The Tropical Atlantic Begins to Stir

Hello everyone!

About a week ago, I dusted off this blog to discuss the potential for some increased tropical cyclone activity in the Atlantic heading deeper into August. This wasn’t exactly a bold prediction given that almost every year activity ramps up as we transition from July to August.

This ramp-up in activity happens around this time every year for a number of reasons, but mostly it has to do with rising ocean temperatures, falling concentrations of dust blowing west from the Sahara Desert, and slackening winds in the upper atmosphere. Each of these trends are typical features of the seasonal cycle, and this year we also have a pulse of the MJO (discussed in some detail in last week’s post, discussed in much more detail by FSU Ph.D. candidate Jake Carstens on his blog this week).

Well the time has come for discussion and predictions of increased activity to give way to, well, actual activity.

Satellite imagery this afternoon shows a few swirls out in the open Atlantic between the Caribbean and Africa. The westernmost swirl, a few hundred miles east of Barbados, has the most thunderstorm activity (red/orange colors on the satellite loop) as of this afternoon. That means it also probably has the best shot at developing into a tropical cyclone as it drifts west-northwest over the coming days. It has been tagged as 94L by the NHC to signify that it bears watching and is worth having some extra information about.

Indeed, a solid majority of ECMWF ensemble members show some development as 94L moves through the Lesser Antilles towards Puerto Rico over the first part of the upcoming week. As a result, islands north of Barbados can expect periods of gusty showers with locally heavy rain resulting in flooding over the next couple days. Thankfully, there are a few forces that should keep 94L on the weaker side even if it does become a TD/TS.

This map showing moisture and wind shear through the middle of the atmosphere indicates that dry air will be present to the west, south, and north of the system’s incipient circulation. With mid-level shear from the southwest, the dry air should make a solid attempt at intruding into the system’s center. This is probably 94L’s biggest hurdle to development, one that it may be able to overcome if it develops a more coherent circulation sooner and is able to keep thunderstorms going around its periphery to insulate the center from the drier environmental air.

After passing through the northern Lesser Antilles, 94L will likely move near or over Puerto Rico. Impacts here should be similar to those in the islands farther southeast: gusty showers with the potential for localized flash flooding. Surge and winds won’t pose much if any concerns as dry air and shear should keep the system’s circulation weak. Forecasts for Hispaniola look pretty similar with the extent of shower/thunderstorm activity dependent on where exactly the center tracks relative to the island. The dry air west and south of the disturbance should keep areas SW of the center’s track fairly dry.

All this interaction with land will pose another challenge to 94L as it moves northwest during the middle part of the week. Interaction with Hispaniola and Puerto Rico isn’t an automatic death sentence for a disturbance like 94L, as we saw with storms like Isaias, Laura, and Irene which strengthened as they tracked near or over the islands, but the odds are not in a tropical cyclone’s favor when passing through this part of the world.

Whatever happens in the Caribbean, a tropical disturbance of some sort should end up in or near the Bahamas by the later part of the upcoming week. As the above ensemble forecast map shows, confidence in a fully-developed tropical cyclone is lower here than down in the Caribbean due to the limiting factors discussed above. Only 11/51 ECMWF ensemble members depict a closed cyclone by the time shown above (five days from now). Only 1/51 members depicts a tropical cyclone of appreciable intensity (the 995mb outlier would be a strong tropical storm).

At this point, the system will face another hurdle.

This forecast map shows a somewhat esoteric parameter (“Potential Vorticity”) that highlights “sneaky” upper-level lows that can be harder to spot in other forecast fields. One such low looks to be located over Florida as 94L enters the Bahamas. These lows are a great source of wind shear and dry air, two things that strongly inhibit tropical cyclone development. Given this upper-level pattern, I would be surprised if anything more than a few gusty showers made it all the way to Florida by next weekend.

What about the rest of the Atlantic?

There’s another swirl embedded within a large trough over the central Atlantic. The NHC gives this system (93L) a 40% chance of developing within the next five days as it follows in the footsteps of 94L. There is less thunderstorm activity associated with 93L this afternoon, and possibly as a result most model forecasts are less enthusiastic about its chances. That said, when one disturbance follows another across the Atlantic, it often has a better shot at development as the lead wave “clears the air” of dust and dryness. So we’ll keep watching it, but right now there isn’t much to say except that another round of gusty showers seems likely in the Lesser Antilles next weekend.

That’s all there is to watch for now, but chances are we’ll have plenty more disturbances to keep an eye on as we get deeper into August. As a reminder, now is a great time to take a few minutes to grease the wheels on your hurricane preparedness while the tropics are still fairly quiet. Whatever steps you can take now to stock up on non-perishable supplies and/or review your plans will make the next hurricane that much less stressful regardless of when it comes.

I’ll have more updates as conditions warrant. More frequent musings can be found on twitter @JackSillin.

-Jack