The tropical Atlantic is expected to continue its highly unusual slumber this week, in what is typically one of the peak weeks for hurricane activity. At 2 p.m. EDT Tuesday, the National Hurricane Center was not tracking any named storms, and was only giving 7-day odds of development of 30% to two tropical disturbances – one in the central Caribbean, and one off the coast of Africa. Both had only lukewarm enthusiasm for development from the 0Z and 6Z Monday runs of the GFS and European model ensembles, with fewer than half of their members showing development.
The only threat to land was the disturbance in the central Caribbean, which was bringing heavy rain showers to the Dominican Republic and Haiti on Tuesday afternoon. Satellite loops showed that the disturbance was disorganized, with a modest amount of heavy thunderstorm activity and not much spin. Conditions were modestly favorable for development, with moderate wind shear of 10-20 knots, a reasonably moist atmosphere, and near-record warm ocean waters of about 30 degrees Celsius (86°F) – though large-scale sinking air was inhibiting development.
The disturbance was headed west at about 15 mph, and will spread heavy rains to Honduras on Friday, when a number of the members of the 6Z Tuesday run of the European ensemble model predicted that a tropical depression could form. Over the weekend, the disturbance will bring heavy rains to Belize and Mexico’s Yucatan Peninsula, and likely emerge into the southwestern Gulf of Mexico (Bay of Campeche) on Sunday. A number of members of the 0Z Tuesday run of the European ensemble model predicted that the disturbance could head more to the north early next week and potentially threaten the U.S., but the GFS model ensemble was cold to this idea in its Tuesday morning runs.
NHC was tracking two other disturbances on Tuesday, one in the central tropical Atlantic, and the one off the coast of Africa mentioned above. Both of these disturbances are expected to move to the west-northwest, eventually recurving, and are unlikely to be a threat to the Caribbean Islands or North America.
Why so quiet? Updates on the head-scratching among forecasters, and why a probabilistic forecast isn’t “wrong”
The Atlantic is getting plenty of side-eye right now from seasonal forecasters and other hurricane experts trying to get a handle on why there’s been so little activity since early August. As we discussed in our “Where are the hurricanes?” post on August 27, there’s no lack of hypotheses, but ultimately it may take a dissertation (or a few of them) to unravel why this period has been so tranquil.
One thing crucial to keep in mind is that seasonal hurricane forecasts are inherently probabilistic, even if they’re not presented that way. NOAA is explicit about its probabilities. In its most recent outlook update (August 8), NOAA called for a 90 percent chance of an above-average Atlantic hurricane season – as well as a 10 percent chance the season would be less active than usual. Those are once-a-decade odds of unexpected calmness, and in fact we saw just such an outcome 11 years ago, in 2013. Like most other seasonal forecasters, NOAA had been sticking to its outlook for a busy 2013 season as late as its August update, which had a 70% chance of an above-average season, 25% chance of near average, and 5% of below average. The year ended up spawning a near-average 14 named storms in the Atlantic, but only two hurricanes and no major hurricanes, with an accumulated cyclone energy of a mere 36.1 compared to the 1991-2020 average of 123.
The most obvious factors in play over recent days, as we noted last week, have been disturbances exiting Africa farther north than usual – often ingesting plenty of Saharan dust and dry air – and then struggling with limited instability over the Main Development Region between the Caribbean and Africa, the result of exceptional warmth being even more dramatic at upper levels than at the ocean surface. (Meanwhile, the Sahara continues to get phenomenal amounts of rain relative to average, a once-in-a-generation event covered in detail on Monday in a post at severe-weather.eu.)
As we segue into northern autumn, upper levels of the atmosphere will cool more rapidly than the surface across the Atlantic subtropics and tropics, so instability will likely tend to increase and give hurricane activity a boost. Sea surface temperatures over the key formation areas remain at scorching record and near-record levels, so there’s every possibility that the second half of the season will kick into gear dramatically. And as many of our readers know all too well, “it only takes one” to inflict untold misery and tens of billions in damage.
For more on the quizzical quiet, see Michael Lowry’s recent Substack posts, as well as the two-week outlook for Atlantic activity issued by Colorado State University forecasters late Tuesday, September 3 (the link appears down the page on the CSU forecast site). The CSU team’s conclusion: “We believe that the most likely category for Atlantic hurricane activity in the next two weeks is below-normal (60%), with near-normal (30%) and above-normal (10%) being less likely.”
At least 14 killed as Yagi’s rains rake the Philippines; a major landfall possible in China
In the Pacific, Tropical Storm Yagi dumped torrential rains as it moved across the northern Philippines on Monday local time. Local officials have reported at least 14 deaths related to floods and landslides, according to the Associated Press. Yagi is predicted to become a strong typhoon as it crosses the South China Sea, heading toward a likely landfall in far southern China around Friday local time and a potential second landfall in northern Vietnam over the weekend.
The Joint Typhoon Warning Center predicted on Tuesday morning that Yagi will peak as a Category 3 equivalent shortly before its first landfall, in line with forecasts from the HWRF and HAFS-A models calling for a Cat 3 and Cat 4 peak, respectively.
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