Recently, there has been a lot of buzz in the meteorological community about “Invest 99L,” a large, disorganized area of convection stretching northeastward from Jamaica and Hispaniola with a weak center of circulation near the Turks & Caicos. On satellite, it looks like a train wreck of a system, but many models take it straight to Florida and into the Gulf of Mexico as a tropical storm or hurricane. It certainly bears watching.
The reason everybody is talking about Invest 99L special is because it could finally end Florida’s “hurricane drought.” After getting pounded by Hurricanes Charley, Frances, Ivan, and Jeanne in 2004 and Dennis, Katrina, and Wilma in 2005, Florida has not gotten hit by any of the 67 hurricanes that have formed since, more than doubling the previous record of 33 between 1979 and 1985. Additionally, no hurricanes have entered the Gulf of Mexico in 1,074 days and counting, the longest drought there in 130 years.
Invests are designated areas of convection that have the potential to develop into tropical cyclones and have been opened for “investigation” by a specific tropical cyclone forecast center like the National Hurricane Center, Central Pacific Hurricane Center, or Joint Typhoon Warning Center. Once this convection has been designated as an invest, additional data is collected by a variety of government and academic institutions to “investigate” the disturbance.
The invests are numbered 90 to 99 before repeating again at 90, and have a suffix to designate the cyclone basin they are located in: L for Atlantic, E for East Pacific, C for Central Pacific, and W for West Pacific. In addition, the Joint Typhoon Warning Center issues invests for the Australian cyclone region, giving invests west of 135°E a S and those east of 135°E a P. They also monitor the Northern Indian Ocean for invests, giving an A to those that form in the Arabian Sea and a B to those that form in the Bay of Bengal.
Now that you know about those funny things called invests, let’s take a closer look at Invest 99L.
As of 00:00 UTC August 26, 2016 (8 pm August 25 EDT), the cyclone was at 21.5°N 73.7°W, with 30 knot sustained winds and a minimum central pressure of 1008 mb. It was moving to the west at 15 mph, with a radius of circulation of 120 nautical miles and maximum wind radius of 80 nautical miles. I obtained the movement of the invest from The Weather Underground, and I got the other measurements from Levi Cowan’s “Tropical Tidbits” website.
As the satellite pictures above show, Invest 99L is extremely disorganized. Convection is occurring throughout the Caribbean, but this convection is not wrapping around the center of circulation, which made its way westward through the Turks & Caicos this evening and is only noticeable on the visible satellite loop above. With 30 knot sustained winds, Invest 99L is not even at tropical storm strength at this point. Even if the winds pick up to tropical storm force, Invest 99L would have to develop a clear center of circulation that was surrounded by convection before it could be designated as a named tropical storm.
Right now, the main threat from Invest 99L is heavy rain over Hispaniola and eastern Cuba, well south of the center of circulation. Since some regions in Hispaniola and eastern Cuba are quite mountainous, heavy rain brings the risk for mudslides and flash flooding, as the terrain funnels runoff, turning small streams into raging torrents. As of Thursday evening, most of the Dominican Republic was on flood alert.
Earlier forecasts had 99L coming ashore as a major hurricane somewhere between New Orleans and Tampa. The European model (which is, on average, the most accurate global weather model) painted an extremely frightening scenario with its 12z run on the 23rd, taking a 938 mb major hurricane just to the east of New Orleans, almost exactly replicating Hurricane Katrina’s track in 2005.
Meanwhile, this morning’s run was far weaker and a bit further east, showing a tropical storm or weak hurricane impacting Florida next Wednesday morning.
There is still a lot of time for this storm to develop, and some models (such as the HWFI) are still quite aggressive in developing it into a major hurricane. However, most models keep it at either a strong tropical storm or a weak category one hurricane, and they are trending weaker.
Invest 99L has struggled to develop because it has encountered a lot of wind shear over the Caribbean, preventing convection from occurring near the center of circulation. Additionally, the terrain over Hispaniola has partially cut off the supply of warm, humid air that has been fueling the invest, reducing the amount of convection to the south of the center of circulation. This thing is hanging on for dear life.
Conditions become more favorable for development as wind shear decreases from 25 knots tonight to 5-15 knots by the weekend and the invest moves away from mountainous Hispaniola toward warm, 86-degree water near the Florida Keys and Gulf of Mexico. According to Weather Underground Blogger Jeff Masters, tropical waves often develop convection overnight, so I’m sure forecasters will keep a keen eye out for convection developing around the center of circulation after the sun sets.
At this point, I’m expecting a Invest 99L to develop into Tropical Storm Hermine and make landfall along the Gulf Coast of the United States sometime next week. There is a significant amount of uncertainty in track at this point in time, but we’ll hopefully have a better idea of where the storm will make landfall by the weekend.
Although conditions will become favorable for development, Invest 99L is such a disaster structure-wise that it will take time for it to organize itself into a named tropical system. As such, I think it will run into land before it has time to develop into a hurricane. The National Hurricane Center gives Invest 99L a 40% chance of developing into a tropical depression by Saturday and a 70% chance of doing so by Tuesday.
I should note that models are notoriously bad at forecasting cyclone strength. While tropical cyclone track forecasts have improved substantially since the 70s, short-term intensity forecasts have remained the same, and longer-term intensity forecasts have only improved slightly. We lack the information to get a detailed initialization of the storm in models, and models are not run with high-enough resolution to model processes like eyewall replacement cycles. Finally, models do a poor job of correctly modeling the interactions between the tropical ocean and atmosphere, and since hurricanes derive their energy from warm ocean water (specifically, the latent heat of condensation of water vapor that has evaporated from the ocean), this weakness is directly applicable to tropical cyclone modeling.
Thanks for reading! I’ll keep you updated on 99L.
Charlie
