Tropical Depression Five Forms in the Far North Atlantic, Ending 60-Day Atlantic Dry Spell

Air Force Hurricane Hunters set to investigate Invest 91L east of the Caribbean starting this afternoon

August ended yesterday without a single tropical depression, tropical storm, or hurricane – only the second August in 60 years without an Atlantic tropical cyclone – but September quickly broke the dry long dry spell with Tropical Depression Five early this morning in the far north Atlantic, the first tropical cyclone anywhere in the Atlantic since Tropical Storm Colin dissipated over North Carolina 60 days ago on July 3rd.

Tropical Depression Five formed at an unusually high latitude for September, the farthest north a system has formed in September since Subtropical Storm Alpha on September 17, 2020. Water temperatures in this part of the Atlantic are much warmer than average, likely contributing to the transition from a frontal low to a tropical depression.

Tropical Depression Five is forecast to gradually strengthen over the next five days, becoming a tropical storm later today and topping out as a high-end Category 1 hurricane early next week. The system is in no hurry to move and should mostly drift over the north-central Atlantic into the weekend.

Meanwhile, much farther south, the system we’ve been following most closely this week – Invest 91L, located about 500 miles east of the easternmost Caribbean islands – hasn’t been able to maintain enough persistent storminess over its broad circulation to graduate to a full-fledged tropical depression or tropical storm. It’s been close at times, but the relentlessly dry and stable atmosphere surrounding it doesn’t want to budge.

The big story this week for 91L has been not as much its organization woes, but the remarkable slowdown in the extended range forecasts into early next week. We discussed both the uphill climb in organization and potential for large track errors with this one in previous newsletters. On Sunday, the acclaimed European model showed 91L roughly 500 miles west of the 4-5 day Labor Day forecast yesterday. That’s a big spread, even for a 7-day forecast.

With the steering regime collapsing, 91L is going to crawl its way north of the U.S. Virgin Islands and Puerto Rico this weekend. By early next week, 91L will begin to pivot northward as it feels the tug of a jet stream dip. Although the southeastern Bahamas and Turks and Caicos should continue to follow the forecasts into the weekend, this one doesn’t appear it will pose a threat to South Florida.

Forecast models for Invest 91L. Blue ellipse indicates model spread out to next Monday. Credit: Tomer Burg/University of Oklahoma.

Elsewhere across the Atlantic, a tropical disturbance just north of the Cabo Verde Islands off Africa has a low chance of development over the next day or two before conditions become less favorable into next week.