The short life of Tropical Depression Four came to an end Friday afternoon when the National Hurricane Center (NHC) designated it a remnant low. On the flip side, Invest 95E in the East Pacific has been upgraded to Tropical Storm Eugene, the fifth named storm of the 2017 Pacific hurricane season. After Tropical Depression Four, another African tropical wave has a chance to develop by late next week near the Lesser Antilles.

Dry air ends the life of Tropical Depression Four, but regeneration not out of the question

At 5:00 p.m. EDT Friday, NHC issued its last advisory on Tropical Depression Four, which has degenerated into a remnant low. An ASCAT pass from Friday morning indicated that the depression no longer had a well-defined circulation, and the low-level center has become impossible to locate on visible imagery. As of 5:00 p.m. EDT, the remnants of Four were located near 16.5°N 52.5°W, and were moving west-northwestward at 21 mph. Maximum 1-minute sustained winds have dropped to 25 knots (30 mph) with an estimated minimum pressure of 1012 mb. Dry air from the Saharan Air Layer (SAL) and strong trade winds are responsible for the depression’s quick demise – usually these two factors limit development in the Tropical Atlantic in July. The NHC expects Four’s remnants to continue moving west-northwestward without regeneration. Regeneration is an unlikely scenario, but is not out of the question, as indicated by the UKMET model. The ECMWF and GFS models do not predict that Four will regenerate. Regardless, we will watch the remnants of Four as they move west-northwestward, and they could be near the Florida coastline by the middle of the next week.

Tropical Storm Eugene forms in East Pacific

Invest 95E in the East Pacific was named Tropical Storm Eugene Friday afternoon by the NHC. The classification was based on improved convective organization and an ASCAT pass which showed a well-defined circulation and winds near tropical storm force. As of 2:00 p.m. PDT, Eugene was centered near 11.9°N 111.2°W and was moving northwest at about 8 mph. Maximum sustained winds were 35 knots (40 mph) with an estimated minimum pressure of 1006 mb. Eugene is expected to remain in an environment with sea surface temperatures above 27°C (80.6°F) for the next 48 hours, with very light wind shear of less than 5 knots and a moist environment. Eugene is a sprawling system that will likely take time to organize. Once Eugene establishes a core, a period of rapid intensification is possible for about 36-48 hours due to the very favorable environment. After 48 hours, while wind shear is expected to remain light, Eugene will traverse over decreasing sea surface temperatures below 26°C (78.8°F) in addition to entering a more stable environment, and Eugene is likely to quickly degrade to a remnant low in about five days. The official NHC forecast predicts Eugene to peak as a category 1 hurricane in about 48 hours, but if it can quickly establish a core, I would not be surprised to see Eugene become a category 2 hurricane due to the very favorable environment. Eugene is expected to take a northwest track remaining well away from any land areas.

Tropical wave expected to exit Africa tomorrow could develop late next week

A strong tropical wave is expected to exit the coast of Africa tomorrow. This wave has the potential of developing into a tropical cyclone by late next week, when it is expected to be near the Windward Islands. The 18z Friday GFS and 12z Friday CMC model runs developed this wave into a tropical cyclone by next Friday. This wave is expected to remain at a low latitude for the next several days, which will keep it south of the strongest Saharan Air Layer (SAL). However, at a latitude this low, a wave is not usually able to get enough spin from the Coriolis Effect to develop into a tropical cyclone until it begins to gain latitude near the Caribbean Sea. Also working against development, trade winds are usually too strong to allow for tropical waves to stack up and develop in this region this time of year, and this could be important to the fate of the wave. The 18z Friday GFS run develops this wave into a tropical depression by next Thursday, and strengthens it into a hurricane by next Saturday when it is east of the Windward Islands. The 12z ECMWF run did not show any development, which is in contrast with the 00z ECMWF run which predicted a similar scenario to earlier GFS runs. It should be noted that predicting track and intensity is very unreliable until a well-defined center forms, especially before the wave exits the African coast. This wave has not yet been mentioned in the Tropical Weather Outlook by the NHC. Regardless, we will continue to monitor it. The next name on the Atlantic naming list is Don, since Tropical Depression Four never became named.

I will be back for another post tomorrow on Eugene, the remnants of Four, and the African wave.