The 2018 hurricane season was busier than normal and featured significant impacts in the Southeastern United States from Hurricanes Florence and Michael, though no direct impacts up the coast into the Northeast. As we approach June 1st, the official beginning of the Atlantic hurricane season, let’s take a look at factors that will influence tropical activity and how the U.S. landfall risk looks compared to normal.
El Niño’s Influence
The water temperatures across the Tropical Pacific, have significant influence on global weather patterns year-round. El Niño (warmer waters) and La Niña (cooler waters) are fluctuations in these water temperatures that impact the large scale weather pattern on a seasonal basis.
The current El Niño developed during the winter, though "floundering around" is a good description of how the event has progressed into an official weak El Niño state. Most of our models maintain this weak El Nino for the bulk of the hurricane season…some weaken it towards a La Nada (near-normal water temperatures) while a select few intensify it to a moderate or stronger event. The wind pattern over the equator over the next couple of weeks favors a bit of an intensification of the event into the early summer, making us question the models that weaken it. However, given its progression so far, it’s difficult to get behind a more significant intensification. Given all of this, a weak El Niño being in place for most of the hurricane season is a sensible forecast.
Model forecasts for El Niño intensity over the next several months.
What does this mean for Atlantic Ocean hurricanes? It means wind shear will increase across the tropical Atlantic... and shear is bad for hurricane development. Because the El Niño looks weak, however, it’s possible (if not likely) that other factors temporarily overwhelm its influence and bring more favorable periods for hurricanes, especially near the peak of the season.
Atlantic Water Temperatures
Current Atlantic water temperature anomalies, courtesy of Tropical Tidbits.
Tropical cyclones are fueled by the waters beneath them, so taking stock of those waters as we approach the season is an important part of the forecast process. There are a couple of things that stand out:
1. Warm to very warm waters from the Gulf of Mexico to off the entire Eastern Seaboard, and east across all of the sub-tropical Atlantic
2. Slightly cooler than normal waters across the deep tropical Atlantic and Caribbean.
What does this all mean? In general, the pattern of cooler waters across the deep tropics and warmer waters across the sub-tropics to the north is a deterrent to long-lived and intense hurricanes that track west across much of the tropical Atlantic. In addition, there is less “fuel” beneath the potential tropical cyclones themselves in the tropics and Caribbean. This, combined with the small negative influence of El Niño, indicates fewer long-lived hurricanes in the tropics than normal. On the flip side, tropical storms and hurricanes do develop farther north in the sub-tropics, and the warmer waters there and near the coastline suggest more of these storms can develop. While these closer-to-home storms aren’t as long-lived and are often not as intense, they do count and can impact land (along with shipping interests).
Where will the Storms Track?
If storms manage to track into the western Atlantic or develop over the Gulf, Caribbean or Bahamas, the pattern in our “analog” (similar) years to 2019 is a bit concerning for U.S. interests during the peak of the season. The analog pattern for August and September shows high pressure extending from the Great Lakes to New England into northern Atlantic. This could steer storms towards the southeast U.S. coast and cause an increased landfall risk.
What it Boils Down to...
This season, we expect a near normal number of named storms and hurricanes, near to below normal number of major hurricanes, and fewer long-lived hurricanes originating from deep tropics. Another measure of hurricane season activity is Accumulated Cyclone Energy or ACE, which not only takes into account the number and intensity of individual storms, but also their longevity. This may be a season where the total number of storms is near normal, but the ACE is below average due to more short-lived storms outside of the deep tropics.
With near-normal activity and a pattern that may steer a few of them in the general direction of the U.S., the risk for a direct landfall is a bit above-average for the Eastern Seaboard. The risk for a direct landfall in a given year is quite low north of the Carolinas, so this modest increase in potential is far from a guarantee that landfall will occur. Instead, it’s more of a “heads up” to keep a closer eye on the tropics this season, especially if you live near the coast.
Outside of the Northeast, the Southeast and Gulf Coast states should be on alert every season, and this season is no different. While a lower risk for long-lived tropical hurricanes is good news for those areas, it only takes one such storm making it into the western Atlantic to be impactful, and storms developing over the warmer waters closer to the coast can contribute to that risk.