The Atlantic basin has recorded 4 named storms, 2 hurricanes, 0 major hurricanes, and a total Accumulated Cyclone Energy (ACE) of just under 14 units in 2018 thus far. This activity is considered above the climatological norm (1966-2009) to begin the month of August with the Atlantic averaging its third named storm by August 13th and its first hurricane by August 10th. Although Beryl and Chris’ formations in early July were a bit of a surprise for some expecting an overall below average season, the significance of their formation during this point in the hurricane season is overall mixed with Beryl’s formation in the Main Development Region (MDR) showing potentially a false signal of what in other years would be a sign of an active year and Chris forming in the subtropical Atlantic via non-tropical origins. The reasoning behind Beryl’s façade of potential favorability in the Atlantic comes from it being aided by a convectively coupled kelvin wave (CCKW), and its small size giving it the ability to manage intensification despite the overall marginal environment much like what was seen with Hurricane Danny in August 2015.
After the formation of Beryl and Chris in early July, the Atlantic went silent with active Saharan Air Layer (SAL) outbreaks in tandem with strong easterly trade winds across the MDR which resulted in further cooling of the tropical Atlantic. These variables resulted in July being the coldest MDR recorded for the month of July since 1950. Additionally, the Madden Julian Oscillation (MJO) resided in the Pacific for much of the month which enhanced convective activity across the basin which in-turn increased vertical wind shear across much of the Atlantic basin. The unfavorable conditions described are reminiscent of what is seen during inactive hurricane seasons, however, it’s worth noting that SAL season peaks in July and the MJO has since propagated out of the Pacific and into the Atlantic. These developments will potentially give the Atlantic an opportunity at becoming more active as we approach September 10th otherwise known as the climatological peak of hurricane season.
Currently, the Eastern Pacific remains convectively active with several tropical cyclones in the basin. This activity has continued to enhance the Subtropical Jetstream (STJ) which causes strong westerly wind shear through the Caribbean Sea. This tropical cyclone activity will have to run its course over the next week or so before we can expect to see any significant change to this. During the next two weeks the MJO is expected to continue to propagate eastward across the Atlantic and into the African continent. As this occurs in accompaniment with Eastern Pacific tropical cyclone activity dimming, vertical wind shear across the Atlantic basin will begin to decrease and African Easterly Waves (AEW) will likely become more enhanced over Africa. The MJO’s presence in the Atlantic basin will also assist in slowing easterly trade winds which
will reduce upwelling of waters in the tropical Atlantic and promote warming of the MDR albeit only gradual. These subtle changes will lead to a more favorable state of the Atlantic basin as late August and September approaches. This period will likely be the most favorable period for hurricane formation in the Atlantic this hurricane season before the MJO progresses out of phase and into the Indian Ocean. This period coincides well with climatology and would favor an uptick in activity during this period before activity potentially begins to trail off again. This is partially a symptom of how MJO-dependent the tropics have been with regards to tropical cyclone formation in the current state of the climate which makes it more likely that whenever the Atlantic enters another suppressed phase in the MJO and/or El Niño begins to become more prominent in the Pacific that the latter part of the season may be less active. However, it is worth noting that storms formed from non-tropical origin like Tropical Storm Debby can form even in generally marginal conditions for tropical cyclone formation. This factor of randomness adds a slightly higher uncertainty specifically to the named storm category given most of these storm never reach hurricane status.
August 20th to September 30th Outlook –
Average (1966-2009): 5 Named Storms, 3 Hurricanes, and 1 Major Hurricane
Forecast: 5 to 8 Named Storms, 2 to 4 Hurricanes, and 1 to 2 Major Hurricanes
The background atmospheric and SST profile for 2018 is a unique challenge for those interested in trying to understand what variables in play will end up dictating hurricane development in the Atlantic basin. The overall state of most SST-driven climate teleconnections across the world remain rather neutral meaning most impacts these may have are subtle and intraseasonal variability such as CCKW and MJO activity can have more prominent impacts on favorability in hurricane development. To find years that were similar to what is being observed currently in 2018, a criteria was needed to help identify these years. This criteria included a warming ENSO from a neutral state through the summer, a neutral Atlantic SST profile or cooler than average MDR SSTs, and a wet Sahel.
The top analogs to 2018 are 2002, 1979, 1993, and 1934. These years specifically had cooler SSTs in MDR during peak hurricane season with an El Niño developing through the fall and a wet Sahel. The mean activity of these years was 10.5 named storms, 5.0 hurricanes, and 1.5 major hurricanes with ACE 75% of the normal 93 units (1951-2000 mean). Inside the spread, 1979 would represent the maximum in the analogs (93 ACE) and 1993 being the minimum (39 ACE). Given this and the aforementioned variables impacting hurricane formation in the Atlantic basin, this would be another indication that this hurricane season will be below the climatological average.
This updated seasonal forecast is meant to revise thoughts discussed in the initial hurricane outlook released June 1. Due to the presence of average to below average SSTs in the tropical Atlantic and warming ENSO, confidence is increasing that the 2018 Atlantic hurricane season will be slightly below average, but within margin of error to be considered near normal. The updated forecast calls for 9 named storms, 4 hurricanes, and 2 major hurricanes with 56 units of ACE for the remainder of the season. This would put the season total at 13 named storms, 6 hurricanes, and 2 major hurricanes with an ACE 75% of normal (~70 units). Although hurricane activity is not expected to be above average it is important to note that it only takes one damaging hurricane to make it a destructive and deadly season. For official information regarding hurricane activity and tropical outlooks please consult with your local National Weather Service office and the National Hurricane Center.