NOAA released their official Atlantic hurricane season outlook Thursday morning for the upcoming 2018 season. Their forecast highlights a near-average to slightly above average hurricane season with 10 -16 named storms, 5-9 hurricanes, and 1-4 major hurricanes (category 3 or higher). Typically 12 named storms, 6 hurricanes, and 3 major hurricanes form in the Atlantic basin each year. NOAA is forecasting “a 75-percent chance that the 2018 Atlantic hurricane season will be near- or above-normal.” A reminder that the forecast numbers only indicate how active the season may be, but it only takes one storm to define a season. There could be 15 hurricanes, but none of them affect land. We just don’t know where any of the storms will form and track in these seasonal forecasts.
Forecasters predict a 35 percent chance of an above-normal season, a 40 percent chance of a near-normal season, and a 25 percent chance of a below-normal season for the upcoming hurricane season, which extends from June 1st through November 30th.
There are numerous factors that play a role in this forecast, including the sea surface temperatures and wind shear. Sea surface temperatures are well below average overall where tropical cyclones tend to form, especially in the MDR (Main Development Region). The MDR spans from the western African coast through the central Caribbean Sea, and is the region where the most intense storms typically form, especially beginning in August. According to Meteorologist Philip Klotzback, this is “the sixth coldest tropical Atlantic (10-20°N, 60-20°W) on record (since 1982) for late May.”
The tropical Atlantic has continued to anomalously cool – now the sixth coldest tropical Atlantic (10-20°N, 60-20°W) on record (since 1982) for late May. Typically a cooler tropical Atlantic means a less active #hurricane season. @TropicalTidbits pic.twitter.com/bXXhkYG3Fc
— Philip Klotzbach (@philklotzbach) May 23, 2018
Thunderstorms and specifically tropical cyclones rely on very warm ocean waters because that’s where they gain their energy from. With below average temperatures in place, this tends to suppress convection. It is still unclear how these oceanic temperatures will evolve as the hurricane season progresses, but it is expected to be a somewhat slow start. NOAA predicts temperatures will warm to near average by the peak of typical activity in the basin. If a storm were to form, it would likely occur close to home over the southwestern Atlantic Ocean, Caribbean Sea, or Gulf of Mexico. This also correlates with the densest tropical cyclone origins during the month of June.
Another component important to tropical cyclone development is wind shear. Tropical cyclones, whether they are tropical depressions, tropical storms, or hurricanes, do not like wind shear. Unlike thunderstorms over land, wind shear rips apart tropical cyclones and prevents the thunderstorms from organizing around a low-level center. If these thunderstorms have a difficult time organizing and wrapping around the center, then it’s unlikely a tropical cyclone develops.
One of this planet’s phenomena is ENSO, El Niños and La Niñas, can have a direct effect on wind shear over the Atlantic basin. During an El Niño phase, ocean waters near the equatorial Pacific Ocean are above average and have an effect on where the thunderstorms over this region set up and the direction of the surface winds. This typically brings above-average wind shear and trade winds to much of the Atlantic, thus making development non-conducive.
On the other hand, La Niñas make the environment in the Atlantic prime for tropical development, at least in terms of wind shear. Shear is generally below average during a La Niña. This is because of the cooler than normal equatorial Pacific waters, which actually increases wind shear over the eastern Pacific basin, therefore limiting the formation of tropical cyclones.
In terms of this year, NOAA declared that La Niña has ended earlier this year after beginning in the summer of 2017. Based on the latest forecast published on May 10th, it is somewhat unclear what the future may hold. We know the La Niña won’t return, but there is currently a 52% chance of neutral conditions and a 38% chance for the development of an El Niño during the peak of hurricane season (August through October). Those chances for an El Niño then reach 50% during the winter. Therefore, variable to possibly increased wind shear is expected across the Atlantic.
Based on this, NOAA is anticipating a near-average Atlantic hurricane season, which deals with these storm names: