Hurricane season winds down, but threat remains
What was once Hurricane Delta is now a remnant batch of showers over the Mid-Atlantic states, and for the time being, the ocean basin is devoid of any other named storms.
The pipeline of storm activity in the tropical Atlantic has mostly gone quiet, with one area to watch and no immediate signs of life. It’s getting late in the season, and in most years, tropical activity begins to taper down into mid-October.
But it’s 2020, and the Atlantic may have other ideas. A weather pattern that encourages air masses to rise, leading to increased showers and thunderstorms, looks likely to overspread the Gulf of Mexico and Caribbean by late in the month into early November. This pattern change could again raise the odds of tropical development, provided other air and ocean ingredients are present as well.
Not a typical season
In a typical storm season, the Atlantic averages one named storm after Oct. 19, which would suggest that even in an average year we wouldn’t be quite done yet. However, this season is anything but typical, considering we are pacing more than a month ahead of the busiest season on record, which occurred in 2005, and have dipped into the Greek alphabet for only the second time.
It’s plausible that the Atlantic basin may cap off its hyperactive season with a robust final act.
While there are no named systems in the Atlantic right now, the week has kicked off with an area to watch about 700 miles east of the Lesser Antilles. The National Hurricane Center gave it a 30 percent chance of developing into a tropical storm during the next five days.
While the system is unlikely to become intense, it did have some of the looks of a tropical depression or tropical storm on Monday.
Visible satellite imagery shows the presence of a low-level center of circulation, with cloudiness pinwheeling around a spiral vortex. To the east, showers and thunderstorms are blossoming, but whether or not those manage to position themselves over that low-level center remains a wild card in the prospects of development.
The system is struggling due to the presence of dry air and harsh upper level winds that are choking some of its thunderstorms, which can be seen in the surging outflow boundaries, or thunderstorm exhaust bands, pushing northwest, away from the main areas of rainfall.
Data from a scatterometer instrument on a satellite showed the possible presence of 35 mph winds with the system, meaning it could be labeled a tropical depression — the precursor to a tropical storm. A scatterometer uses electromagnetic pulses to derive wind speeds based on ocean surface roughness and the dynamics of how winds affect waves.
Downpours from whatever becomes of the system should arrive in the Leeward Islands late Wednesday into Thursday. In the off chance the system picks up a name, Epsilon is the next one up.
Looking ahead, it appears that tropical activity should remain at a relative minimum for the next week to week and a half. Thereafter, things may start to perk back up as we head into late October and early November.
That’s when the next area of broad rising motion associated with a convectively-coupled Kelvin wave arrives in the western Atlantic.
Get ready for the wave
A convectively-coupled Kelvin wave is a midsized overturning circulation in the atmosphere that meanders around the tropics; on one side of such a wave, air is more prone to rise, while it sinks on the other side. (To get a sense of such a feature, picture fans in a crowded stadium doing “the wave,” with everyone throwing their hands up as the wave arrives and then back down as it departs.)
When that rising motion overspreads the western Atlantic, including the Gulf of Mexico and Caribbean, there will be an increased risk of tropical development with any organized thunderstorm clusters that can become established.
This time of year, the most likely areas for development are the Caribbean and Gulf of Mexico.
While water temperatures in the western and north-central Gulf of Mexico have been cooled markedly by storms like Laura, Sally and Delta as well as cold fronts that have swept cooler air southward, the waters of the eastern Gulf are still relatively warm, thanks in large part to ocean currents.
Putting it all together, it looks like the period of roughly Oct. 25 through Nov. 12 could feature another flurry of activity, although such a late season active period is not as likely to produce as big of a burst of storms as we saw earlier this season.