A very busy period lies ahead across the entire Atlantic

Not surprisingly, the second week of September is shaping up to be quite active.  There are still only two named storms lurking out there, Paulette and Rene, but there are also four other areas of interest in the coming days scattered from the Gulf of Mexico to Africa.

Tropical weather outlook over the next five days. The track and "cone of uncertainty" are shown for Paulette and Rene. The other four areas have Xs at their current location and shading where development is possible within the next five days. (NHC)

Aside from Tropical Storms Paulette and Rene in the central Atlantic, there are two weak waves in the western Atlantic -- one approaching the Bahamas and heading west, and another west of the Florida peninsula and heading west. They don't have very high chances of becoming tropical cyclones, but it's not out of the question either. Then there are two much stronger waves in the far eastern Atlantic -- one just exiting the African coast today, and one about three days behind it over continental Africa. These are far more likely to become named storms.

The animation below shows the output from the European model's 50-member ensemble (each area of interest could have 50 trackable lows) over the next ten days. Don't pay attention to any details, but it's easy to see the general patterns and tracks.  The tracks are colored by intensity... and note that weaker storms are steered by different layers of the atmosphere than stronger storms.

Rene is forecast to track toward the west-northwest and encounter lower wind shear, allowing it to strengthen a bit more, but by early next week, its future becomes very hazy. Paulette is expected to reach hurricane intensity in a few days and reach the Bermuda area on Monday night into Tuesday morning. The two storms are close enough to each other that the larger one (Paulette) can influence the environment that Rene moves into... that's where the haziness comes in.

The next two names on deck are Sally and Teddy.  After that, only two more names remain before we switch to the Greek alphabet. The 2005 season reached the Greek alphabet on October 22 and went on to use six names from that alphabet.

In terms of ACE (Accumulated Cyclone Energy), this season is just about average -- 102% of average for this date.  It will likely slip below average on September 11 for the first time this season.

And speaking of the peak of the season, the first two weeks of September are historically it, though the actual peak depends on which period one uses as the climatology!  In the time series below, I plot the daily average ACE (with a centered seven-day smoothing) from four different time periods.  There are humps, bumps, dips, and wiggles, and if you really hunt for the absolute maxima, you get September 11 (purple), September 12 (green), September 13 (blue), or September 14 (orange). But it's clear from looking at these that those exact dates don't really mean much.

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Paulette and Rene usher in peak week of season

Tropical Depressions 17 and 18 were upgraded to Tropical Storms Paulette and Rene on Monday.  These are the 16th and 17th named storms of the season, and both set new records for earliest 16th and 17th named storms. Due to a weakness in the subtropical ridge, both of these storms are expected to turn toward the northwest and remain over the open ocean. 

While the official forecast from NHC keeps Paulette as a tropical storm through the next five days, there is support among the regional hurricane models for it to strengthen to a hurricane by the weekend, by which time it could be a threat to Bermuda.

NHC is forecasting Rene to become a hurricane on Thursday, and this is also supported by model guidance.  There have already been 5 hurricanes this season (Hanna, Isaias, Laura, Marco, Nana), so Paulette and Rene could be the 6th and 7th.

Looking further east, there is a strong wave over western Africa that many models favor for development in the coming days.  The next name on the list is Sally, and the current record earliest 18th named storm formation is October 2 (that was Stan in 2005).  Unlike Paulette and Rene, this one is more likely to remain in the deep tropics; if it does, it would reach the Lesser Antilles on Tuesday-Wednesday.  It's far too soon to be certain that it will stay south, and some models and ensemble members do turn it to the north like the two before it.

The map below shows track from the European model's 50-member ensemble, with the "L"s valid next Wednesday.  Paulette is the cluster in the upper left, Rene is the cluster in the upper right, the wave of interest is the cluster in the lower left (Sally?), and the next wave behind it is the cluster in the lower right (Teddy?). If everything comes to fruition, next week will fit the description as the peak of the season! As with any model run that's valid over a week out, never take the details seriously, just look at the big picture for general placement and timing; the intensity is an even bigger question mark.

Even with the formation of Paulette and Rene, the ACE (Accumulated Cyclone Energy) has slipped down even closer to average today, now at 105% of an average season through September 8.

And speaking of the peak of the season, the first two weeks of September are historically it, though the actual peak depends on which period one uses as the climatology!  In the time series below, I plot the daily average ACE (with a centered seven-day smoothing) from four different time periods.  There are humps, bumps, dips, and wiggles, and if you really hunt for the absolute maxima, you get September 11 (purple), September 12 (green), September 13 (blue), or September 14 (orange). But it's clear from looking at these that those exact dates don't really mean much.

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Two new tropical depressions form

Finally, from a mess of scattered easterly waves in the deep tropics that people have been watching for days, two tropical cyclones have emerged: Tropical Depressions 17 and 18.  Both are located in the far eastern Atlantic, and both are likely to become tropical storms soon. The next two names on the list are Paulette and Rene -- and the record earliest dates for 16th and 17th named storm formation are September 17 and 18 (Philippe and Rita in 2005).

TD17 is the western one, and faces a more hostile environment in the coming days. While it could easily become a tropical storm, there is no model guidance that even remotely suggests it could become a hurricane.  It looks likely that it will make a turn toward the northwest in 5-6 days.

TD18 is still east of Cabo Verde, where a tropical storm warning is in effect.  Storms can and do form between the African coast and Cabo Verde, but it's impressively quick (recall that Fred passed over the islands as a hurricane in 2015!). This one does have several models that bring it up to hurricane -- and even major hurricane -- intensity in a few days. It looks like it will turn safely to the north well before becoming a concern to land, but that will depend on the strength of the subtropical ridge to its north, which has had notable breaks in it lately.

If these don't start cranking out of ACE (Accumulated Cyclone Energy) real soon, then for the first time this year, the Atlantic hurricane season will be "average" -- in terms of ACE. As of this writing, 2020 is at 45.0, and the average (over the past fifty years) through today is 42.6. We cross below average on the 9th if nothing else happens. But the odds are good that TD 17 and/or 18 will be TS Paulette and Rene by then and delay that crossing a bit.

And while 2020 is smashing records for the earliest date of the Nth named storm formation, it really does look quite average when compared to historical values of ACE through today.

Aside from TD 17 and 18, there are two other areas of interest coming up this week: another easterly wave exiting the African coast, and a weak low pressure system near Bermuda that is drifting toward the U.S. mid-Atlantic coast.  NHC is giving both of these just a 30% probability of becoming at least a tropical depression within the next five days.

I was also reminded of a unique moment that occurred three (and ten) years ago today. In the anxiety and haste of preparing for Hurricane Irma's potential arrival on September 10th, 2017, I was struck by the satellite presentation on the morning of the 7th and knew it reminded me of something. And it did... I remembered the same set and placement of active named storms just a few years prior (K, I, J in 2010) and made this comparison image that I shared on Twitter: https://twitter.com/BMcNoldy/status/905770545899286528

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