24 cyclones attaining tropical depression status,
22 cyclones attaining tropical storm status,
12 cyclones attaining hurricane status, and
6 cyclones attaining major hurricane status.
This prediction far exceeds the 1991-2020 averages of 14.4 tropical storms, 7.2 hurricanes, and 3.2 major hurricanes each season. In short, I expect 2024 to be very active, with a moderate probability of breaking the top 5 most active seasons ever recorded, at least in terms of number of named storms. Below, I'll discuss the various factors that went into making this prediction.
The biggest story of the year so far in hurricane forecasting is the unprecedented warming of the tropical Atlantic ocean. For many decades, climate scientists have documented the increasing ocean temperatures caused by anthropogenic global warming, but 2024 stands out in particular.
The chart above shows the global averaged sea surface temperature since 1981. The second half of 2023 and 2024 thus far have seen average ocean temperatures far above what had been previously observed. On its own, a global ocean temperature increase should correlate to more fuel for tropical cyclone formation, but this isn't all.
The above diagram gives a snapshot from earlier this month of global sea surface temperature anomalies relative to the current mean. This means that the the large positive average anomaly from the previous image is accounted for, and we're looking at deviations from that average. One area that stands out is the tropical Atlantic, which is experiencing unusual warmth on top of an unprecedented global mean. In previous years, I've often commented on warm ocean temperatures and their contribution to more active seasons, but this year's signal is stronger than any I've seen. This factor alone is enough to predict a very active hurricane season.
One other area that stands out in the previous global ocean temperature map is the equitorial eastern Pacific, which is relatively much cooler. This signals the return of La Niña conditions, which have ramifications for global weather.
The chart above is the model forecast of the El Niño Southern Oscillation (ENSO) index, a composite measure of the sea surface temperature anomalies in the equitorial Pacific. Most of 2023 saw a positive index, called El Niño, but the index has recently been dropping, and the official conditions for a La Niña event should be met by peak hurricane season. Typically such an event is correlated with lower wind shear and hence less disruption to developing cyclones. Therefore, this is another signal in favor of an active hurricane season.
I'll run through a few more quick factors. Early indications point to a west African monsoon which is more robust than usual (this is captured, for instance, by the long-term model precipitation forecast above for the three month period August-October 2024). Many hurricanes have their origin in disturbances crossing west Africa, so this could mean that there are more "seeds" for hurricane formation; this is yet another positive factor for an active 2024. Lastly, I often consider the strength of trade winds and the magnitude of zonal wind shear across the Atlantic basin. Roughly, this is a measure of how consistent the direction and strength of the east-west component of wind is at different altitudes in the atmosphere. As mentioned above, La Niña usually leads to lower wind shear, but it's worth noting that these measures have remained close to average for 2024 thus far. This factor therefore doesn't have a significant forecast impact right now.
Next, I'll give a finer analysis of the risks by region. My estimates are on a scale from 1 (least risk) to 5 (most risk).
U.S. East Coast and Atlantic Canada: 5
The east coast of North America is at extreme risk from hurricanes this year. Ocean warmth was already discussed above, but on top of that, the likelihood of La Niña conditions and current model data both point to stronger than usual ridging over the subtropical Atlantic this summer. This means that tropical cyclone tracks have less chance of curving out to sea and an increased risk to land.
Yucatan Peninsula and Central America: 4
The same factors pointing to an active hurricane season suggest an above-normal risk for these regions. However, the predicted shear and atmospheric moisture anomalies aren't quite as favorable in the southern and western Caribbean, so I'll assign a "4" rather than a "5" here.
Carribean Islands: 5
For the last few years, many tropical cyclones forming in the tropical Atlantic had some struggles with dry air and wind shear as they approached the Caribbean; others curved out to sea before impacting the islands. This year, the risk seems much higher: a more robust African monsoon will lead to more long-track hurricanes and ample atmospheric moisture. Further, the developing La Niña will suppress shear and keep storms on a more westward track.
Gulf of Mexico: 4
As with every other region, the coast of the Gulf of Mexico is at high risk. The choice of "4" rather than "5" is due to some expected dryness, especially in the western Gulf where extreme heat and drought may prevail for significant parts of the season. The eastern Gulf should be on high alert, especially near the peak of the season in August and September.
Overall, I expect the 2024 Atlantic hurricane season to be very active, potentially historically so. Nevertheless, this is just an amateur forecast. Individuals in hurricane-prone areas should always have emergency measures in place. For more on hurricane safety sources, see here. Remember, devastating storms can occur even in otherwise quiet seasons.
Sources: https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf, https://tropical.colostate.edu/Forecast/2024-04.pdf, https://www.tropicaltidbits.com/analysis/models/, https://www.noaa.gov/news-release/noaa-predicts-above-normal-2024-atlantic-hurricane-season
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