On Wednesday morning, the National Oceanic and Atmospheric Administration released their latest hurricane outlook calling for even greater odds of an above-average season, which runs through November. That would make 2021 the sixth consecutive year to feature above-average tropical storm activity.
“NOAA’s updated outlook … indicates an above average season is likely,” said Matthew Rosencrans, a meteorologist at NOAA’s Climate Prediction Center, in a news conference Wednesday. “The number of named storms is likely to be 15 to 21, [which includes] seven to 10 hurricanes and three to five major hurricanes.”
That’s an increase from its May prediction for 13 to 20 named storms and six to 10 hurricanes; its forecast for the number of major hurricanes was unchanged.
In recent weeks, the Atlantic has been virtually silent, without any named systems present anywhere in the ocean basin since the dissipation of Elsa on July 9. Elsa brought heavy rain and tornadoes to parts of the East Coast.
“Given an increase in predicted number of named storms and hurricanes, there is now a 65 percent chance of an above-average season … and only a 10 percent chance of a below-average season,” Rosencrans said.
Driving the predictions are a number of factors that take into consideration large-scale features of both the atmosphere and ocean.
In the short term, the National Hurricane Center is monitoring three disturbances that it says has low chances, 30 percent or less, to develop.
One tropical wave exists off the coast of Africa near Cabo Verde. That one will swing north and remain disorganized and broad, with no real potential for its circulation to tighten. A second system still over Mali looks a bit more interesting and could slowly acquire tropical characteristics as it exits offshore late week.
A third tropical wave is meandering west over the central Atlantic and could begin to organize late this weekend as conditions become marginally favorable.
The three systems point to the awakening of the Atlantic’s MDR, or Main Development Region. This imaginary box stretches through roughly the eastern two-thirds of the tropical North Atlantic several degrees north of the equator, and encompasses the more classic formation locations for tropical systems that compose the bulk of long-track, long-lived Atlantic hurricanes.
At present, hurricanes are hard to come by because of widespread sinking air over the Atlantic. That has suppressed upward motion and inhibited updrafts, preventing the type of clustered thunderstorm activity that occasionally self-aggregates into a fledgling storm. Moreover, intrusions of the SAL, or Saharan Air Layer, draped a layer of hot, dry air over much of the eastern Atlantic, capping the vertical development of thunderstorms in the MDR.
That all looks to change in the coming weeks thanks to the combined effects of several large-scale overturning circulations that will bring lift, or foster rising air, over the Atlantic.
The Madden-Julian Oscillation, or MJO, can be envisioned as a large wave that sweeps around the global tropics every 30 to 60 days. Air rises on the leading edge of the waves, which is characterized by a packet of unsettled weather and widespread thunderstorm activity, while sinking air on the back side marks the “suppressed phase” of the wave.
Similar, albeit smaller, features known as “convectively coupled Kelvin waves” will periodically overlap with the MJO to bring occasional enhancements or temporary lulls in the season. It’s ordinarily possible to predict these subseasonal upticks about a month in advance before a swarm of storms brew.
All indications are that beginning around Aug. 10 to 14, the Atlantic will awaken. Later in the month, the tropics could become quite busy as rising motion really takes hold of the Atlantic Basin. It’s a time of year when hurricane activity generally ramps up anyway, so there’s every reason to believe that the latter two weeks of August and the start of September will be busy.
Moreover, the Atlantic has warmed significantly in the past couple of weeks. Previously, waters off the Gulf of Mexico coastline were running about a degree or two below average. Now the entire gulf is a degree or two above average. That bolsters the odds of tropical cyclones, which feed off warm water, rapidly intensifying assuming they enter the gulf.
Ocean waters over the open Atlantic have also experienced a warming trend across the tropical belt, save for immediately surrounding the Cabo Verde islands. Rosencrans did note that sea surface temperatures, while anomalously warm, are not as hot as in 2020, meaning the season, though anticipated to be active, is not expected to rival the hyperactivity of 2020.
In addition, some semblance of a La Niña pattern — the opposite of the more well-known El Niño — could begin to develop as we head deeper into the autumn. That would mean cooler waters over the east equatorial Pacific, fostering sinking there. That would trigger an opposite reaction over the Atlantic, reinforcing rising motion and, in turn, favoring hurricanes. (That can also sometimes weaken high altitude winds over the Atlantic, making it easier for storms to form.)
If La Niña does develop, it could extend the end of hurricane season later, potentially keeping activity going into November.
Upper-level steering currents could be more supportive of hurricanes affecting the East Coast this year, too, thanks to a swath of blocking high pressure that may become established over the Canadian Maritimes later in August and into September.
It’s important to remember that, even though forecasts like this are generally reliable across the entire Atlantic, hurricane season’s impacts are manifest on an individual storm level. In other words, it only takes one storm. Hurricane Andrew formed in 1992, and put an end to what, until that point, had been a cakewalk of an early season. The Category 5 storm leveled entire communities in south Florida.
Preparing ahead of time, rather than scrambling when a storm develops and warnings are issued, is the best way to make sure you’re ready for whatever this hurricane season holds.