97L Growing More Organized as it Approaches the Lesser Antilles Islands

A tropical wave located about 1000 miles east-southeast of Barbados in the Lesser Antilles Islands late Monday morning (Invest 97L) was headed west at 15 - 20 mph, and has the potential to become a dangerous storm in the Caribbean later this week. Satellite loops on Monday morning showed 97L was growing considerably more organized, with more curvature to the cloud pattern, an increasing amount of heavy thunderstorm activity, low-level spiral bands that were getting more defined, and upper-level outflow that was becoming better established to 97L’s north. The storm’s organization was being aided by low wind shear of 5 knots, a very moist atmosphere (relative humidities at mid-levels of the atmosphere near 70%) and warm ocean waters of 30°C (86°F). Significant negatives for development included the storm’s forward speed of 15 - 20 mph, which was too fast for the storm to get itself properly aligned in the vertical, plus 97L’s nearness to the equator. The system was centered near 9.5°N, which was too far south to be able to leverage the Earth’s spin and acquire much spin of its own.


Figure 1. Latest satellite image of 97L.

Forecast for 97L
Invest 97L will continue west to west-northwest at 15 - 20 mph through Tuesday, reaching a latitude of about 12°N by Tuesday afternoon. This is far enough away from the equator to give 97L an extra boost of spin that may allow it to become a tropical depression on Tuesday. With the SHIPS model predicting wind shear remaining low, mid-level moisture staying high at 65 - 70%, SSTs remaining a very warm 29 - 30°C (84 - 86°F), and 97L slowing its forward speed to about 10 - 15 mph, conditions will be ripe on Tuesday for 97L to become a tropical depression or tropical storm before it reaches the Lesser Antilles Islands. By Tuesday night, the outer spiral bands of 97L will begin spreading over the Lesser Antilles, bringing high winds and heavy rains. The core of the storm will pass through the islands on Wednesday afternoon. It is unlikely that 97L will have time to intensify into a hurricane by then, though a strong tropical storm with 60 - 70 mph winds is quite possible.

Invest 97L may pass very close to the coast of South America on Thursday and Friday, which would interfere with development. In addition, the southeastern Caribbean is a well-known tropical cyclone graveyard, where scores of healthy-looking storms have died or suffered severe degradation. This is often due to the fact that the southeastern Caribbean is a place where the surface trade winds tend to accelerate, due to the geography and meteorology of the area. A region of accelerating flow at the surface means that air must come from above to replace the air that is being sucked away at the surface. Sinking air from above warms and dries as it descends, creating high pressure and conditions unfavorable for tropical cyclones. In addition, tropical cyclones passing near the coast of South America often suck in dry continental air from the land areas to the south. The last hurricane to pass through the southeastern Caribbean, Hurricane Tomas of 2010, degraded from a Category 1 hurricane to a tropical depression due to high wind shear and dry air as it moved across the region. Recent runs of the SHIPS model have been predicting that 97L will increase its forward speed to 25 mph on Thursday in response to the acceleration of the trade winds over the southeastern Caribbean, and this will likely interfere with development. The model is also indicating that 97L will draw in dry air from northern South America, further slowing intensification. Once 97L manages to separate itself from the coast of South America early next week, more significant intensification can occur.

Model support for development of 97L continues to remain high. Our top three models for predicting hurricane genesis—the GFS, UKMET and European models—all predicted in their 00Z Monday runs that 97L would develop into a tropical depression or tropical storm between Tuesday and Thursday. About 80% of the 20 forecasts from the members of the 00Z Monday GFS ensemble showed development into a tropical storm, with 50% predicting a hurricane. The European model ensemble was less aggressive developing the storm, probably because of a predicted track too close to the coast of South America—about 60% of its 50 ensemble members predicted a tropical storm in the Caribbean, with 30% predicting a hurricane. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 60% and 90%, respectively. The Hurricane Hunters are scheduled to make their first flight into the storm on Tuesday afternoon. The next name on the Atlantic list of storm names is Matthew.





Figure 2. Forecasts from the 00Z Monday European (ECMWF) model ensemble (top) and GFS model ensemble (bottom) had a number of their 70 members predicting a hurricane for late in the week in the Caribbean (light blue dots.) The operational versions of the models, run at higher resolution (red lines), also showed the storm becoming a hurricane by ten days into the future. The European model showed a more westerly track for 97L, with a long-range threat to the Gulf of Mexico, while the GFS model predicted more of a threat to the U.S. East Coast.

Long-range forecast for 97L
The steering currents for 97L will be complex over the coming ten days, and it is very difficult at this point to narrow down where the storm will be 5 - 10 days into the future. A large upper-level low pressure system is expected to separate from the jet stream and settle over the Mid-Atlantic states late this week, and the steering currents associated with this low are expected to be strong enough to pull 97L more to the northwest by the weekend, according to a majority of the Monday morning runs of the models. In this scenario, Haiti, the Dominican Republic, Jamaica, or eastern Cuba would be at risk from a direct strike by 97L on Sunday or Monday. Early next week, this upper level low is expected to lift out to the northeast as a strong trough of low pressure passes its north and captures it. This trough may be strong enough to pull 97L to the northeast with it, if the storm is far enough to the north. Otherwise, 97L will likely continue on a west-northwesterly path. As one can see from the latest set of ensemble model runs (Figure 2), 97L could eventually make landfall anywhere from Nicaragua to Newfoundland, Canada, so we really can’t narrow things down much at this point. If 97L ends up consolidating its center at a latitude significantly different from what these models are expecting, or on a day different from what is expected, the forecast tracks may change dramatically. Making an accurate long-range track forecast from a tropical wave in the process of transitioning into a tropical depression is notoriously difficult.

Roslyn forms off the Pacific coast of Mexico
In the Eastern Pacific, Tropical Storm Roslyn, the seventeenth named storm of this very busy 2016 Eastern Pacific hurricane season, formed on Monday morning. An average season in the Eastern Pacific has just fifteen named storms during the entire year. Roslyn is expected to head north into an area of high wind shear and dry air, and will dissipate late this week without affecting any land areas.


Figure 3. Radar image of Typhoon Megi taken at 10:40 am EDT September 26, 2016 (22:40 local time.) Image credit: Taiwan Central Weather Bureau.

Typhoon Megi takes aim at Taiwan
Category 2 Typhoon Megi was holding at 105 mph sustained winds late Monday morning as it headed west-northwest at 10 mph towards Taiwan. Satellite images and radar images indicated that Megi was undergoing an eyewall replacement cycle late Monday morning, where the inner eyewall collapses and is replaced by a larger-diameter outer eyewall, and this process will likely keep Megi from intensifying before the storm makes landfall in Taiwan near midnight Monday (U.S. EDT.) Megi is the fourth significant typhoon to affect Taiwan this year. Super Typhoon Nepartak hit Taiwan on July 7 as a Category 4 storm with top sustained winds of 150 mph. Earlier in September, Super Typhoon Meranti passed just to Taiwan's southwest, killing two and leaving nearly a million without power, and Typhoon Malakas passed just to Taiwan's northeast a few days later. Taiwan averages 3 to 4 typhoon strikes per year, according to the Central Weather Bureau. On its predicted course, Megi would make a second landfall as a tropical storm along the coast of southeast China, not far south of where Meranti claimed at least 29 lives and caused at least $2.6 billion in damage. Storm chaser James Reynolds is in Taiwan and will be making live updates on Megi’s landfall on his Twitter feed.

Bob Henson will be back this afternoon with an update on major flooding occurring on the Cedar River and Mississippi River in Iowa.

Jeff Masters