91L May Develop; Gert a Strengthening Tropical Storm

A tropical wave in the central tropical Atlantic (Invest 91L) has the potential to develop into a tropical depression late this week as it moves westwards at about 15 mph, but the forecast has a lot of uncertainty. Satellite images on Monday morning showed that 91L was a complex, elongated system that stretched along a 700-mile long portion of the central tropical Atlantic between 9°N and 14°N latitude. Heavy thunderstorm activity had increased since Sunday, but remained disorganized, and only a modest degree of spin was apparent in the visible satellite imagery. Wind shear was moderate, 10 – 20 knots, and sea surface temperatures were 27.5°C (82°F)--about 0.5°C to 1°C above average for this time of year, and adequate for development. The Madden-Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently in a phase that is favorable for supporting tropical cyclones in the Atlantic, as pointed out by tweets from The Weather Company's Michael Ventrice.

Figure 1: The Saharan Air Layer (SAL) analysis from 8 am EDT Monday, August 14, 2017, showed that dry Saharan air lay well to the north of tropical wave 91L, and should not be a major impediment to development early this week. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Forecast for 91L

Conditions appear favorable for development through at least Thursday. The atmosphere surrounding 91L will be moist (mid-level relative humidity of 65 – 75%), and the dry air of the Saharan Air Layer (SAL) is far enough north that it should not pose any immediate issues. Wind shear along 91L’s path is predicted to be mostly moderate, 10 – 20 knots, through Thursday, and sea surface temperatures will be 27.5°C (82°F).

All three of our top models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—predicted in their 0Z Monday runs that 91L would develop into a tropical depression this week. However, the forecast is a very complex one, since it is not obvious where the center of 91L will end up forming along its elongated axis. The 0Z UKMET model had 91L forming along the western edge of this disturbed weather area, resulting in a tropical depression arriving in the Lesser Antilles Islands on Thursday. The European and GFS models had a more easterly formation location, resulting in arrival of the storm near or just north of the Lesser Antilles on Friday. And we should not be confident that 91L will develop at all this week, since the most recent 6Z Monday run of the GFS model predicted no development of the storm. The 0Z Monday ensemble runs reflect the ongoing uncertainty. 91L becomes a tropical storm in about 75% of the Euro ensemble members, but only about 20% of the GFS members--and there are major differences among ensemble members in the various projected tracks as soon as 3 or 4 days from now. In its tropical weather outlook issued at 8:00 am EDT Monday, the National Hurricane Center gave 91L 2-day and 5-day odds of development of 20% and 60%, respectively.

Figure 2. Tropical Storm Gert as seen by the GOES-16 satellite at 10:45 am EDT August 14, 2017. At the time, Gert had top sustained winds of 60 mph. Image credit: NOAA/CIRA/RAMMB. NOAA’s GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.

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 Monday morning showed that Gert’s heavy thunderstorm activity had increased in intensity and organization, but was mostly confined to the southeast side of the center of circulation, thanks to strong upper level winds out of the north that were creating a moderate 15 knots of wind shear. Gert was embedded in an environment of dry air with a relative humidity at mid-levels of the atmosphere near 50%, and the strong upper-level winds were pushing this dry air into the northern portion of the storm, limiting development of thunderstorms on that side. Sea surface temperatures of 29°C (84°F) were favorable for development.

Forecast for Gert

The 12Z Monday SHIPS model predicted that wind shear over Gert would mostly be in the moderate range, 10 – 20 knots, through Wednesday morning, which should allow at least modest intensification. The official NHC forecast from 11 am EDT Monday calls for Gert to become a hurricane by Wednesday morning, and so does our top intensity model, the HWRF, although it is unlikely that Gert will attain the major-hurricane status predicted by HWRF for several runs. By Wednesday night, Gert will begin encountering very high levels of wind shear above 30 knots, which should convert the storm to a hurricane-strength extratropical storm on Thursday. The models are in excellent agreement on a track arcing to the north then northeast this week, taking Gert midway between North Carolina and Bermuda, then out to sea well south of the Canadian Maritime Provinces.

#Gert in all its glory Monday morning 8/14 pic.twitter.com/L4xldxW2da

— NOAA Satellites (@NOAASatellites) August 14, 2017

Yet another African tropical wave to watch later this week

Another tropical wave with the potential to develop into a tropical depression is predicted to emerge from the coast of Africa on Thursday. The 0Z Monday operational runs of both the European and UKMET models predicted development of this new wave by early next week, as did about 30% of the 50 members of the European ensemble forecast. This wave was predicted to take a more west-northwesterly track than 91L. Yes, the tropical Atlantic is heating up!

Bob Henson contributed to this post.