The Atlantic’s first tropical cyclone in the deep tropics formed on Tuesday – Tropical Depression Seven. TD Seven is likely to become a tropical storm over the next few days, but faces a very uncertain future. It may threaten the Windward Islands, but its intensity remains highly uncertain. In addition to TD Seven, Invest 91L has a chance to develop over the Northwestern Gulf of Mexico late this week.

As of 5:00 p.m. EDT Tuesday, Tropical Depression Seven was centered near 9.8°N 40.4°W. Maximum sustained winds were 30 knots (35 mph), with an estimated minimum pressure of 1009 mb. An ASCAT pass on Tuesday morning confirmed that a well-defined, tight circulation has formed, and deep convective organization has now met the definition of a tropical depression. Seven is the first tropical cyclone in the Atlantic basin to form in the Atlantic’s Main Development Region (MDR) in 2020, and the first to form directly from an African tropical wave. The SHIPS guidance indicates that conditions will likely be somewhat favorable during the next 3 days or so, with wind shear light (less than 10 knots) and sea surface temperatures near 28°C (82.4°F). It is interesting to note that the global (UKMET, ECMWF and GFS) models show TD Seven weakening, and possibly opening up into a wave, as it approaches the Windward Islands. This is likely a result of dry air intrusion and an acceleration in the depression’s forward speed late this week. Interestingly, though, they have been trending stronger and longer-lived in recent runs. The 12z HWRF guidance was very aggressive, bringing TD Seven to major hurricane status east of the Lesser Antilles – but at this moment, this scenario seems unlikely. The SHIPS guidance is also somewhat aggressive, making TD Seven a strengthening hurricane over the eastern Caribbean by Day 5. July climatology would strongly favor the global models being right in this case, but TD Seven is a small cyclone, and small cyclones are difficult to forecast intensity.

Although it is a distinct possibility Seven never strengthens significantly, tropical cyclones in the MDR during July are often a harbinger of very active Atlantic hurricane seasons. Normally, during July, the Atlantic MDR is too cool, dry and stable for tropical cyclogenesis to occur. However, in some cases, thermodynamics are anomalously favorable, allowing cyclones – often weak – to form. For instance, in the hyperactive 2017 season, three weak tropical cyclones formed in the Atlantic MDR before August, though none exceeded 45 mph maximum sustained winds. Considering sea surface temperatures in the Atlantic MDR are much warmer than normal, and wind shear over the tropical Atlantic has been below normal so far this season – it appears likely a busy peak of the season will be upon us. Currently, the record for the earliest 7th named storm for the Atlantic basin is held by Tropical Storm Gert of 2005, which formed on July 24. If TD Seven or 91L becomes a named storm prior to then, it will be named Gonzalo and will be the earliest 7th Atlantic named storm on record.

Invest 91L also may develop over the Northwestern Gulf of Mexico

Another tropical wave, Invest 91L, has emerged over the southeastern Gulf of Mexico and has potential to develop into a weak, short-lived tropical cyclone late this week. As of 18:00 UTC Tuesday, Invest 91L was centered near 25.2°N 84.7°W. Maximum sustained winds were 25 knots (30 mph), with an estimated minimum pressure of 1014 mb. The ECMWF and GFS models have occasionally developed 91L into a weak tropical cyclone prior to moving inland into southeastern Texas. The National Hurricane Center (NHC) currently gives 91L a 30 percent chance of development within 48 hours, and a 40 percent chance within five days. An Air Force Hurricane Hunter aircraft is scheduled to visit 91L tomorrow, if considered necessary by NHC. The most likely window for 91L to develop will be on Thursday or Friday, just prior to moving inland.