August 19, poses a threat as a tropical storm this weekend to the Leeward Islands, Virgin Islands, and Puerto Rico. TD 13 could affect the Bahamas and Florida as a hurricane on Sunday and Monday, respectively. A Tropical Storm Watch was up for most of the northern Leeward Islands on Thursday.

At 11 a.m. EDT Thursday, August 20, TD 13 was located about 750 miles east of the northern Leeward Islands and headed west-northwest at 21 mph. Conditions for development were favorable, with sea surface temperatures (SSTs) near 28 degrees Celsius (82°F) and light wind shear of about 5 knots. However, the system had moved into a drier region of the atmosphere, with a mid-level relative humidity of 60%, and this dry air was interfering with development. Satellite images showed TD 13 with a modest amount of heavy thunderstorm activity slowly growing in organization and areal coverage.

Heavy rain threat to Leeward Islands, Virgin Islands, Puerto Rico

A west-northwest motion will take TD 13 through or just north of the Leeward Islands Friday evening through Saturday, and the system is predicted to bring the islands 1 – 3 inches of rain, with isolated amounts up to 5 inches. The Virgin Islands and Puerto Rico are likely to see higher rainfall amounts of 3 – 6 inches through Sunday. With the islands on the weaker (left) side of TD 13’s circulation, wind damage should be limited. For a fast-moving system like TD 13, winds on the right (north) side likely will be at least 20 mph higher than those on the left (south) side.

The Bahamas and Florida at highest risk early next week

Assuming that TD 13 develops into a tropical storm and does not remain weak, a more northerly track is likely, which would keep the system north of the high mountains of Hispaniola and Cuba. On that track, the Turks and Caicos Islands, Bahama Islands, and Florida are at highest risk of impacts early next week. The Bermuda high, which is steering TD 13, will be strong and will extend far to the west, though a weakness in the high may allow TD 13 to turn more to the northwest on Monday, when it will be near Florida. At that time, TD 13 may have to contend with higher wind shear from a trough of low pressure over the U.S., which might interfere with intensification. Until that time, wind shear is predicted to be a light to moderate 5 – 15 knots. Ocean temperatures will steadily warm over the next five days, reaching a very warm 30 – 31 degrees Celsius (86 – 88°F) by Monday. Warm waters extend to great depth over the Bahamas, giving the ocean a high heat content ideal for fueling rapid intensification.

A NOAA hurricane hunter aircraft is scheduled to investigate TD 13 Thursday evening. The aircraft will be feeding data from its Doppler radar in real time to the HWRF model, the only model currently configured to use this important source of extra data to make forecasts.

Most top intensity models predict TD 13 will become major hurricane and threaten Florida

The five main intensity models used by NHC are the regional/dynamical HWRF, HMON, and COAMPS-TC models (which subdivide the atmosphere into a 3-D grid around the storm and solve the atmospheric equations of fluid flow at each point on the grid), and the statistics-based LGEM and DSHP models. Two of the top-performing global dynamical models for tracking hurricanes – the European (ECMWF) and GFS models – typically are not considered by NHC forecasters when making intensity forecasts, and both generally make poor intensity forecasts.

In 2019, the official NHC forecast did the best job predicting intensity, except for 5-day forecasts, which the COAMPS-TC model did a better job at. (See this review I wrote on hurricane model performance in 2019 for Yale Climate Connections.) Thus, NHC’s four-day and five-day intensity predictions that TD 13 will be a category 1 hurricane on Monday, when it makes its closest approach to Florida, is worth respecting. However, it is concerning that the 6Z Thursday run of the COAMPS-TC model, which outperformed the official NHC forecast at 5-day forecasts last year, predicted a category 3 hurricane as of Monday, August 24. In addition, two of the other top intensity models, the HWRF and HMON, also were predicting that TD 13 would be a category 3 hurricane on Monday. The other top intensity models, the LGEM and DSHP models, were predicting that TD 13 by then would be no more than a strong tropical storm.

Bottom line: the official NHC forecast of a category 1 hurricane near Florida on Monday has high uncertainty, but residents should anticipate that TD 13 could be a major hurricane that will hit Florida that day. That said, it is also possible TD 13 could be a disorganized tropical storm at that time, if significant land interaction with Cuba and Hispaniola occur.

