Tropical Depression Eight strengthened into Tropical Storm Gert Sunday afternoon, and is likely to strengthen to near hurricane strength by Tuesday as it turns northeastward into the mid-latitude Atlantic. Gert is the seventh named storm of the 2017 Atlantic hurricane season, and is unusually early for a “G” storm. Only three seasons since reliable records began in 1851 – 1936, 1995, and 2005 – had their seventh named storm form earlier. The average seventh named storm in the Atlantic does not form until September 16. The Atlantic could see Tropical Storm Harvey by the end of this week from Invest 91L, which has potential to develop into a long-tracked storm that could pose a possible threat to the Lesser Antilles later this week.

Tropical Storm Gert named, likely to strengthen and could become a hurricane at a high latitude

As of 5:00 p.m. EDT Sunday, Tropical Storm Gert was centered near 28.1°N 71.7°W, and was moving north-northwest at about 10 mph. Maximum sustained winds were 35 knots (40 mph), with an estimated minimum pressure of 1011 mb. Gert is a well-organized cyclone with numerous spiral bands. Although the central convection is not very deep due to some dry mid-level air entrained into the storm, deep convection has begun to increase near the center during the past few hours, possibly signaling intensification.

The track forecast for Gert is straightforward, and the cyclone is expected to turn to the northeast out to sea about halfway between the United States and Bermuda. The cyclone is no threat to land, and no watches or warnings have been issued. The intensity forecast for Gert is a little more tricky. Conditions should support slow intensification of Gert through the next 48 hours, with wind shear expected to be light to moderate, around 10-15 knots, mid-level relative humidity values of 50-55 percent, and sea surface temperatures above 28°C (84.2°F). However, by Wednesday, wind shear is expected to increase to over 30 knots, while the cyclone crosses the 26°C (78.8°F) isotherm as well as enters a drier environment. Much of the intensity guidance brings Gert to hurricane strength by Tuesday or Wednesday, as well as the HWRF model, which predicts that Gert will rapidly intensify into a category 4 hurricane along the Gulf Stream. Global models are much less bullish and predict that Gert will only strengthen into a moderate to strong tropical storm. The official intensity forecast from the National Hurricane Center (NHC) sides more with the global models, and predicts that Gert will peak as a 65-mph tropical storm. However, based on the current structure of the cyclone, I would not be surprised to see Gert attain Category 1 hurricane strength, but the HWRF model solution is almost certainly overdone. Gert is expected to transition to an extratropical cyclone by Thursday, eventually dissipating by Friday.

Invest 91L: A new tropical wave to watch

A complex and broad area of low pressure, associated with a tropical wave – designated Invest 91L – has a chance to gradually develop into a tropical cyclone this week as it moves generally westward across the tropical Atlantic at about 15 mph. As of 18:00 UTC Sunday, Invest 91L was centered near 13.4°N 21.1°W. Maximum sustained winds were 25 knots (30 mph) with an estimated minimum pressure of 1010 mb. NHC gives 91L a 10 percent chance of development within 48 hours, and a 40 percent chance of development within 5 days. The tropical wave is located east of the broad area of low pressure, and these systems are likely to merge early this week. 91L is expected to be in favorable conditions throughout the next five days, with wind shear light (under 10 knots), mid-level relative humidity values of over 60 percent, and sea surface temperatures above 27°C (80.6°F). Although the environment is expected to be favorable, the merger of the two systems will be a complex process, and will likely slow development. The 18z Sunday GFS model run, as well as the 12z Sunday ECMWF, CMC and UKMET model runs, all predicted development of 91L over the next five days, in addition to most GEFS and EPS ensemble members. Many of these model runs indicated the possibility that 91L could eventually become a hurricane in the long range. It is far too soon to predict the exact track and intensity of 91L if development occurs, and this will be highly uncertain until a well-defined circulation develops. I think the most likely outcome is that 91L struggles to consolidate quickly due to its large size, and that 91L could approach the Northern Lesser Antilles in about five days as a tropical storm. 91L is a possible long-range threat to the United States, but as mentioned earlier, it is way too soon for exact details in track.

The active part of the Atlantic hurricane season is approaching, and I expect the Atlantic will be very busy for the next few weeks with multiple systems to track. Originally, global models expected a tropical cyclone outbreak in the Eastern Pacific, but have recently backed off this idea and are now trending towards multiple tropical cyclones in the Atlantic instead. I am expecting the Atlantic to have at least two to three more named storms this month after Gert.

I will be back tomorrow for an update on Gert and 91L. The East Pacific is becoming quiet, with Tropical Depression Jova expected to become a remnant low tonight.