Are the typical bounds of Cabo Verde season changing?

[CyclonicFury]

For years, the Cabo Verde part of hurricane season was considered to primarily occur from mid-August to late September, with the occasional rare storm outside of the period.

But since 2017 or so, we have begun to see a lot more early and late MDR activity, with less strong MDR activity near the peak. 2017 had Tropical Storm Bret in June, the first June MDR tropical storm in 38 years.

2018 followed with Hurricane Beryl in early July, which was at the time the earliest MDR hurricane in the satellite era. It also had Tropical Storm Nadine in October, which set records for easternmost formation at the time of the year.

In 2019, there was Hurricane Lorenzo in late September, which became of the latest long-track CV major hurricanes on record. Tropical Depression 15L formed off the coast of Africa in mid-October, which was even farther east.

In 2021, the Cape Verde season started very early with Hurricane Elsa, which broke Beryl's record for earliest MDR hurricane in the satellite era. It also had Hurricane Sam, which became a powerful Cape Verde hurricane at the very end of September, and Tropical Storm Victor, which lasted into October.

In 2022, Tropical Depression Twelve formed over the eastern MDR in early October.

In 2023, TWO tropical storms formed in the MDR in June: Bret and Cindy. There was also the formation of two storms in the MDR in October: Sean and Tammy, the latter of which became the easternmost MDR hurricane on record in the basin in October.

But 2024 was where this recent phenomenon was taken to an entirely new level. Hurricane Beryl became a CATEGORY 4 in the MDR on June 30, breaking the record for earliest MDR hurricane in the satellite era set by Elsa 3 years earlier. No hurricanes formed in the MDR in August or September. Now, we have Hurricane Kirk in the eastern MDR, which shattered Tammy's record set just last year for easternmost MDR October hurricane. And 91L has potential to break Kirk's record if models are correct! We are likely to see two legitimate Cape Verde type hurricanes in October, which is unprecedented.

There's two main theories I have as for why this has been the case: 1) the MDR becoming more stable over time in Aug/Sep, which peaks around early-mid August and 2) Active WAM shifting the African monsoon trough northward, leading to tropical waves exiting too far north during August. We'll have to see if what has happened in the last few years is an aberration, or an actual trend to a change of CV season timing.

[aspen]

Hard to tell if this is a trend or just a temporary activity phase, because each season with early or late MDR activity was a little different in terms of its background conditions and overall distribution of activity, therefore each has different explanations for why they exhibited their MDR TCG.

2017 was your textbook hyperactive MDR year, peaking in September with only minimal TCG in the region before ASO. Bret was a sign of things to come but the MDR didn’t go crazy until climo peak, and then proceeded to shut down once October rolled around.

2018 was a solid MDR year overall (Beryl, Florence, Helene, and Issac all became MDR hurricanes), which is surprising given its weak +ENSO and mediocre SSTAs. Beryl was definitely unusual given the circumstances.

2019 also didn’t have any impressive ENSO/SSTA indicators, so Lorenzo at the time was an anomaly (and still kind of is). However, peak MDR activity still didn’t extend much further outside its normal bounds, as 15L remained weak. Early-season activity was also fairly mediocre, and the basin didn’t ramp up until late August as per usual.

Might as well mention 2020. While it didn’t have any super early MDR development like all the other seasons, it did still produce Gonzalo and Isaias in the back half of July, which is a little earlier than normal. Plus, as I’ve mentioned before, July MDR activity is usually correlated with an above-average season…which definitely panned out lol. Also unlike the other seasons, 2020 had near-continuous activity with only minor breaks.

2021 is when we start seeing the ongoing trend of a small late June/early July peak in activity, followed by a drought for much of July into August. Elsa turned out to be an indicator of another solid MDR year, with Larry and Sam becoming 30+ ACE MDR majors.

2022 famously had Bonnie massively under-perform and then go quiet until early September. Bonnie may have been another Beryl ‘24 if its precursor wave was only a degree or 2 further north, but contrary to most of the models, it stayed quite far south. 2022 was a delayed peak season caused by unforeseen suppressive factors, hence why MDR activity went later than normal.

2023’s early and late activity was likely fueled by the record SSTs and the surprisingly non-suppressive El Niño. I still don’t fully understand why/how we get a 21 NS Atlantic hurricane season with a MDR Cat 5 in an El Niño, but suffice to say it exhibited both that early season peak and an extension to climo peak.

2024 is in a similar boat as 2022, except even more extreme. Massive peak in late June/early July with 35 ACE Beryl, then sporadic TCG until another peak in late September/early October. So unlike all other years discussed here, 2024 seems to be a double-peaked season surrounding normal climo peak due to the northerly displaced ITCZ/AEJ.

