Tomorrow marks the beginning of the 2019 Atlantic Hurricane Season, which has been shrouded in questions over the past several months. Since the discussions in April, the overall outlook for the season is trending more active than once thought. That being said, there still remains high amounts of uncertainty going forward. For reference throughout the article, please refer to the graphic posted below.
Over the past several weeks, the state of the ENSO has become less certain, as the current El Nino-like pattern is not as pronounced as it was earlier this spring. The westerly wind burst that occurred earlier in the month did not fully progress across the entire equatorial Pacific, leaving the portion near South America with stronger trades relative to the remainder of the ENSO regions. Subsurface heat anomalies within the Kelvin wave have decreased dramatically, especially within the Nino 1+2 regions. Cooler subsurface anomalies have especially thrived in the eastern equatorial Pacific due to this and the downwelling of cooler water further south along the western South American coastline. Some warm pockets are still present, but have been diminishing gradually. As a result, the highest anomalies are centered around the central Pacific and are currently forecast to remain that way. Climate models are now setting up the current El Nino, which is already on the cusp of being qualified as such, as more of a Modoki El Nino towards the summer and fall, which is defined by the highest SSTAs concentrated in the central equatorial Pacific. In addition, the ENSO support in the North Pacific is unclear, given that the PDO signature is not well defined given the cool anomalies off the West Coast of the U.S. and scattered warm and cool anomalies east of Japan; climate models do show this anomalously cool pocket dissipating as troughing becomes less persistent, amplifying the PDO. If a Modoki El Nino does pan out, then all of the normal impacts of an El Nino shift westwards, so shearing convective outflow and descent would not be directly focused over the Atlantic, but rather, closer towards the EPAC. This would leave more room open for vertical ascent over the tropical Atlantic, possibly enhancing tropical activity. Modoki El Ninos are quite rare, so their exact impact on Atlantic tropical activity is unknown, but in general, these years do produce more tropical activity in the Atlantic than typical El Ninos.
The state of the Atlantic has also been trending more favorable towards the development of tropical activity. The most notable difference is that the pattern has trended towards a more positive AMO signature, with an AMO index number increasing to -1.35. This is still in the negatives because the overall pattern is displaced to the north of where it typically stands. The overall anomaly configuration has trended towards a warming tropical Atlantic, with the exception of a recent trade burst directly off of Africa cooling the waters. These warmer waters extend northward across waters west of the whole of Africa and Europe. Cool anomalies in regions near and south of Maritime Canada have also spread out towards the east, and waters near Greenland and Iceland have also warmed anomalously. The warm pool across the western Atlantic has cooled quite drastically, although it is still running above average. This pattern, along with climate models indicating lowering shear due to the weakening and westward displacement of the El Nino, is becoming more favorable for tropical waves to thrive and develop in classic fashion. However, this northward displacement of the AMO may mean that the highest heat anomalies will be concentrated farther north, which could cause the ITCZ to be slightly weaker. This, however, may be counteracted by a stronger West African Monsoon, which enhances the vorticity and amplification of tropical waves as they progress offshore. This has enhanced moisture and precipitable water anomalies across the tropics and especially over western Africa. It still remains to be seen how the state of the monsoon impacts the African Easterly Jet and the quantity of dust within Saharan Air Layer.
Given the conditions we have in place going into this hurricane season, we can look at previous years that had analogous setups to this one and see how they ended up. We call these “analog years”. There are no perfect analogs, and there never are, but the given that uncertainty this year is higher than normal, the range of possibilities is higher. Some analog years chosen were very active years, such as 1969, 2004, and 2017. 1969 had a lot of homegrown storms and a few Cabo Verde hurricanes. 2004 featured high amounts of MDR and Caribbean activity with lots of Cabo Verde hurricanes based almost entirely within August and September. 2017 featured less stricken activity than 2004, but still featured multiple long-tracked, intense, and destructive hurricanes across the tropical Atlantic, Caribbean Sea, and Gulf of Mexico. Alternatively, there are a some analog years, such as 2002 and 2014, that were near or below normal. 2002 saw lots of weak, short-lived, and homegrown activity with few storms forming in the tropics. 2014 featured very few named storms, but average amounts of hurricanes, all of which performed some sort of recurve across the western Atlantic. In these years, the highest concentration of tracks occurred across the central MDR, the western Caribbean, the central and southern Gulf of Meixco just off the U.S. East Coast, and the central North Atlantic.
Putting all of these factors into consideration, the most likely outcome will be a season that features average or above average activity. At this point in time, most agencies are pointing to an average or slightly above average season. Uncertainty still exists and will do so until tropical activity begins to ramp up later this summer. My personal prediction states that there will be anywhere from 12-16 named storms, 6-8 hurricanes, 3-4 majors, and an ACE value between 100 and 150 totals units. Note that some of these observations can and will change as the peak of the season gets closer. Occasional updates will be provided if they may adjust the given prediction.