09/09/2025
So what's with our doorknob-dead tropics at the heart of the season? A bit longer write up this morning.
Climatologically, we are entering the "peak" days of hurricane season. If you graphed all activity we see, September 10th is the statistically most active day of the year in the basin. But the NHC's map is blank on our side, and the whole basin looks like a graveyard. Will it stay that way?
We can start with the current Atlantic pattern. We can look at upper winds and sea temps. The upper-air pattern is currently quite hostile. We have several instances of waves slipping under the upper ridge in the Atlantic, creating upper troughs and disturbances that spread wind shear and dry air across the Sub-tropical and tropical part of the basin. Waves moving east are hitting firehoses of shear, like we just saw with Invest 91L, that likely develops in 9 out of 10 other seasons. This hostility isn't unusual in May or June but to see it peak season is strange.
We also see cooler than normal water temps out in the Eastern Atlantic, and even the Gulf of Guinea. Broadly this gives waves less development chances off Africa, and the G of G cool-ness messes with localized shear and moisture patterns. When you kick the waves as soon as they leave, makes life much harder down the road.
More broadly, there's widespread general stability in the Atlantic this year. Mid-latitude warming + meh SSTs create stability via marginal lapse rates across the basin. You need temperatures cooling with height to lift air (warm air rises), and the tropics just haven't had ton of that. Warmer aloft, cooler beneath. Background factors like the MJO and Kelvin Waves haven't helped much, only really lining up that week Erin came through, and early season for that run of 2-3 junk storms. Forcing has largely stayed either in the Pacific, or just generally dormant on the whole. Besides an active East Pac year, neither the West Pac or Atlantic have done much of anything.
Which begs the question: Does this mean the season's rip, or that we're heading for a back-loaded Late Sep-Oct?
Short-term, I think the Atlantic has a small window, but it's narrow. You see ensembles lighting up, which is a product of climatology mostly. There are a few waves, but this current-day pattern is not going to let them do much. I think working into around that Sep 16th-20th window, much better forcing shows as a stronger CCKW shows up, and the MJO meagerly rotates back to 8-1-2 phases, giving us a better window of activity. Will need some sort of waves or pattern to take advantage, but that's your next real opportunity.
Longer term, one thing that often saves these seasons in terms of activity (IE "backloaded seasons") is the transition to fall. The previously mentioned lapse rate/stability issue self-corrects as upper air temps cool with loss of solar insulation. Troposphere cools, while waters stay hot via heat retention (and lack of storms). This is what drove storms like Ian, Helene, Milton and others in the last few years. It could again this year. CPC longer-term outlooks suggest a more zonal pattern over the Eastern US, which could lower pressure elsewhere with less wave-breaking in the Atlantic. We might wind up with a "Late summer" look at month's end.
So basically, the season is almost certainly going to fall short of lofty expectations, but like the last few years, it could still make up for lost time. Don't discount the threat of a strong hurricane in late Sep or early-mid Oct in the basin. But for now, it's dead and there really aren't any signs of life yet.
