Tropical Storm Gert Forms, But is Not a Threat to Land

Tropical Storm Gert formed on Sunday afternoon in the Atlantic waters between the Lesser Antilles Islands and Bermuda, becoming the seventh named storm of the season. Gert is not expected to be a threat to any land areas. The seventh named storm of the Atlantic hurricane season occurs, on average, on September 16, so we’ve seen a lot of early-season activity this year. The record earliest seventh Atlantic named storm was 2005’s version of Tropical Storm Gert, which got its name on July 24 of that year.

Satellite images on Sunday evening showed that the heavy thunderstorm activity associated with Gert had expanded to surround the entire center of circulation; earlier on Sunday, the thunderstorms had been limited to the east side of the circulation, thanks to dry air interference. Gert was embedded in an environment of very dry air with a relative humidity at mid-levels of the atmosphere near 45%. Otherwise, conditions were favorable for development, with low wind shear of 5 – 10 knots and warm sea surface temperatures of 29°C (84°F.) Gert was fairly symmetric with good upper-level outflow, and could easily be a strong tropical storm with 65 mph winds by Tuesday, as predicted by NHC.

Forecast for Gert

The 18Z Sunday SHIPS model predicted that wind shear over Gert would mostly be in the moderate range, 10 – 20 knots, through Tuesday, which should allow some modest intensification. By Wednesday, the storm will begin encountering very high levels of wind shear above 20 knots, which should kill the system by Thursday or Friday. The models are in excellent agreement on a track arcing to the north then northeast early this week, taking Gert midway between North Carolina and Bermuda, then out to sea well south of the Canadian Maritime Provinces.

Figure 1. The Saharan Air Layer (SAL) analysis from 5 pm EDT Sunday, August 13, 2017, showed that dry Saharan air lay well to the north of tropical wave 91L, and it should not be a major impediment to development early this week. Image credit: University of Wisconsin/CIMSS/NOAA Hurricane Research Division.

New African tropical wave 91L may develop

A tropical wave that moved off the coast of Africa early Sunday was designated Invest 91L by NHC, and has the potential to develop into a tropical depression later this week as it moves west to west-northwest at about 15 mph. Much like its predecessor (the wave that became Gert), 91L is starting out as a complex, elongated system. The wave, located a few hundred miles south of the Cabo Verde Islands on Sunday evening, may merge with a separate area of low pressure a few hundred miles to its west by Tuesday. Alternatively, the two areas of low pressure may remain separate entities, with one or both of them developing into tropical depressions. All three of our top models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—predicted in their 12Z Sunday runs that 91L would develop into a tropical depression this week, and potentially move into or north of the Lesser Antilles Islands as early as Friday. However, the forecast is a very complex one, and we should not put much stock in the track and intensity forecasts until we see how the two areas of low pressure end up interacting.

Conditions appear quite favorable for development through at least Thursday. The atmosphere surrounding 91L, and downstream of it, is consistently moist (mid-level relative humidity of 65 – 70%). The dry air of the Saharan Air Layer (SAL) is far enough north that it should not pose any immediate issues for 91L. Wind shear along 91L’s path is predicted to be mostly light (less than 10 knots) through Thursday, and sea surface temperatures will be 27-28°C (82-84°F), about 0.5°C to 1°C above average for this time of year. In its tropical weather outlook issued at 8:00 pm EDT Sunday, August 13, 2017, the National Hurricane Center gave 91L 2-day and 5-day odds of development of 10% and 40%, respectively.

Bob Henson contributed to this post.