By: Jeff Masters and Bob Henson From Weather Underground

A large but nearly thunderstorm-free tropical wave (Invest 99L) was located in the tropical Atlantic about 1100 miles east of the Lesser Antilles Islands late Sunday morning, and was headed west at 15 – 20 mph. This disturbance is not likely to develop into a tropical depression or tropical storm before moving into the Lesser Antilles Islands on Tuesday night, but will bring some heavy rains and gusty winds to the islands on Tuesday and Wednesday. The storm could be a long-range threat to the U.S. East Coast from Florida northwards in about 7 – 10 days.

The most impressive thing about 99L when viewing satellite loops is its very large size and excellent spin. A large region of the atmosphere has been put in motion by this disturbance, which is both good news and bad news: good news because such large disturbances typically take a long time to spin up into a tropical cyclone, but bad news because once they do, they affect a large area and will resist rapid weakening. The other notable feature of the storm on Sunday morning was the lack of heavy thunderstorm activity, due to dry air from the Saharan Air Layer (SAL), as seen in water vapor satellite imagery. However, this dry air was only moderately dry, with humidities at mid-levels of the atmosphere between 500 – 700 mb running about 60%, and the circulation of 99L was getting more defined late Sunday morning, potentially signaling that the storm might be about to build some heavy thunderstorms near its center. Other conditions were generally favorable for development, with wind shear a moderate 10 – 15 knots and sea surface temperatures (SSTs) near 27.5°C (82°F), which was close to average.

Figure 1. Latest satellite image of 99L.

Track forecast: 99L a potential threat to Hispaniola, the Bahamas, and the U.S. coast from Florida northwards
A strong ridge of high pressure will keep 99L headed north of due west over the next few days, and the storm should pass through the northern Lesser Antilles Tuesday night through Wednesday, track close to Puerto Rico on Wednesday night, and affect Hispaniola and the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, as a weak trough of low pressure passing to the north of 99L late this week may be strong enough to turn the storm to the north before it can reach the U.S. East Coast. The track of 99L may also be affected late this week by tropical wave 90L (see below), which could grow into a hurricane that comes close enough to exert a steering influence.

Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Sunday, August 21, 2016. The SAL was interfering with both Fiona and 99L, but was not as concentrated as we saw early in August. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Intensity forecast for 99L
99L has a lot of hurdles to overcome to become a named storm. The 8 am EDT Sunday run of the SHIPS model showed moderately favorable conditions for development through Thursday, with wind shear in the moderate range, 10 – 15 knots, a relatively moist atmosphere, and SSTs near 28°C (83°F.) The total heat content of the ocean will steadily increase as 99L moves westwards, as well. But working against development of 99L will be the large size of the storm, dry air of the SAL, potential interaction with the land areas of Puerto Rico and Hispaniola, and large scale sinking air over the tropical Atlantic imparted by an unfavorable phase of the Madden-Julian Oscillation (MJO). The active portion of the MJO is currently located in the Western Pacific, which is leading to increased tropical cyclone activity there–three named storms were active there on Sunday morning. This positioning of the MJO typically leads to compensating sinking air and surface high pressure over the tropical Atlantic, with reduced chances of tropical cyclone development there.

None of the Sunday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis–the European, GFS and UKMET models–showed development of 99L into a tropical depression or tropical storm over the next five days. However, beyond five days, when the storm will likely be near or just north of the central Bahamas, the models are predicting a more favorable environment for development. The 00Z Sunday runs of the GFS and European model ensembles had 5% and 12% of their members predicting that 99L would eventually become a hurricane, after seven days. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 20% and 50%, respectively.

Figure 3. MODIS visible satellite image of 90L off the coast of Africa, taken on Sunday morning, August 21, 2016. Image credit: NASA.

90L off the coast of Africa likely to develop
A large tropical wave with plenty of spin (Invest 90L) emerged from the coast of Africa on Saturday night, and appears poised to become a tropical depression by Tuesday as it heads west-northwest at about 10 mph. Satellite images on Sunday morning showed that 90L was well-organized, with low-level spiral bands and an increasing amount of heavy thunderstorms. Wind shear was light, 5 -10 knots, and SSTs were warm enough for development, 27°C (81°F).

The Sunday morning operational runs of the European, GFS and UKMET models all showed development of this wave into a tropical depression in 1 – 3 days. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 70% and 90%, respectively. The wave will head west-northwest through Tuesday, skirting the Cabo Verde Islands to the south, then turn more to the northwest on a path similar to Fiona’s. The storm will be moving into a region of ocean where very few tropical cyclones ever make the long trek westwards to hit the United States–though 90L could be a long-range threat to Bermuda. The next name on the list of Atlantic storms is Gaston, and we can expect 90L to be called Hurricane Gaston by this weekend.

Figure 4. With its shell of convection displaced far to its east by strong vertical wind shear, Tropical Storm Fiona’s low-level center was left completely exposed (left) at 0745Z (3:45 am EDT) Sunday, August 21, 2016. Less than seven hours later, at 1415Z (10:15 am EDT), a new burst of convection had developed near Fiona’s center (right). Image credit: NOAA/NESDIS.

Fiona the fighter lives to see another day
The epitaph was almost written Saturday night on Tropical Storm Fiona. The storm had intensified Saturday afternoon, with a substantial core of convection helping the storm’s top sustained winds to increase to 50 mph as measured by scatterometer. Overnight, though, strong wind shear (around 30 knots) ripped away the convection and left Fiona’s low-level circulation completely open. Early Sunday morning, yet another burst of convection erupted near Fiona’s center. Fiona’s small size and the typical overnight maximum in tropical convection are helping to facilitate these ups and downs in intensity. As of 11 am EDT Sunday, Fiona’s top winds were 40 mph, barely keeping it a tropical storm. Working its way across the open Atlantic, at 22.9°N, 53.3°W–roughly 700 miles northeast of the Leeward Islands–Fiona was continuing west-northwest at 16 mph.

The main question in Fiona’s future is whether the storm will survive the 20-30 knot wind shear projected for the next 1-2 days. After that point, wind shear will drop below 20 knots, and the storm will be traveling over very warm SSTs of around 30°C (86°F), around 1-2°C above average for this time of year. The contrast between these warm waters and cold upper-level temperatures in the subtropics would allow for strong convection, and Fiona’s small size could allow the storm to intensify more quickly than a larger storm might. Recent runs of the European model weakens Fiona into an open trough, while the GFS, UKMET, and HWRF maintain it as a weak low through the 5-day forecast period. The official NHC outlook downgrades Fiona to depression strength by Monday and makes Fiona a remnant low by Tuesday. If Fiona does survive, it could become a respectably strong tropical storm late in the week, potentially passing near Bermuda on a recurving path north and northeast.

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