11:45 AM (Thursday) | *A very active tropical scene in the Atlantic Basin at the climatological peak of the season with numerous systems to monitor…Pacific Ocean remains unusually quiet*
Paul Dorian
By one measure, the climatological peak of the Atlantic Basin tropical season falls on today’s date of September 10th. Plot courtesy NOAA
Overview
If you try to pinpoint the climatological peak in the Atlantic Basin tropical season to one particular day in terms of the number of active storms, it actually comes down to today, September 10th. Well, here we are and right on queue, there are numerous waves to monitor in the Atlantic Basin from the eastern Gulf of Mexico to just off the west coast of Africa. A few of these tropical waves are relatively close to the US and will have an impact in the near-term while another few systems are potential concerns for the second half of September. Meanwhile, the Pacific Ocean remains unusually quiet in terms of tropical activity in what should also be an active time of year. In fact, the overall tropical activity in the world’s largest ocean has been way below-normal in 2020 and this has contributed to a below-normal tropical season so far for the Northern Hemisphere as a whole.
The climatological peak of the Atlantic Basin tropical season features six different waves to monitor from the eastern Gulf of Mexico to just off the west coast of Africa. Map courtesy NOAA/NHC
Mid-Atlantic
There are currently as many as 6 tropical systems on today’s weather map extending from the eastern Gulf of Mexico to just off the west coast of Africa. One of these tropical waves has pushed slowly northwestward during the past few days over the western Atlantic and right towards the Carolina coastline. While it has not strengthened enough to meet tropical storm status, it has enhanced overall moisture levels in the Mid-Atlantic region and that has contributed to some heavy rainfall today in the DC-to-Philly-to-NYC corridor. In fact, the threat for heavy rainfall will continue into tonight in this same area and there are numerous “flash flood watches” posted by the National Weather Service to account for the possibility of a quick few inches of rain in some spots.
Tropical moisture will enhance total rainfall amounts over the next few days in both of the Mid-Atlantic and Gulf of Mexico regions. Map courtesy NOAA/WPC
Florida/Gulf of Mexico
Meanwhile, two tropical waves currently surround the Florida Peninsula with one to its west over the eastern Gulf of Mexico and the other to its east near the Bahamas. Both of these systems will drift westward over the next few days so that by the early part of next week they may be together over the warm waters of the Gulf of Mexico. There is some chance of intensification in both cases, but reaching tropical storm status is no guarantee for either. The combination of the two systems will, however, contribute to some significant rainfall in Florida over the next few days as well as along portions of the northern Gulf coastal region.
La Nina (i.e., colder-than-normal sea surface temperatures) conditions in the equatorial part of the Pacific Ocean tend to favor less tropical activity in the Pacific and more in the Atlantic Basin. Map courtesy NOAA
Eastern Atlantic
Meanwhile, in the eastern Atlantic there are two already-named tropical storms, Paulette and Rene. Both currently feature 50 mph maximum sustained winds and are moving along in a west-to-northwest direction. Not far behind, yet another tropical wave is just now pushing off the west coast of Africa and it is producing a large area of showers and thunderstorms. This particular wave has an excellent chance of reaching tropical depression status and then tropical storm status by the early part of next week as it likely pushes in the same general west-to-northwest direction as TS Paulette and TS Rene. All three of these eastern Atlantic tropical systems will have to be monitored closely in coming days to see if they can ever reach the US east coast - way too early to say anything conclusive on those prospects.
ACE is a metric used to measure overall tropical activity which takes into account the strength and longevity of a tropical system. While the North Atlantic has features an extremely high number of tropical storms this season, the ACE value is not too far from normal. The Pacific Ocean has been running at well below-normal in terms of ACE and this has contributed greatly to a below-normal ACE value for the Northern Hemisphere as a whole (boxed in region). Data courtesy Colorado State University/NOAA
Accumulated Cyclone Energy (ACE)
One of the best ways to measure overall tropical activity is with a metric known as the “Accumulated Cyclone Energy” (ACE) which takes into account not only the number of tropical systems, but also the strength and longevity. Since ACE includes both the intensity and longevity of tropical systems, a long-lived tropical storm could contribute as much as a short-lived storm that reached hurricane intensity.
While the Atlantic Basin has seen an extremely high number of storms this tropical season, many of them have been rather short-lived and, in some cases, rather weak. As a result, the North Atlantic ACE is currently running at levels not too far from normal – despite the very high number of named systems. Of course, this could increase quite a bit in coming days given the activity levels seen now across the Atlantic Basin. By the way, if three of these systems manage to attain hurricane status at the same time then that would be the first time since 2018 when Florence, Isaac and Helene formed. If five of these systems in the Atlantic Basin managed to attain tropical depression or greater status at the same time then that would be the first time since September of 1971.
Meanwhile, the Pacific Ocean remains rather quiet for this time of year. In fact, there are currently no systems at all west of the international dateline and only two relatively weak systems in the eastern Pacific in what is normally a much more active time of year. La Nina conditions have ruled this season (i.e., colder-than-normal sea surface temperatures) in the equatorial part of the Pacific and this has contributed to what has been well below-normal tropical activity. In fact, the ACE value for the Northern Hemisphere as a whole is quite a bit below-normal - dominated by the quiet year in the Pacific Ocean.
Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com
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