A vigorous tropical wave associated with a broad area of low pressure, located over the central tropical Atlantic, has been designated Potential Tropical Cyclone Five by the National Hurricane Center (NHC). The NHC initiates Potential Tropical Cyclone advisories to disturbances that are not yet a tropical cyclone, but both 1) have a high chance of development within 48 hours and 2) tropical storm watches or warnings are required for an area within 48 hours. Five is likely to become the Atlantic’s first cyclone in the deep tropics in 2021, and as I have mentioned before, pre-August named storms in the deep tropics are often correlated with above-average Atlantic hurricane activity.

As of 5:00 p.m. EDT Wednesday, Potential Tropical Cyclone Five was centered near 9.6°N 43.7°W, and was moving west-northwestward at about 21 mph. Maximum sustained winds were 30 knots (35 mph), with an estimated minimum pressure of 1008 mb. The disturbance looks better organized than this morning, but since there has not been any definitive proof yet of a well-defined circulation, it is not classified as a tropical cyclone yet. An ASCAT pass from this morning suggested that the low-level circulation of Five remained elongated and not well-defined. We may get another ASCAT pass this evening, which could suggest whether or not the disturbance has become a tropical cyclone. Regardless of its current status, the disturbance should become a tropical depression or storm by tomorrow morning. In general, environmental conditions look quite conducive for intensification in the short term, which is unusual for so early in the season. The 18z GFS-based SHIPS guidance predicted that wind shear would remain light (less than 10 knots) for the next 48 hours, with sea surface temperatures warmer than 27°F (80.6°F) and 700-500 mb relative humidity values near 70 percent. This should allow for some intensification to occur as the disturbance (likely to become Tropical Storm Elsa) approaches the Lesser Antilles. As a result, Tropical Storm Warnings have been issued for the following islands: Barbados, St. Vincent, the Grenadines, St. Lucia, and Martinique. Although the environment appears generally conducive for intensification, there is one significant factor working against Five’s intensity potential: forward speed. The weak upper level flow and strong easterly flow near the surface contributes to significant “speed shear”, which may make it difficult for Five to maintain a closed circulation in the Caribbean Sea if it is not an established tropical cyclone by the time it approaches the Lesser Antilles. This is fairly typical for early in the hurricane season, and is likely the only factor that could prevent Five from becoming a hurricane. Although nearly all of the global models develop Five, they have vast disagreement in strength. The 12z GFS model develops Five into a Category 1 hurricane by the time it approaches the Lesser Antilles, and the HWRF and HMON models are similar but even stronger. The 12z ECMWF model was its strongest run for Five yet, making it a moderate tropical storm as it approaches the Lesser Antilles, but it was still weaker than the other models and suggests that it remains a possibility that Five could open up into a tropical wave over the eastern Caribbean Sea. The NHC intensity forecast currently only predicts modest intensification into a 50-knot (60 mph) tropical storm as it enters the eastern Caribbean, but this intensity forecast remains highly uncertain. The more Five intensifies in the short term, the more likely it is to survive. Five’s long term future remains very uncertain as well, but the Greater Antilles (such as Hispaniola, Jamaica and Cuba) should closely monitor its progress, as it is likely at least a tropical storm could affect that region this weekend. It remains too soon to tell if Five will impact the United States, but it is possible if land interaction does not cause the system to dissipate.