23 September 2021

Tropical Depression Eighteen (future-Sam) forms in deep tropics

Since my previous post on Monday, Peter and Rose have both fizzled out with little fanfare, they were both mid-range tropical storms that were around for 2.5 days.  The easterly wave that had just left the African coast is farther west now of course, and was upgraded to Tropical Depression 18 on Wednesday afternoon.

This latest tropical cyclone is all-but-certain to become the 18th named storm, Sam, and then the season's 7th hurricane after that. As we see in the satellite image above, the storm appears to be well-organized, though there is a lot of dry dusty Saharan air immediately to its north and east -- that has probably been limiting a more rapid intensification.  The depression is centered about 1800 miles east of the Lesser Antilles.

Over the next week, TD18/Sam is going to move relatively slowly around the subtropical High... but the exact strength and placement of that High (ridge) will determine how close it gets to the northern Lesser Antilles and eastern Greater Antilles.  The NHC forecast splits the difference between some models that keep it more to the south and some that are quicker to recurve it.  So the Leewards, Virgin Islands, and Puerto Rico should absolutely be paying very close attention to this, as it could be about a week away, but could also be a major hurricane at that point.

The latest example of that spread is illustrated by the European global model ensemble (ECMWF, left) and American global model ensemble (GFS, right) below.  While some forecasts among these two are fairly similar (the eastern part of the ECMWF spread and the western part of the GFS spread), the western part of the ECMWF spread has some serious implications for land impacts.

It's worth noting that over the past few days, the ECMWF deterministic runs have been far more consistent with the more western/southern track, while the GFS deterministic runs have drifted around more and have had a northern bias.

Anything beyond this 10-day timeframe shown above is not worth thinking about yet, as even this is clearly stretching the limits of predictability.

We're also still watching ex-Odette... yes, the Odette that became a post-tropical cyclone five days ago.  It's still out there in the north-central Atlantic between Newfoundland and the Azores, and has a shot at transitioning back to a subtropical or tropical low in the coming days as it moves south over warmer water.  It is not expected to drift back closer to land.

Future-Sam and even reborn-Odette should start contributing to the season's ACE (Accumulated Cyclone Energy) shortly, but in the meantime, the ACE as of today is down to 111% of average for the date.  If Sam were not in the cards, the 2021 season would cross the "average" line for the first time on October 1st.  To put the current ACE value in recent historical perspective, the 2017 season was 2.1x higher by now, and even 2020 was 1.3x higher by now.  The season with the most ACE by this date, 1933, was 2.6x higher by now and we likely missed some of the activity!

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