96L Off the Coast of Africa Growing More Organized

By Jeff Masters
Published: 2:01 PM GMT on July 28, 2016

A strong tropical wave that moved off the coast of Africa on Wednesday morning has become more organized over the far eastern Atlantic, and has the potential to develop into a tropical depression in the coming days as it tracks west-northwestward at 10 - 15 mph into the middle Atlantic. NHC designated this disturbance Invest 96L on Wednesday morning--the first "Invest" of the year for an African tropical wave. Satellite loops on Thursday morning showed 96L had a compact area of heavy thunderstorms, and this activity had acquired a modest degree of organization. Some spin was evident in the cloud pattern, and low-level spiral banding features had begun to appear. Wind shear was a light 5 - 10 knots, and sea surface temperatures (SSTs) were warm, near 28°C (82°F), which was about 1°C (1.8°F) above average. Water vapor satellite imagery showed that the eastern tropical Atlantic was quite moist, with the dry air of the Saharan Air Layer (SAL) several hundred miles north of 96L. These conditions are favorable for development of a tropical depression.


Figure 1. Analysis of the Saharan Air Layer (SAL) from 8 am EDT (12 UTC) Thursday, July 28, 2016, showed that the dry air and dust of the SAL lay a few hundred miles to the north of 96L. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Forecast for 96L
Steering currents favor a west to west-northwesterly motion at 10 - 20 mph for 96L over the next five days, and the storm should reach a point near 40°W, midway between the Lesser Antilles Islands and Africa, on Sunday. The 8 am EDT Thursday run of the SHIPS model predicted modestly favorable conditions for development through Saturday morning, with wind shear in the light to moderate range, 5 - 15 knots, a moist atmosphere, and warm SSTs near 27.5 - 28°C (81 - 82°F.) However, on Saturday and Sunday, 96L will encounter cooler waters, with temperatures a marginal 26.5 - 27°C (80 - 81°F). The SHIPS model also predicts that wind shear over the weekend will become moderate to high, 15 - 25 knots, and the atmosphere will get very dry, due to an intrusion of the Saharan Air Layer (check out the 10-day African dust forecast from NASA.) These unfavorable conditions would stymie any development of 96L.

The Thursday morning operational runs of our three reliable models for predicting tropical cyclone genesis, the European, GFS and UKMET models, all supported some limited development of 96L, but stopped short of predicting it would become a tropical depression. The 00Z Thursday run of the GFS ensemble forecast, done by taking the operational high-resolution version of the model and running it at lower resolution with slight perturbations to the initial conditions in order to generate a range of possible outcomes, had more than 50% of its twenty ensemble members predict that a tropical depression would form this weekend or early next week in the eastern Atlantic. Most of these forecasts had the storm dying out the middle Atlantic, due to unfavorable conditions, and none had it becoming a hurricane. Between 10 - 20% of the 50 members of the 00Z Thursday European ensemble model forecasts showed 96L becoming a tropical depression. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave 96L 2-day and 5-day development odds of 30% and 40%, respectively. Though the long-range uncertainty on what 96L might do is high, one reasonable scenario is for the system to steadily grow in organization the next few days, come close to or achieve tropical depression status by Saturday, then get ripped up by wind shear and dry air well before reaching the Lesser Antilles Islands by the middle of next week. Should 96L become a tropical storm, the next name on the Atlantic list is Earl.

Eastern Pacific getting less active
For the first time since July 2, there is only one active tropical cyclone in the Eastern Pacific: Tropical Storm Frank, which peaked as a Category 1 storm on Tuesday. Frank is degenerating quickly over 23°C waters and is likely to dissipate by Thursday evening, potentially giving us on Friday our first tropical cyclone-free day in the Eastern Pacific since July 1. Beginning on July 2, the Eastern Pacific had Tropical Storm Agatha form, followed by Category 4 Hurricane Blas, Category 2 Hurricane Celia, Category 3 Hurricane Darby, Tropical Storm Estelle, Category 1 Hurricane Frank and Category 4 Hurricane Georgette. This puts us far ahead of climatology: the Eastern Pacific usually does not see its seventh named storm until August 7, its fifth hurricane until August 26, and its third major hurricane until September 20. An average season has 15 named storms, 8 hurricanes, and 3 major hurricanes. The quiet may not last long: in their 8 am EDT Thursday Tropical Weather Outlook, NHC gave a new tropical disturbance a few hundred miles south of the southern tip of Mexico's Baja Peninsula 2-day and 5-day development odds of 20% and 70%, respectively. This storm--which would be named Howard if it gets to tropical storm strength--is expected to move west-northwest and not impact Mexico.

Jeff Masters

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About The Author
Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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