Tropical Disturbances Smothered, Covered, and Capped by Near-Record July Dust

It’s been the second dustiest July in over 20 years across the tropical Atlantic, but the peak is behind us

Saharan dust plume blowing off western Africa on Sunday, July 21^st, as seen by the low-orbiting NOAA-20 satellite. Credit: NOAA

If it seems like we’ve been talking a lot about Saharan dust this July, it’s because we have. So far the tropical Atlantic has seen higher dust concentrations than any July in the continuous satellite record (since 2002) with the exception of July 2018.

Dust cover over the Atlantic typically peaks in late June or July so the arrival isn’t unusual but the amount this July is notably high. The dust topped out earlier this month and will decrease quickly once August arrives.

The veil of dust is most prominent at between 5,000 and 20,000 feet above the surface, where it’s known as the Saharan Air Layer or SAL. The SAL is generally warmer than the cooler air near the ocean and the increasing temperature with height – known as an inversion – caps or stunts thunderstorm growth. Additionally, along the edges of the SAL, wind shear can be high, another deterrent to tropical activity.

For much of this July, the warm, dusty Saharan air has left us with little to talk about in the tropical Atlantic.

Why dusty Julys offer little consolation for August and September

Unfortunately, the amount of dust in June and July is a lousy predictor of seasonal hurricane activity, mainly because the dust falls off so suddenly by August as zesty tropical disturbances roll off Africa.

Sporadic dust outbreaks still happen in August and September and can affect individual systems from time-to-time, but the extent and magnitude of the outbreaks don’t knock down the entire basin as they can do in the early part of the hurricane season.

Watching the (tropical) waves as August approaches

As we’ve discussed in newsletters this week, the sinking air that’s moved over the Atlantic in recent weeks should be replaced by rising air and more stormy conditions in about 2 weeks. This means we’ll need to start keeping closer tabs on “dry” disturbances and tropical waves that may start off without much fanfare in the eastern Atlantic, but perk up as they get closer to the Caribbean or U.S. in a week or two.

There’s a weak signal for some development of a tropical wave nearing the islands later next week, but we’re still a ways off to say much about this scenario. The overall upper-level wind pattern will be trending toward a more conducive configuration for development as we turn the calendar, however, so by the first full week of August, we’ll need to sharpen our sights on what could be a busy stretch into the peak of the season.