Jeff Masters Weather Blog

An early start to the Atlantic’s Cabo Verde season? » Yale Climate Connections

A tropical wave designated Invest 95L, now racing through the central tropical Atlantic, may pose a threat to the Lesser Antilles and the northern coast of South America as early as Sunday, June 30. Although it is very early in the season to be watching waves in this region for development, the record-warm ocean temperatures present across much of the tropical Atlantic give 95L a chance of developing into a named storm this weekend, and there could even be another system on its heels next week. Some models are predicting that 95L will become a dangerous long-track Cabo Verde-type hurricane, though there will be significant obstacles for it to overcome to achieve that.

After emerging off the coast of Africa early this week, 95L has been moving west at 15-20 mph. Satellite images on Thursday morning showed 95L had only a modest area of heavy thunderstorms, but the cloud pattern was beginning to show organization, with some spin developing and several low-level curved bands. 

Conditions were quite favorable for development. Sea surface temperatures were near record warm, about 28 degrees Celsius (82°F), wind shear was a light 5-10 knots, and the atmosphere was moist, with a midlevel relative humidity of 70%. The system was embedded in the Intertropical Convergence Zone (ITCZ) and was relatively close to the equator (10°N), and these factors will slow development. A large area of dry air associated with the Saharan Air Layer (SAL) lies to the north of 95L, and this dry air may become a significant hindrance to its development as approaches the Windward Islands.

The operational versions of the GFS and European model and many of their ensemble members have predicted the development of 95L over multiple cycles, with some operational and ensemble runs suggesting 95L could reach hurricane strength by Sunday or Monday. Moreover, the Thursday morning (6Z) runs of three finer-scale regional hurricane models – the HWRF, HMON, and HAFS-B – all intensified 95L to hurricane strength prior to its reaching the islands; only the HAFS-A run was notably weaker. The models all agreed that 95L would pass through the Windward Islands between Monday morning and Monday night, then enter the Eastern Caribbean.

The environment ahead of 95L is unusually supportive for late June. Along with record-warm sea surface temperatures (see below), wind shear is projected by the SHIPS model to remain light, 5-10 knots, and mid-level relative humidity will remain fairly high, at 60 to 70 percent – though some drying of the atmosphere will likely occur on Saturday and Sunday, according to the SHIPS model.

In their 8 a.m. EDT Thursday Tropical Weather Outlook, the National Hurricane Center gave 95L 2-day and 7-day odds of development of 40% and 70%, respectively. Watches could be required for parts of the Lesser Antilles as soon as late Friday or Saturday if current trends continue.

Beyond the islands, forecast ensembles were in some disagreement on whether 95L might start recurving to the northwest, take a more west-northwesterly track toward the Greater Antilles, or move west into the central Caribbean. A stronger and earlier-developing storm would be more likely to take a more west-northwesterly track, threatening the northern Caribbean islands, while a weaker and slower-developing storm would be more likely to track due west.

The eastern Caribbean is notorious for being a “hurricane graveyard” early in the season, as low-level trade winds are usually divergent (thus acting to weaken showers and thunderstorms) and the prevailing upper-level winds typically produce high wind shear. These factors may not be as storm-disruptive next week as they usually are in late June and early July.

A gif showing where tropical storms form in late June through early AugustA gif showing where tropical storms form in late June through early August
Figure 1. Weekly formation locations (from Jun. 24 to Aug. 4) of all tropical depressions and tropical storms since 1851. In June, only about 1 in 15 tropical cyclones develop in the portion of the Main Development Region (MDR) east of the Lesser Antilles Islands. By early July, though, these odds grow to about 1 in 3. (Image credit: Michael Lowry’s June 26 post on Substack)

Record-warm waters extend from Africa to the Caribbean

Tropical cyclones spawned by west-moving tropical waves emerging from the coast of Africa were traditionally called Cape Verde-type storms. They are more properly called Cabo Verde-type storms now, since their namesake – an island nation off the African coast – is now called Cabo Verde. Many of these storms intensify as they move west across what’s known as the Main Development Region (MDR), which stretches roughly from the west coast of Africa to the Lesser Antilles.

It’s usually July before the upper-level flow and sea surface temperatures in the MDR are conducive to tropical development. This year, however, persistent record-melting temperatures across the Atlantic are helping to give the MDR a slight head start. Sea surface temperatures in the western MDR are running around 28-29 degrees Celsius (82-84 degrees Fahrenheit), which is 1-2°C (2-4°F) warmer than average for late June.

A map of sea surface temperatures showing that the Atlantic is very warm.A map of sea surface temperatures showing that the Atlantic is very warm.
Figure 2. Departures from average sea surface temperature (degrees Celsius) as of June 25, 2024. Most of the tropical North Atlantic, including the Main Development Region (outlined in black) between the Caribbean and Africa, was running 1°C to 2°C (2-4°F) warmer than average for this time of year. (Image credit: National Hurricane Center)

Another threat may follow 95L

Longer-range runs of the GFS and European models indicate that another wave close behind 95L could develop early next week as it approaches the Lesser Antilles, perhaps reaching the islands on Wednesday. Such development could be quashed if 95L does strengthen and the two systems end up being close enough to interfere with each other.

Meanwhile, a tropical wave designated 94L in the northwest Caribbean could organize gradually as it heads toward the Yucatan Peninsula. Any development should be modest, but 94L would have another chance for brief development as it moves into the Bay of Campeche over the weekend on a track similar to that of Tropical Storm Alberto. In both cases, strong wind shear and land interaction seem likely to overwhelm the very warm sea surface temperatures and moist atmosphere present. A hurricane-hunter mission into 94L has been tentatively set for Friday afternoon. In their 8 a.m. EDT Thursday Tropical Weather Outlook, the National Hurricane Center gave 94L 2-day and 7-day odds of development of 10% and 30%, respectively.

The next two names on the Atlantic list of storms are Beryl and Chris. The 2024 hurricane season got off to its slowest start since 2014 on Jun. 19, when Tropical Storm Alberto formed. The typical formation date of the season’s first named storm is Jun. 20, and the second, Jul. 17. The record-earliest formation date of the season’s second named storm came on May 16, 1951, when Tropical Storm Able formed.

A slow start to tropical cyclones across most of the Northern Hemisphere

Apart from the Atlantic, most of the Northern Hemisphere tropics should remain unusually calm over the next few days. As of June 26, the Eastern Pacific had yet to see a single tropical cyclone – not even a tropical depression, much less a named storm. In 59 years of satellite-era records dating back to 1966, the only later start to the Eastern Pacific season was just last year, when the eventual Hurricane Adrian first became a tropical cyclone on June 27, 2023. The latest “A” storm on record was Ava, which formed on July 3, 1969, following two tropical depressions about a month earlier. No development is predicted in the Eastern Pacific for at least the next week, so both of these records appear likely to be broken.

According to statistics kept by Colorado State University, the Northern Hemisphere has produced only three named storms and just one hurricane-strength system so far this year. This compares to the average values through June 26 (across the period 1991-2020) of 9.1 named storms and 4.0 hurricane-strength systems. Accumulated cyclone energy for the entire hemisphere was at a paltry 12.4, compared to the average to date of 61.1.

Important caveat: early-season activity has little to no correlation with peak-season activity, so it’s best not to assume that the forecasts of a hyperactive 2024 Atlantic season are out to lunch.

Updates on this system will occur within this post daily until our next full post.

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