The next two names on the Atlantic list of storms are Laura and Marco. The earliest twelfth storm in Atlantic tropical history was Luis on August 29, 1995. There is a tie for earliest thirteenth storms, with Lee on September 2, 2011, and Maria on September 2, 2005. (Lee was originally the twelfth storm of the 2011 season, but an unnamed system that reached tropical storm strength on September 1, just before Lee, was discovered in post-season analysis.)

Tropical Depression 14 forms in the central Caribbean

A tropical wave in the central Caribbean, which NHC earlier this week designated 97L, developed into Tropical Depression Fourteen (TD 14) at 11 a.m. EDT Thursday, August 20. TD 14 was headed west at 21 mph with top winds of 35 mph, and a Tropical Storm Watch was posted for the northern coast of Honduras.

Dry air, which had been the bane of this system earlier this week, had diminished on Thursday morning, and TD 14 was in a relatively moist atmosphere with a mid-level relative humidity of 65%. Conditions for development were favorable, with SSTs near 29.5 degrees Celsius (85°F) and light wind shear of 5 – 10 knots. Satellite images showed that TD 14 had an impressive amount of heavy thunderstorm activity steadily growing in areal coverage and organization. An Air Force hurricane hunter aircraft is scheduled to investigate TD 14 Thursday afternoon.

Forecast for TD 14

As TD 14 progresses west-northwestward through Saturday, its forward speed will slow as a result of the steering influence of a large trough of low pressure over the central U.S. The upper-level southwesterly winds ahead of this trough will turn TD 14 more to the northwest, and the system is likely to pass over Mexico’s Yucatan Peninsula on Saturday night and enter the Gulf of Mexico on Sunday.

Approaching the Yucatan Peninsula on Friday, TD 14 will find very favorable conditions for development. The 12Z Thursday run of the SHIPS model predicted that the atmosphere surrounding the system would moisten to a relative humidity of 75%, wind shear would be a light 5 – 10 knots, and SSTs would be a very warm 30 degrees Celsius (86°F). The waters of the western Caribbean have the highest heat content of any place in the Atlantic, providing ample fuel for any tropical cyclone that passes across. These conditions likely will allow TD 14 to be near category 1 hurricane strength at landfall Saturday in the Yucatan Peninsula.

Crossing the Yucatan will weaken TD 14, and it may take a day for the storm to reorganize over the warm waters of the Gulf of Mexico, where SSTs are a very warm 30 degrees Celsius (86°F). An upper-level trough of low pressure over the Gulf at that time will bring dry air and high wind shear to TD 14, limiting how much re-intensification can occur. The long-range intensity forecast is uncertain, but TD 14 could be a strong tropical storm or a category 1 hurricane when it makes its expected landfall on Tuesday along the U.S. Gulf Coast.

Next up: a new tropical wave off the coast of Africa

A strong tropical wave located near the coast of Africa on Thursday morning has the potential to develop into a tropical depression this weekend or early next week. The disturbance will move west-northwest at 15 – 20 mph over the tropical Atlantic, passing to the north of the Lesser Antilles Islands around Wednesday, August 25. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave the new African tropical wave two-day and five-day odds of formation of 20% and 40%, respectively.

Hurricane Genevieve scrapes the Baja Peninsula

Still clinging to Category 1 strength, Hurricane Genevieve passed dangerously close to the southern Baja Peninsula of Mexico on Thursday, August 20. As of 8 a.m. PDT Thursday, Genevieve was centered about 50 miles southwest of the peninsula, about halfway between Magdalena Bay and Cabo San Lucas and moving northwest, parallel to the peninsula, at 12 mph. The center’s closest approach to land may be when it passes just west of Magdalena Bay and the town of Puerto San Carlos on Thursday afternoon before angling slightly toward the open northeast Pacific.

The most reliable hurricane models, according to their 2019 performance

Intense rainbands were pushing across the southern Baja Peninsula on Thursday. Widespread 2 – 4 inch rains are expected, with a foot or more possible at higher elevations. Sustained winds could briefly hit tropical storm strength (39 mph or more). At 8 a.m. PDT, winds were 25 mph at the Cabo San Lucas International Airport, and 28 mph at the Manuel Marquez de Leon International Airport in La Paz.

Genevieve has weakened steadily since attaining its peak category 4 on Tuesday, August 18. Its track is bringing it over substantially cooler water, and the rugged terrain of the peninsula is disrupting flow around the northeast side of the storm. Genevieve will also be ingesting drier air over time. As a result, further weakening is expected, and Genevieve will likely be a tropical storm by late Thursday and a tropical depression by Saturday.

Bob Henson contributed the Hurricane Genevieve portion of this post.

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