The eastern United States is enjoying a brief reprieve from the brutal humidity that’s maintained its stranglehold over the sweat-soaked region since early this summer. Clear skies and drier air put the weather toward the back of everyone’s mind. But even though things are pretty calm here on land, the big weather story this week is out to sea.

A tropical disturbance that began just east of the Caribbean is slowly making its way toward the southeastern United States. Whether the disturbance strengthens into a tropical cyclone and where it travels is something everyone living along the East Coast and Gulf Coast needs to watch carefully over the next week or so.

A satellite image of the Atlantic Ocean on August 23, 2016, showing Invest 99L just east of the Caribbean. Image credit: NOAA/NHC

 
We have several tropical systems out in the Atlantic this week, but the one we need to watch is a tropical disturbance named Invest 99L. “Invest” is short for "investigative area"—a title the National Hurricane Center uses to track areas of interest that could develop into a tropical cyclone. Invest 99L is currently moving through the Caribbean, bringing heavy rain. It's heading in a general west-northwest direction that should take it toward the Bahamas by this weekend.

A Hurricane Hunter aircraft flew into the disturbance yesterday, August 23, and found that its thunderstorms weren’t well organized and it didn’t have a closed wind circulation at the surface—two requirements a system needs in order to be called a tropical system. Despite its ragged appearance yesterday, many weather models develop the disturbance into a tropical storm or hurricane over the next five to seven days.

Determining how strong the system will get and where the system will go aren’t easy at this point. It’s one of the most frustrating things about forecasting tropical cyclones that some of the biggest things we’re uncertain about are also the most important bits of information to people who are potentially in harm’s way. Take, for instance, the August 23 runs of two major and widely respected weather models, the GFS, which is run by NOAA, and the ECMWF, the famous European model. The GFS model doesn’t do much with this storm—the model shows the disturbance fizzling out near the Bahamas. If that winds up happening, everyone will forget about it by the end of the weekend. The European model, on the other hand, spins it up into a formidable hurricane, one that could be concerning to folks who live along or near the U.S. coast.

Now, which model do you believe—the one that shows nothing, or the one that predicts a disaster? That’s the problem here, and it’s something meteorologists will struggle with for a couple of days until we know a) whether or not the disturbance will get itself organized at all, and b) how well it can get its act together once (and if!) it does so. The next couple of model runs will give us a much better idea of what’s going to happen, if anything at all.

The spaghetti model plot (above), created on August 23, shows the disturbance’s possible tracks as predicted by various weather models. It shows a pretty strong consensus among the different models that the system will likely move in that general west-northwest motion, skirting the Caribbean islands and heading toward the Bahamas and eventually Florida.

This general track could change if the system stays weaker than expected or begins to strengthen. The intensity of the possible storm is also key to which way it will move. Weak tropical systems are steered by winds closer to the surface, while stronger tropical systems—those with denser and more intense thunderstorms around their core—are steered by winds from the surface all the way through the top of the atmosphere. This is why meteorologists can sometimes get the track forecast for a tropical storm so wrong; if a storm stays weaker than forecast, it’ll go in a completely different direction than it would have if it were stronger.

We’ve been through this kind of a scenario before. In 2015, Tropical Storm Erika formed in almost the exact same spot as Invest 99L and it was forecast to graze north of the Caribbean islands and reach Florida as a hurricane. The storm never strengthened as forecast, so it kept moving west and drenched the Caribbean islands with devastating amounts of rainfall.

Regardless of its development, anyone who lives along or near the coast from Texas to Virginia needs to keep a close eye on how this disturbance develops over the next week. We’re quickly approaching the peak of hurricane season, so now is the time to get your emergency plans ready. Some things to think about include where you’ll go if you have to evacuate; how much food, water, medicine, money, and supplies you’ll need to make it through an evacuation or extended disruption at home; and what you’ll do if you have to ride out the storm at home.

Remember that the effects of a tropical storm or hurricane don’t stop at the coast. Extensive flooding and wind damage can extend for many miles inland, even days after a storm makes landfall. A system doesn’t even need to be a tropical storm in order to create extensive damage, either—the historic flooding in Louisiana earlier this month occurred as a result of a disturbance that never got a name or a title. Keep up with your local forecasts and the forecasts issued by the National Hurricane Center, which come out every six hours.