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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Watching far eastern Atlantic for next storm... Sam?

Since my previous post on Friday, we have had Odette, Peter, and Rose form in the Atlantic!  And, we're likely going to see the season's 18th named storm form this week: Sam.

Odette (15L) was a named storm for only a day, and came from the area of interest that was off the North Carolina coast on Friday.  The other two areas that were highlighted in that post became Peter (16L) and Rose (17L), and they are both still active. 

Neither Peter nor Rose will be able to strengthen much, and both will remain far out in the ocean.  Both are facing wind shear and dry air to different degrees.

Peter is located just north of the Leeward Islands and is close to a strong upper-level low pressure system which is forcing the thunderstorm activity east of the circulation center.  Rose is west of Cabo Verde and is close to ingesting lots of dry Saharan air.  Both are forecast to track toward the northwest then north in the coming days.  The only potential concern would be Bermuda this weekend, but impacts from Peter would not be significant.

It's worth pointing out that Rose became the season's 17th named storm on September 19th.  Since 1851, only two other seasons had the 17th named storm so early in the year: 2005 and 2020.  Rose is also still a name from the original 1979 list and has never been used before now! 

Now on to what will almost certainly become Sam: Invest 98L.  This easterly wave left the west coast of Africa on Sunday morning and has been strongly favored by the model guidance to develop and to maintain a track in the deep tropics for a while.

The model guidance is generally bullish on this system, eventually.  It's in a tight spot right now, with Rose immediately to its northwest and Saharan air streaming off the continent to the north. 

The latest deterministic (single higher-resolution run compared to the ensembles) European model run brings this storm very close to or over the northern Leeward Islands next Wednesday as a hurricane.  The latest American model run is two days slower and 275 miles farther north.  Clearly, there is some disagreement to sort out in the coming days, but they both have a hurricane tracking through the deep tropics next week.

European (top) and American (bottom) deterministic runs, both valid when the storm is at the longitude of the Leeward Islands.  For the European run, that's next Wednesday, but next Friday for the American model.

Looking at the updated ACE (Accumulated Cyclone Energy) chart, 2021 is now at about 117% of average for the date.  This time of year, it takes a decent amount of activity just to keep up with climatology, and a couple of low-end tropical storms don't quite cut it, so we're creeping closer to average every day... for now.

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A lull after nonstop action with Ida through Nicholas

Since my previous post on Tuesday morning, nothing has happened!  That in itself is odd to say when there are multiple features of interest and it's mid-September.  Neither the disturbance that was near the Bahamas nor the wave in the far eastern Atlantic have developed yet, though both are still expected to.

The western system, Invest 96L, is now centered just 150 miles off the North Carolina coast and appears quite close to becoming the season's next tropical or subtropical cyclone.  In the satellite animation below, it's easy to see the clouds rotating around a low-level center, but all of the thunderstorm activity is displaced to the north and east. It is expected to head off toward the northeast, intensifying as it does so... transitioning to a strong extratropical cyclone by the end of Saturday.

If it develops into a tropical or subtropical storm in the short window remaining, the next name on the list is Odette. Reaching 15 named storms is above average for an entire season, let alone by mid-September... the only seasons with 15+ named storms before September 19th are 2020, 2011, and 2005!

The wave that we've been watching since it left the African coast four days ago, Invest 95L, is now centered about 1200 miles east of the Lesser Antilles and is zipping westward at 20 mph.  It has struggled to develop, but NHC still gives it a 70% probability of formation within the next five days, somewhere in the shaded area in the map at the top. 

If it does form, the model guidance generally keeps it pretty weak and just north of the Caribbean on Monday-Tuesday, then some start showing intensification as it turns north.  There's not a lot of confidence in anything longer-range than Monday-Tuesday because it's so weak and disorganized now.  Certainly the northeast Caribbean should be watching it closely, but there's a lot of time to wait and watch for any potential US east coast or Bermuda encounters.

Finally, a new easterly wave has existed the African coast, but it's likely to turn to the northwest and find itself over cool water in less than a week.

The ACE (Accumulated Cyclone Energy) is now at about 124% of average for the date... it would actually slip below average for the first time this year on September 28th if nothing else is named by then, which is unlikely.

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