Gert a Hurricane; 91L and 92L Disorganized

Hurricane Gert achieved Category 1 hurricane status on Monday afternoon in the Atlantic waters between North Carolina and Bermuda, becoming the second Atlantic hurricane of the 2017 season. Gert is not expected to be a threat to any land areas. The second hurricane of the Atlantic hurricane season occurs, on average, on August 28, so we are about two weeks ahead of average for that statistic.

Satellite images on Tuesday morning showed that Gert had a respectable amount of heavy thunderstorm activity and a prominent eye, though moderate wind shear of about 15 knots was stretching out the hurricane from north to south. 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 intensification. Sea surface temperatures of 28.5°C (83°F) were favorable for development.

Figure 1.  Infrared GOES-16 image of Hurricane Gert as of 11:57 am EDT August 15, 2017. Image credit: NASA/MSFC Earth Science Branch. NOAA’s GOES-16 satellite has not been declared operational and its data are preliminary and undergoing testing.

Forecast for Gert

The 12Z Tuesday 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. Our top dynamical intensity models, the HWRF, COAMPS-TC, and the new HMON model (the replacement for the old GFDL model), all predicted in their 6Z Tuesday runs that Gert would undergo a period of rapid intensification through Wednesday afternoon, peaking as a major Category 3 hurricane by Wednesday afternoon. However, our best statistical intensity models, the DSHIPS and LGEM, were more restrained, predicting that Gert would maintain Category 1 strength, reaching a peak intensity of 85 mph winds on Wednesday afternoon. Given the dry air surrounding Gert and the continued presence of moderate levels of wind shear, the official NHC forecast follows the less aggressive intensity forecast models, with the 11 am EDT Tuesday NHC advisory calling for Gert to peak as an 85 mph Category 1 hurricane on Wednesday afternoon. 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 by Friday. The models are in excellent agreement on a track arcing to the northeast this week, taking Gert out to sea well south of the Canadian Maritime Provinces.

Figure 2. MODIS true-color satellite image of 91L on Tuesday morning, August 15, 2017. Image credit: NASA.

91L still disorganized

A tropical wave located near 14°N, 41°W in the central tropical Atlantic at 8 am EDT Tuesday (Invest 91L), remains disorganized, but has the potential to develop into a tropical depression late this week as it moves westwards at about 15 mph. Satellite images on Tuesday morning showed that 91L had only a limited amout of disorganized heavy thunderstorm activity, though the disturbance had acquired more spin since Monday. 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 atmosphere surrounding 91L had dried since Monday, with relative humidity at mid-levels of the atmosphere at 60%, down from 70% on Monday morning. This drier air was likely keeping heavy thunderstorm activity down.

Forecast for 91L

Conditions appear adequate for development through at least Friday, though the atmosphere surrounding 91L will be a bit on the dry side (a mid-level relative humidity of 55 - 60%). Wind shear is predicted to be mostly moderate, 10 – 20 knots, through Thursday, and sea surface temperatures will be warm, 27.5 – 28.5°C (82 - 83°F).

Our top models for predicting tropical cyclone genesis—the GFS, European, and UKMET models—were much more restrained about developing 91L in their 0Z Tuesday runs, compared to their Monday morning runs. The European model did not develop 91L; the GFS model showed weak development on Thursday and Friday, then the storm dying in the Eastern Caribbean. The UKMET model was the only one of the three that supported the idea of 91L becoming a weak tropical storm, predicting that 91L would attain that status by Friday as it moved through the Lesser Antilles Islands, then weaken as it traversed the eastern and central Caribbean early next week. In its tropical weather outlook issued at 8:00 am EDT Tuesday, the National Hurricane Center gave 91L 2-day and 5-day odds of development of 20% and 40%, respectively. Their forecast from 24 hours previous had given 5-day development odds of 60%.

Figure 3: The Saharan Air Layer (SAL) analysis from 8 am EDT Tuesday, August 15, 2017, showed that 91L was beginning to encounter dry Saharan air; 92L was in a moister environment. A new tropical wave to watch was still over western Africa, poised to emerge from the coast on Wednesday. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

92L, a disturbance 700 miles east of 91L, is worth watching

The elongated area of disturbed weather in the Central Atlantic that we were discussing on Monday split into two separate areas of low pressure that could potentially develop into tropical depressions: 91L, and a disturbance about 700 miles to its east. This disturbance (92L) was located near 13.5°N, 30°W at 8 am EDT Tuesday. Satellite images on Tuesday morning showed that this eastern twin of 91L also had a limited amount of disorganized heavy thunderstorm activity, but a respectable amount of spin. Conditions were favorable for development, with light wind shear less than 10 knots, a moist atmosphere with a mid-level relative humidity of 70%, and warm SSTs of 27.5°C (82°F). The disturbance was headed west to west-northwest at 15 - 20 mph. The 0Z Tuesday run of the GFS model ensemble had about 30% of its members show development into a tropical depression over the next five days, but less than 10% of the European model ensemble members showed development. In its tropical weather outlook issued at 8:00 am EDT Tuesday, the National Hurricane Center gave 92L 2-day and 5-day odds of development of 10% and 20%, respectively. Based on the latest satellite appearance of 92L, these odds should be bumped up to 20% and 30%, respectively. 91L and 92L are close enough together that they could affect each other’s track and intensity; in particular, if 91L develops, its upper-level outflow could weaken 92L.

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 Wednesday. The 0Z Tuesday operational runs of our three reliable models for predicting tropical cyclone genesis—the GFS, European and UKMET models—all predicted development of this new wave by early next week, as did about 30% of the 50 members of the European ensemble forecast and 60% of the 20 members of the GFS ensemble forecast. This wave was predicted to take a more west-northwesterly track than 91L, but may pose a long-range threat to the Lesser Antilles Islands.