RE: INVEST 97L
Quote:97L Potentially a Dangerous Storm for the Caribbean
By:Jeff Masters , 5:38 PM GMT on September 25, 2016
[/url] [url=undefined] [/url] [url=undefined]
A tropical wave located midway between the Lesser Antilles Islands and the coast of Africa on Sunday afternoon was headed west at 15 - 20 mph, and has the potential to become a dangerous storm in the Caribbean later this week. NHC designated this system Invest 97L on Sunday morning. After looking remarkably unimpressive on satellite loops for the previous few days, 97L was turning that situation around on Sunday. The system had a large circulation at middle levels of the atmosphere, with an increasing amount of heavy thunderstorm activity. Some low-level spiral bands were beginning to develop, and upper-level outflow was becoming established to 97L’s north. The storm’s organization was being aided by low wind shear of 5 - 10 knots, a very moist atmosphere (relative humidities at mid-levels of the atmosphere near 75%) and warm ocean waters of 29°C (84°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 vertically aligned, plus 97L’s nearness to the equator. The system was centered near 8°N, which was too far south to be able to leverage the Earth’s spin and acquire much spin.
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. 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 70 - 75%, SSTs remaining a very warm 29°C (84°F), and 97L slowing its forward speed to about 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.
Invest 97L may pass very close to the coast of South America, 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 primarily 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.
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 Sunday runs that 97L would develop into a tropical depression or tropical storm between Monday and Wednesday. About 70% of the 20 forecasts from the members of the 00Z Sunday GFS ensemble showed development into a tropical storm, with 40% 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 40% of its 50 ensemble members predicted a tropical storm in the Caribbean, with 30% predicting a hurricane. In their 2 pm EDT Sunday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 30% 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 out to ten days from the 00Z Sunday European 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.
Will 97L threaten the U.S.?
Forecasts of what might happen to 97L beyond five days from now are speculative, but let’s go ahead and speculate. A large upper-level low pressure system is expected to form 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 Sunday morning runs of the models. In this scenario, Haiti, Jamaica, Cuba, the Bahamas, and Florida would be at greatest risk for a strike by 97L. According to the Sunday morning Extended Forecast Discussion from the NWS (thanks to WU member nrtiwlnvragn for posting this link), the models are in substantial disagreement on the evolution of this upper-level low, with the GFS model being judged to have the best handle on it. If this analysis is correct, the long-range forecasts from the GFS model may be better than the European model’s. However, you can throw all these forecasts out the window 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. Making an accurate long-range track forecast from a tropical wave in the process of transitioning into a tropical depression is notoriously difficult.