Invest 91L struggles to organize

In a year when everything’s seemed to struggle — when the Atlantic’s gone an incredible 59 days and counting without a recorded tropical cyclone, in likely the first season without a hurricane through August since 2013, and perhaps even the first season since 1997 without an August tropical cyclone of any kind — it’s no surprise Invest 91L — the central Atlantic disturbance garnering high development odds — is struggling to organize.

Although showers and thunderstorms percolated around 91L overnight, the storminess remains disjointed on either side of the still-broad and elongated low pressure circulation.

Dry air is a big obstacle for the disturbance to overcome in the days ahead, and due to the difficult environment, only gradual development is expected.


Another positive development over the last 24 hours has been a general trend toward a much slower-moving and poleward-seeking system down the line.

This is shifting the threat away from the southeastern Bahamas and Turks and Caicos. Additionally, dozens of dropsonde observations from Monday’s high-altitude hurricane hunter mission were assimilated into the overnight global forecast models.

While interests in the Bahamas will want to stay atop the latest forecasts, the added data and better model agreement should help our neighbors rest a little easier Tuesday.

Deterministic forecasts from the European ECMWF model from late Saturday to Monday. Lighter lines indicate older forecasts. Forecast models have generally slowed and trended eastward since earlier runs. (Brian Tang/University at Albany.)

For us in South Florida, 91L is not an immediate concern for now. With dry air woes continuing to delay near-term formation and hostile wind shear cranking up in a few days, the disturbance has some work ahead.

A dip in the jet stream by late weekend looks to weaken high-pressure steering and open a channel for 91L to begin to pull northward early next week. We’ll keep an eye to the trends – as we’ve discussed in previous newsletters, forecasts this far out are subject to large errors – but for now the trend is our friend.


Elsewhere in the tropics, we’re watching another tropical wave roll off Africa. Some development is possible with this one, but the conducive environment may be short-lived as it moves deeper into the Atlantic. Either way, it’s no concern for us in South Florida this week.

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