08 September 2020

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.


  1. Regarding ACE, if we factor in contributions from E/C/NW Pacific basins, we actually are below average to date ...And the ENSO conditions are neutral to weak negative. Do we get more extreme activity in the NATL if stronger La NiƱa conditions are present, historically speaking?

  2. Yes, while El Nino is a suppressing factor for Atlantic activity, La Nina is an enhancing factor... both acting via increased/decreased wind shear across the deep tropics.