So out of these seasons, I think only 2021 and 2023 show a true extension of MDR activity in both directions. 2017 mostly adhered to climo timing, 2018 and 2019 had it in only one direction (early for 18 and late for 19), 2020 was just a constant spam-fest, and 2022 and 2024 had late-shifted peaks due to anomalous unforeseen atmospheric factors. There has seemingly been a trend towards a peak in late June/early July, which is backed up by a post someone made not long after Beryl showing this more pronounced early peak in the last 20 years compared to the 90s and 2000s. But we’d need to see more seasons to see if this trend persists or if it’s just yet another multi-Decadal phase in TCG.

[CyclonicFury]

Hard to tell if this is a trend or just a temporary activity phase, because each season with early or late MDR activity was a little different in terms of its background conditions and overall distribution of activity, therefore each has different explanations for why they exhibited their MDR TCG.

2017 was your textbook hyperactive MDR year, peaking in September with only minimal TCG in the region before ASO. Bret was a sign of things to come but the MDR didn’t go crazy until climo peak, and then proceeded to shut down once October rolled around.

2018 was a solid MDR year overall (Beryl, Florence, Helene, and Issac all became MDR hurricanes), which is surprising given its weak +ENSO and mediocre SSTAs. Beryl was definitely unusual given the circumstances.

2019 also didn’t have any impressive ENSO/SSTA indicators, so Lorenzo at the time was an anomaly (and still kind of is). However, peak MDR activity still didn’t extend much further outside its normal bounds, as 15L remained weak. Early-season activity was also fairly mediocre, and the basin didn’t ramp up until late August as per usual.

Might as well mention 2020. While it didn’t have any super early MDR development like all the other seasons, it did still produce Gonzalo and Isaias in the back half of July, which is a little earlier than normal. Plus, as I’ve mentioned before, July MDR activity is usually correlated with an above-average season…which definitely panned out lol. Also unlike the other seasons, 2020 had near-continuous activity with only minor breaks.

2021 is when we start seeing the ongoing trend of a small late June/early July peak in activity, followed by a drought for much of July into August. Elsa turned out to be an indicator of another solid MDR year, with Larry and Sam becoming 30+ ACE MDR majors.

2022 famously had Bonnie massively under-perform and then go quiet until early September. Bonnie may have been another Beryl ‘24 if its precursor wave was only a degree or 2 further north, but contrary to most of the models, it stayed quite far south. 2022 was a delayed peak season caused by unforeseen suppressive factors, hence why MDR activity went later than normal.

2023’s early and late activity was likely fueled by the record SSTs and the surprisingly non-suppressive El Niño. I still don’t fully understand why/how we get a 21 NS Atlantic hurricane season with a MDR Cat 5 in an El Niño, but suffice to say it exhibited both that early season peak and an extension to climo peak.

2024 is in a similar boat as 2022, except even more extreme. Massive peak in late June/early July with 35 ACE Beryl, then sporadic TCG until another peak in late September/early October. So unlike all other years discussed here, 2024 seems to be a double-peaked season surrounding normal climo peak due to the northerly displaced ITCZ/AEJ.

So out of these seasons, I think only 2021 and 2023 show a true extension of MDR activity in both directions. 2017 mostly adhered to climo timing, 2018 and 2019 had it in only one direction (early for 18 and late for 19), 2020 was just a constant spam-fest, and 2022 and 2024 had late-shifted peaks due to anomalous unforeseen atmospheric factors. There has seemingly been a trend towards a peak in late June/early July, which is backed up by a post someone made not long after Beryl showing this more pronounced early peak in the last 20 years compared to the 90s and 2000s. But we’d need to see more seasons to see if this trend persists or if it’s just yet another multi-Decadal phase in TCG.

What's also been surprising is the lack of MDR hurricanes during the month of August. There hasn't been a hurricane in the MDR during August since Irma in 2017, which became a hurricane on August 31. Yet since then there's been 3 July MDR hurricanes, and 2 (likely 3 soon) October MDR hurricanes.

[al78]

2018 followed with Hurricane Beryl in early July, which was at the time the earliest MDR hurricane in the satellite era. It also had Tropical Storm Nadine in October, which set records for easternmost formation at the time of the year.

If you look at the MDR region in isolation rather than storms which formed in the MDR, the MDR activity in 2018 was suppressed. Only short-lived cat 1 hurricanes tracked through there and Florence did not fully mature until it moved north of the MDR into the sub-tropics. If you look at storm tracks overall the sub-tropics were considerably more active than the MDR. I remember 2018 from the anticipation of an active season due to an MDR hurricane in July but August activity was very low (two short-lived tropical storms), then a switch flipped as we went into September and activity skyrocketed.