Tropical Storm Gert strengthened into the second hurricane of the 2017 Atlantic hurricane season late Monday over the subtropical western Atlantic. Despite a rather unimpressive convective structure, microwave imagery indicates that Gert has nearly developed a closed eyewall. Gert is likely to strengthen some more over the next two days, possibly becoming a category 2 hurricane as it accelerates to the northeast. The average second hurricane in the Atlantic does not form until August 28, continuing the fast start to the 2017 Atlantic hurricane season.

Gert becomes a hurricane, could become a category 2 hurricane before extratropical transition

As of 11:00 p.m. EDT, Hurricane Gert was centered near 31.2°N 72.3°W and was moving northward at about 8 mph. Maximum sustained winds were 65 knots (75 mph) with an estimated minimum pressure of 986 mb. Gert was upgraded to a hurricane based on Dvorak estimates as well as microwave imagery indicating the presence of an eye. Gert is expected to be affected by northerly wind shear of about 15 knots for the next day or so, and this should slow intensification in the short term. However, on Wednesday, Gert is expected to accelerate to the northeast and strengthen as wind shear decreases. Sea surface temperatures are expected to remain at or above 27°C (82.4°F) for the next two days, and this should allow strengthening. Gert is also expected to enter an environment with slightly more mid-level moisture. The official National Hurricane Center (NHC) forecast brings Gert to category 2 strength by Wednesday evening with 105-mph winds. It is not out of the question that Gert strengthens into a category 3 major hurricane, and if it did manage to do so, it would be a rare feat for a hurricane at that high of a latitude. Gert should transition into a hurricane-force extratropical cyclone by Thursday, eventually dissipating by Saturday. Gert is no threat to land, but could cause rip currents along the United States East Coast. No reconnaissance missions are currently scheduled to investigate Gert since it is not a threat to land.

Invest 91L: A challenging forecast

An elongated, complex area of low pressure has the potential to slowly organize into a tropical cyclone by late this week as it moves westward across the tropical Atlantic at about 15 mph. As of 00:00 UTC Tuesday, Invest 91L was centered near 13.2°N 37.0°W and was moving westward at about 15 mph. Maximum sustained winds were 25 knots (30 mph) with an estimated minimum pressure of 1011 mb. The National Hurricane Center (NHC) gives 91L a 30 percent chance of development within 48 hours, and a 60 percent chance of development within five days. Throughout the forecast period, 91L is expected to be in a somewhat conducive environment for development. Mid-level relative humidity values are expected to remain above 55 percent, sea surface temperatures will be around 27-28°C (82.4-84.2°F), while wind shear is expected to remain less than 20 knots. The 12z Monday UKMET model run did not show development, while the 18z Monday GFS and 12z Monday ECMWF model runs supported some slight development before the Antilles. The vast majority of EPS ensemble members developed 91L into a tropical depression east of the Lesser Antilles. However, based on its current large size and elongated nature, I expect development of 91L to be very slow, and I would not be surprised if it does not develop before the Lesser Antilles. Regardless, residents of the Lesser Antilles should closely monitor the progress of 91L. It is too soon to predict if 91L will pose a threat to the United States.

Another tropical wave behind 91L to watch

A tropical wave is expected to the exit the coast of Africa on Wednesday or Thursday. This wave has a chance to develop into a tropical cyclone over the eastern tropical Atlantic by late this weekend. The NHC gives this wave a near zero percent chance of development within 48 hours, and a 20 percent chance of development within five days. The 12z Monday ECMWF, UKMET and CMC model runs, as well as the 18z Monday GFS run, predicted development of this wave. I believe this wave has potential to develop into a classic, long-tracked recurving “Cape Verde” type hurricane that will likely not impact land, but it is too far out to speculate on exact details on track or intensity since the wave still has not yet emerged into the Atlantic.

I will be back tomorrow with another update on the tropics.