Disorganized 99L May Develop Before Reaching the Caribbean; Fiona No Threat

A large but disorganized tropical wave (Invest 99L), located in the tropical Atlantic midway between Africa and the Lesser Antilles Islands on Saturday morning, was headed west at 15 - 20 mph. Residents of the islands in the eastern Caribbean should closely monitor this disturbance, as it has the potential to develop into a tropical depression or tropical storm before moving into the Lesser Antilles Islands on Tuesday night. The most impressive thing about 99L when viewing satellite loops is its very large size and impressive amount of 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 Saturday morning was the relative 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 65 - 70%. Other conditions were generally favorable for development, with wind shear a moderate 10 -15 knots and sea surface temperatures (SSTs) 27.5 - 28°C (82 - 83°F), which was about 1°C (1.8°F) above average.


Figure 1. MODIS visible satellite image of 99L and a new tropical wave about to emerge from the coast of Africa, taken on Saturday morning, August 20, 2016. The next name on the list of “Invests” is 90L. Image credit: NASA.

Track forecast: 99L a potential long-range threat to Hispaniola, the Bahamas, Cuba, and the U.S.
A strong ridge of high pressure will keep 99L headed slightly north of due west over the next few days, and the storm should enter the Lesser Antilles Islands by Tuesday night, spreading heavy rains and gusty winds. By Wednesday, the storm will likely respond to a weakness in the ridge of high pressure steering it, taking a more west-northwesterly track close to Puerto Rico, reaching Hispaniola or the Southeastern Bahamas by Thursday. The uncertainty about the track increases greatly thereafter, and the entire U.S. coast from Texas to Maine could potentially be a target for 99L in the 7 -10 day time frame.


Figure 2. The dry air of the Saharan Air Layer (SAL) as analyzed by satellite at 8 am EDT Saturday, August 20, 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: NBD or OMG?
As usual, the intensity forecast is a challenging one. The 8 am EDT Saturday run of the SHIPS model showed moderately favorable conditions for development through Wednesday, 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. Working against development of 99L will be the dry air of the SAL to its north, plus large scale sinking air over the tropical Atlantic imparted by 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 Saturday 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. However, by Wednesday, forecasts from the GFS model predict that the MJO will weaken, allowing large scale rising air to set up over the eastern Caribbean—potentially increasing the chances for 99L to develop.

Some of the runs of the GFS model on Friday showed development of 99L into a tropical storm or hurricane in the eastern Caribbean, but the Saturday morning (00Z and 06Z) forecasts from the model have backed off on that idea. Given 99L’s large size and current state of disorganization, the earliest it is likely to become a tropical depression is Monday, so the Lesser Antilles Islands are not going to see a hurricane from this storm. The Saturday morning (00Z) operational runs of our three reliable models for predicting tropical cyclone genesis—the European, GFS and UKMET models—had only one of the three--the UKMET model—showing development of 99L into a tropical storm over the next five days. However, at least five of the twenty members of the 00Z Saturday GFS ensemble showed 99L eventually becoming a hurricane after seven days, as did eight of the fifty members of the European model ensemble. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 10% and 50%, respectively. The next name on the Atlantic list of tropical storm names is Gaston—but it is possible that the tropical wave emerging off the coast of Africa on Saturday night could become a named storm before 99L develops, in which case 99L would grab the name Hermine.

A new tropical wave emerging from the coast of Africa may develop
A large new tropical wave with plenty of spin will emerge from the coast of Africa on Saturday night, and the Saturday morning operational runs of the European, GFS and UKMET models all showed development of this wave into a tropical depression in 3 - 5 days. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 50%, respectively. The wave will head west-northwest to northwest over the next five days on a path similar to Fiona’s, into a region of ocean where very few tropical cyclones ever make the long trek westwards to hit the U.S.


Figure 3. Infrared image of Tropical Storm Fiona as of 1515Z (11:15 am EDT) Saturday, August 20, 2016. Image credit: NOAA/NESDIS.

Tropical Storm Fiona is having a rough weekend
Tenacious Tropical Storm Fiona is fighting its way across the central Atlantic in the midst of moderate to strong vertical wind shear (20-30 knots) that is forcing dry Saharan air into the storm. As of the 11 am EDT Saturday advisory, Fiona was centered near 20.3°N, 47.8°W and moving west-northwest at 14 mph. Fiona is now a minimal tropical storm, with top sustained winds estimated of just 40 mph (at best). The storm’s convection had largely dissipated until early Saturday morning, when a new burst of showers and thunderstorms developed on the northeast side of the low-level center of circulation. Although the HWRF model (currently the most reliable at short-term intensity forecasts) strengthens Fiona briefly over the weekend, the strong consensus among models, including HWRF, is that Fiona will weaken below tropical storm strength by early next week. It could maintain its identity as a tropical depression for at least another day or two after that.

Fiona’s west-northwest motion should continue till around Tuesday, when we can expect it to begin arcing northward between around 60°W and 65°W, toward a weakness in the broad ridging that covers most of the Atlantic subtropics. By this point, wind shear may relax and SSTs beneath Fiona will have risen to 29-30°C, which could help keep Fiona clinging to life if it survives till then. The stronger Fiona is at this point, the more northward a track it would take. Fiona could end up passing near Bermuda around the middle of next week, although it seems unlikely there would be much more than gales and squalls (if that) associated with it.


Figure 4. Residents flee flooded Baton Rouge as waters rose on Saturday, August 13, 2016. (Phin Percy)

Louisiana disaster survivors with disabilities need your support after historic flooding
The Portlight.org disaster relief charity, founded and staffed by members of the wunderground community, is responding to this week's devastating floods in Louisiana. The disaster is particularly troublesome for a state that is still in recovery from major flooding just last March, and many resources are completely depleted because of the March flooding. That storm left more than 5,000 homes damaged or destroyed and cost $1.5 billion across a three-state area. With at least 40,000 homes damaged or destroyed, this week's flooding will cost at least $2 billion, and perhaps much more, according to insurance broker Aon Benfield. There is an urgent need for durable and consumable medical supplies as well as housing. Portlight will be working with the American Red Cross, local stakeholder organizations, and federal partners to respond to this historic flooding event. Your support is needed to make this happen! Please consider making a donation to Portlight's disaster relief fund at the portlight.org website to further their reach and response in the state of Louisiana. Thank you for any support you can offer!

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Jeff Masters and Bob Henson