There's a point in the
CSU discussion about reaching hyperactive ACE that I wanted to explore more about here:
Obviously if the season were to take a more active trajectory, there would still be time to reach the hyperactive ACE threshold (>159.6 ACE). Again assuming no ACE generated through 9 September, the Atlantic would need to produce ~104 additional ACE. Seven years in the satellite era (since 1966) have produced >=104 ACE from 10 September-onwards: 1967, 1998, 1999, 2004, 2005, 2017, and 2020.
(They had previously noted that to hit above-average 126.3 ACE, the season just needs to roughly follow climo after September 10.)
Here's how each of the 7 seasons produced most of their 104 ACE after September 10 (which would give us a hyperactive season if we followed them exactly).
1967:
- Cat 5 Beulah in the Caribbean and Gulf, starting from its first peak as Cat 4 (though its intensity fluctuated significantly as C4-TS-C5)
- Three long-lived Cat 1/2s: Chloe and Doria in September, Heidi in October
1998:
- Cat 4 Georges in the MDR, Caribbean and Gulf (very southernly track for a storm that formed on September 15)
- Three early-recurving or subtropical Cat 1/2s in late September (Ivan, Jeanne, Karl)
- Cat 5 Mitch, a classic October Caribbean monster
1999:
- Most of Floyd's lifetime, as a recurving Cat 4 in the SW open Atlantic that still hit NC
- Cat 4 Gert, a strong long-lived CV storm that formed on September 11
- Cat 4 Lenny, a late-season Caribbean storm but in the wrong month and direction
- Two other October Cat 2s (Irene and Jose)
2004:
- Slightly more than half of Ivan's lifetime, starting from south of Haiti (maintained Cat 3-5 from there until landfall)
- Cat 3 Jeanne, whose time as a hurricane was during the cyclonic loop and approach to Florida
- Cat 4 Karl, a recurving CV storm
2005:
- Rita and Wilma
- Long-lived but weak storms: Ophelia (Cat 1 meandering offshore NC), the trio of weird late-season storms (TS Delta, C1 Epsilon, TS Zeta)
- Other strong but short-lived storms: Beta (Cat 3 in SW Caribbean)
2017:
- Irma after Cuba, moving north as Cat 4 and landfalling in SW Florida
- Jose after its peak, already weakening on Sep 10 that stopped being Cat 4 soon after, but then becoming a long-lived Cat 1
- Maria's entire lifetime, both as an intense Cat 5 and long-lived recurving Cat 1-3
- Two shorter-lived majors in the subtropics: Lee (late September), Ophelia (October)
2020:
- Cat 4 Teddy as a CV storm
- Delta, Eta and Iota, each having enough strength and lifespan to contribute moderate ACE
- Other shorter-lived Cat 2/3s: Sally (mid-September), Epsilon, Zeta (late October)
How you interpret this is up to you. But to me, these seasons fall broadly into three categories:
- A strong hurricane was already in place earlier, but still contributed significant ACE after September 10 during or past its peak (1967, 2004, 2017)
- An active CV season in mid to late September, preferably with a classic Cat 4+ (1998, 1999, 2004, 2017, 2020), but some with long-lived but weaker systems (1967)
- Significant Oct-Nov activity in the Caribbean, preferably a Cat 4+ (1998, 1999, 2005, 2020)
2005 also had an anomaly in Rita, as no other years on this list had a
home-grown Gulf major, much less a sub-900. (However, Beulah and Zeta came close, as it entered the Gulf as Cat 1 and TS respectively.) Meanwhile, some years also got help from short-lived majors or meandering Cat 1/2s, though they don't contribute as much as the above.
What does this mean for 2024? We will almost certainly miss out on #1. All years that didn't have #1 (which is still the majority of the 10 seasons) relied heavily on #2 and #3, almost always both (except 2005). In other words, to hit hyperactive ACE,
we need both a very active late MDR season (preferably with storms like Lorenzo 2019 and Sam 2021)
and a strong Caribbean season (with classic monsters like Wilma, Mitch, Matthew, Delta, Eta and Iota; a 2010- or 2022-like late season with a bunch of Cat 1-2s won't make the cut).
(Also note that some of the #2 years had a storm forming immediately after September 10 or having already formed, such as Floyd and Teddy. We're unlikely to have that at least until September 15.)
My opinion? This is hard, but far from impossible. Yes, it does require the flip to switch almost immediately after September 15-20, and continue well into late season (at least through October, even if we don't get a full-on 2020 repeat). Both the MDR and the Caribbean need to be productive for their late seasons, preferably with several majors. Yet, precedents exist: Late-season CV storms have become slightly more common in recent years (Lorenzo, Sam, and also Tammy formed in late
October), and while the Caribbean doesn't always produce (such as a 2021-like October TUTT or 2022-like random Pacific MJO), its potential is also typically insane during late season.
CSU had explicitly noted in their discussion that most concrete negative factors that we know to date will become much less detrimental, if not beneficial, as we move to late season (La Nina with better atmospheric coupling, AEJ shear, north-displaced ITCZ, warm tropopause that implies stability; I'd also add +NAO and SAL to this list). Meanwhile, the "hardware" is still there to support a more extended peak season than usual. If you're optimistic, late September may even behave like a typical early September, as the SSTs clearly support it; but even with more conservative expectations, that's still not a time when MDR suddenly becomes incapable of pumping out storms. The Caribbean has also largely been spared from MDR's troubles: shear has been generally below-average there, and its current issues are mostly lack of waves and strong trades, both of which should (hopefully) improve over time.
Unless there are
still inhibiting factors that were not mentioned above... But I haven't seen any evidence suggesting that, unlike 2013 when the THC collapse eventually became identifiable, even during the season.
