The Bell has Rung, So Why are Forecast Models So Muted?
Late August marks the traditional ramp up in hurricane activity, but long-range models show very little development potential. Is something amiss with the hurricane season?
Dr. Bill Gray, the larger-than-life seasonal prognosticator and forefather of tropical meteorology, had a tradition every August 20th in his academic department at Colorado State University in Fort Collins. He’d stroll through the halls of his labs ringing a bell heralding the start of the most active part of the hurricane season. Hurricane season isn’t a steady climb but a rapid ascent that, as we’ve shown in this newsletter over the years, usually kicks into high gear around the last week of August.
The bell has rung, but our forecast models over the next few weeks are lackluster to say the least. As we discussed in newsletters last week, the upper-level configuration of the Atlantic right now should favor tropical development and of course water temperatures remain at record levels throughout the basin. Which begs the question: are all those early forecasts calling for a hyperactive hurricane season overcooked?
The bait and switch
Hurricane season is notorious for fooling us around this time in August each year. Without fail, each August, we begin to wonder “where are the hurricanes?”, even in years like this one where we’ve already had hurricanes – including the earliest Category 5 hurricane on record – and when the hurricane count is already running weeks ahead of what’s typical in mid-August. Every hurricane that’s formed so far has made landfall, including already two U.S. hurricane landfalls (the first U.S hurricane landfall doesn’t typically occur until around August 23rd). We expect the hurricanes to go full tilt when history tells us they come in fits and starts.
The same head fake came in 2023 before the outbreak of storms that preceded a major hurricane strike in Florida’s Big Bend, and in 2022, which went 60 days without a single tropical system and didn’t notch its first hurricane until September 2nd, but culminated with Category 4 Fiona and Category 5 Ian that devastated southwest Florida.
Sometimes the first big ramp up in hurricane activity happens in August but some years it doesn’t happen until September, but it almost invariably happens.
What’s behind the short-term lull?
Several factors are putting up temporary road blocks to development this week. The most obvious of these is a northward displacement of a collision zone where many of our tropical seedlings take shape over Africa. This band of storminess is known as the Intertropical Convergence Zone or ITCZ and snakes north and south throughout the year.
The extreme northward position means that disturbances are launching from Africa so far north that they’re getting embedded in loads of Saharan dust. We’re seeing this today with a disturbance that just pushed off Africa north of the Cabo Verde islands.
These large seedlings are having trouble shaking their dusty origins and either struggle to mix out the dry environment like Hurricane Debby’s precursor disturbance or simply wash out as they move westward like the forecast for the current tropical wave.
The Saharan dust has been running above average since July and is especially high in the eastern North Atlantic this August (2nd highest on record behind 2011) near the coast of Africa where many of our tropical waves have been splashing into the Atlantic in recent weeks.
The development barrier should naturally fall, especially into September as the ITCZ drops southward and Saharan dust quickly tails off.
Too much of a good thing?
As regular readers of this newsletter know, wind shear – the change in wind direction or speed with altitude – is one of the big hurdles to tropical development. Typically it’s the strong winds at upper levels (30,000-40,000 feet) which blow from west to east (known as upper-level westerlies) through the deep tropical Atlantic and the Main Development Region or MDR that create most of the wind shear issues.
So when upper-level winds in the tropical Atlantic turn more east-to-west (easterly winds) and against the west-to-east grain, it usually helps to reduce wind shear, which in turn favors hurricane development. This is especially true during La Niña years like we’re experiencing and when rising air settles in over Africa and the North Indian Ocean as it has recently – a big upper-level disturbance we track across the tropics called the Madden Julian Oscillation or MJO.
The combination of La Niña with the part of the MJO that usually reduces wind shear would normally suggest a conducive period for development. However, the upper-level winds have switched so far out of the east that they’re creating easterly wind shear issues across the Atlantic, an unusual problem not often seen in the basin.
It means that the MJO will need to move to a less conducive phase to bring back the seasonal upper-level westerlies and tamp down on easterly shear issues plaguing the basin right now. The easterly shear is also worsened by the low-level westerly winds from the northward displaced ITCZ we discussed earlier.
The MJO naturally propagates eastward across the globe and extended range models are already showing the easterly shear issues abating toward the end of the month.
So when will activity pick back up?
Putting all the pieces together, it should be another week or two before the current impediments clear out.
It’s possible our next spurt of activity is delayed until the beginning of September, but we should see the transition as August comes to a close late next week.
Traditionally 70% of tropical activity, including most hurricane formations, happens from September onward. Still a long way to go in the game, but for now we’ll soak up the unforeseen late August lull.
I vote for more wind shear and more Saharan Dust!
Thanks again for the clear explation and to mother nature for saharan dust and wind shear. I'd like it to stay this way until Nov. 30th!!