By now most readers have heard about the tropical low pressure system in the Caribbean Sea, which is approaching Puerto Rico. It is not a tropical storm yet, as a reconnaissance aircraft on Wednesday failed to find a well-defined circulation. It nonetheless appears to be developing stronger thunderstorms, and is producing tropical storm-force winds. It likely will become a depression and then a storm later this week as it passes near the Bahamas and then tracks toward Florida.
The purpose of this post is to discuss what may happen after that, but before doing so, I want to discuss the fallibility of forecast models at this time, and why we don’t have that much confidence in where the system, Invest 99L for now, will go in five to seven days, if it does head toward the Gulf of Mexico.
Model inaccuracy—it’s considerable
Let’s focus on track models. They tell us where a storm will go. Although models can do a fairly good job simulating the large scale steering currents that guide storm motion, such as upper-level pressure systems, these forecasts remain sketchy when a tropical system is poorly defined. And as I noted earlier, Invest 99L has yet to develop a center of circulation.
With that said let’s look at track errors for National Hurricane Center forecasts for three classes of storms: hurricanes (which have well defined circulations), strong tropical storms (likewise) and depressions and weak tropical storms (not so much). Actually, the category we’re interested in isn’t even on this chart—”stuff that ain’t formed yet.” Thus at five days we can expect the track error to be at a very minimum of 350 nautical miles for undeveloped tropical systems, and likely much, much more.
