15 cyclones attaining tropical depression status,
14 cyclones attaining tropical storm status,
8 cyclones attaining hurricane status,
4 cyclones attaining major hurricane status.
This prediction is near the 1991-2020 averages of 14.4 tropical storms, 7.2 hurricanes, and 3.2 major hurricanes each season in the Atlantic basin. I expect that 2023 will thus be an about average season. Below, I'll discuss the various factors involved in making this prediction. Note that the the NOAA identified a unnamed subtropical storm that formed in January 2023 in the western subtropical Atlantic, which was not designated operationally at the time. This storm is included in the above total, and will be covered on this blog once a tropical cyclone report on it is released.
I'll begin as usual with the El Niño Southern Oscillation (ENSO) index. Though simply a measure of sea surface temperature anomalies in the equitorial Pacific Ocean, it is correlated with trends in tropical cyclone activity all around the globe. For the Atlantic specifically, warmer than normal Pacific waters (El Niño) tend to give quieter Atlantic seasons, while cooler than normal waters (La Niña) lead to busier Atlantic seasons. It's worth noting that our understanding of this phenomenon, as well as possibly the ENSO cycle itself, has changed over the last few decades.
Nevertheless, there are some signs that point in the opposite direction.
Moving from the Pacific to the Atlantic, we also see above average sea surface temperatures in the main cyclone formation regions, including the tropical Atlantic, the Caribbean, and the Gulf of Mexico. The effect of these anomalies on hurricane formation is more straightforward: warmer oceans mean more favorable conditions for development and intensification. It's worth noting that some of the warmest areas are in the tropics rather than the subtropics this year, where they will likely give a greater boost to tropical cyclones. The best recent historical analog for this sea surface temperature profile in May is 2010, a very active hurricane season.
One last major factor to examine is wind shear. The figure below, from Colorado State University's annual hurricane forecast report, shows different possible atmospheric wind profiles and how they might influence Atlantic hurricane activity.
The graphic above shows two possible scenarios for wind shear across the MDR (Main Development Region) of the Atlantic. The vertical direction indicates height in the atmosphere, and the horizontal axis is the component of the wind speed in the west-east direction (left is west, right is east) averaged across the indicated area. At lower altitudes, the east-to-west trade winds dominate the scene, but the wind direction may often change several miles aloft. The change of west-east wind speed with height is known as zonal wind shear and has significant implications for cyclone activity. In the unfavorable (blue) profile, the surface trade winds are stronger but the winds switch east near the top of the troposphere, the layer of Earth's atmosphere where most weather occurs. This makes it difficult for vertically aligned vortices to form, because different levels of a nascent circulation are pushed in opposite directions.
The red profile, in contrast, is a relatively uniform westerly wind at every level, meaning wind shear is low and conditions are favorable for tropical cyclone development. CSU takes spring zonal wind shear measurements into account when making their forecast; this shear has been a bit lower than normal for 2023 so far. Of course, this could change: El Niño also sometimes brings about a high zonal shear regime across the Atlantic. Putting all this together, I'm forecasting a near average hurricane season. 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: 2
I'll put the east coast at lower than usual risk this year. The subtropical Atlantic has consistently been dominated by troughs this year so far, so cyclones reaching the latitude of the east coast have a good chance of recurving to the east. The oncoming El Niño may bring an early fall to the continental U.S., shutting down development in this region by October.
Yucatan Peninsula and Central America: 3
A more active Pacific hurricane season could help suppress cyclone formation on the other side of central America this summer. However, Atlantic waters are particularly warm in the southwestern Caribbean, which is always a danger zone for rapid intensification. I expect most of the season to be quiet in this region, but the possibility of a few strong storms brings the risk up to a 3.
Carribean Islands: 4
El Niño aside, the tropical Atlantic looks as favorable as it has looked in a while: temperatures are very warm, and the Saharan air layer is less prominent than usual near Africa. Look for possible long-track hurricanes forming and affecting the Caribbean islands, before likely turning northward. I'll put the eastern islands at greater risk than the western ones.
Gulf of Mexico: 2
The same factors at play for the U.S. East Coast give a similar story here. Though waters are warm, I think shear and atmospheric steering currents should shield the Gulf coast from the worst impacts, particularly during the latter half of the season. July and August may be the riskiest months for a Gulf hurricane landfall if one does occur.
Overall, I expect the 2023 Atlantic hurricane season to be around average, with perhaps a slightly larger number of hurricanes and major hurricanes than usual (though I expect most of these to go out to sea). 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://yaleclimateconnections.org/2023/04/a-mystery-in-the-pacific-is-complicating-climate-projections/, https://www.nhc.noaa.gov/sst/rsst_atl_anom_loop.php, https://tropical.colostate.edu/forecasting.html
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