• Bryan Norcross

August begins with some signs of tropical activity near Africa

The beginning of August is working out like the odds say it should. The tropics are, on average, fairly quiet this time of year, and nothing appears threatening through this week, at least.

A robust disturbance is just moving off Africa, but Saharan dust and a generally unfavorable atmospheric pattern will likely keep it from developing very much. The computer forecast models show little if any development.

Another disturbance is forecast to cross the African coast later in the week. Off and on, the long-range computer forecast models have indicated it might develop, but the forecasts have been inconsistent. In any case, predictions for a system that hasn't even begun to organize are intrinsically problematic. We'll see what the models say when the system is over the Atlantic waters late in the week.

There are at least two ways to look at the progress of a hurricane season. Usually, we tick off the named storms as they occur and compare that to average activity. Five named storms have developed so far this season, and the average date for the third storm getting named – averaging over the years 1990 to 2019 – is August 4th. So we're way ahead of schedule in the named-storm department.

When we plot named storms, the peak of the curve is September 10th – the date when we are most likely to have a named storm active. That's the hurricane-season graph you usually see on TV.

Another way to look at how active the hurricane season has been, however, is to calculate how much energy has been produced by the storms that have formed. The "Accumulated Cyclone Energy" or ACE takes into account the strength of a storm and how long it is strong. The longer and stronger, the more ACE it accumulates.

Hurricane Irma was an example of a hurricane that generated a lot of ACE. It was extremely strong and survived for almost two weeks. It generated the second-largest ACE number in the record book. Ivan in 2004 is number one.

Brian McNoldy, a hurricane researcher at the University of Miami, made a plot of the average ACE generate each day during 50 years of hurricane seasons from 1971-2020. Notice how low the graph is at the beginning of August – meaning the odds of a strong hurricane developing are low. But the plot shoots up in a hurry by the middle of the month. Then it falls off quickly again in October.

The graph tells us when the strongest storms are most likely to occur. In fact, about 75% of the energy generated by tropical storms and hurricanes occurs between August 15th and October 15th. That's the heart of hurricane season.

Damaging storms can occur outside of that 2-month window, of course. Witness Hurricanes Wilma and Sandy. But the odds drop off significantly after the middle of October.

For now, we'll keep an eye on Africa, but nothing is imminent.