20 September 2021

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.


17 September 2021

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.


14 September 2021

Hurricane Nicholas hits Texas, and keeping a close eye on Africa

Since my previous update two days ago, Tropical Storm Nicholas formed on September 12th in the Bay of Campeche, then rapidly intensified to a hurricane just as it made landfall near Galveston in the early morning hours of the 14th. Nicholas is the season's 14th named storm and 6th hurricane... the average dates for those to occur: November 18th and October 15th.  This is quite an exceptional level of activity so early in the season.


[By the way, Category 2 Hurricane Ike made landfall in Galveston too, on September 13th, 2008... Nicholas missed that anniversary by a few hours.]

Throughout the day on Monday, Nicholas had some really favorable conditions to work with, and some not-so-favorable. The center reformed in different locations, shear made the storm lopsided, dry air eroded large portions of it away, but at the end of the day (literally), the super-warm Gulf of Mexico provided it with enough fuel to just barely cross the Category 1 hurricane threshold.

http://bmcnoldy.rsmas.miami.edu/tropics/radar/

By far, the greatest threat from Nicholas has been and continues to be the heavy rain.  As you can see in that radar loop above, most sectors of the storm are devoid of rainfall, but where it does exist, it's persistent and slow-moving.  The areas at highest risk for flash flooding are highlighted below:


We've been watching pre-Nicholas for the past 5-6 days, since it was near Nicaragua. Although some of these storms aren't named systems for very long, or it seems like they pop up out of nowhere, they most definitely do not. Some just take a long time to fester and once they finally get their act together they're already near landfall.

As my friend and colleague Bob Henson pointed out in a blog post, Nicholas is now the 19th named storm to make landfall in the U.S. since May 2020... the average is about three per year.

Now shifting our attention to Africa, recall that easterly wave I mention in Sunday's update.  It has wasted no time getting better organized once it left the coast.  Currently tagged as Invest 95L, it is quite close to becoming Tropical Depression 15, and then Tropical Storm Odette would be the next name.


The GFS and ECMWF global model ensembles both favor this for development, but neither are alarmingly bullish on intensifying it too soon.  It's in no rush to head westward, so even if it were to develop and reach the Lesser Antilles, it wouldn't reach the islands until the Sunday-Tuesday timeframe.  There's plenty of time to wait and let this do its thing.

One thing the ensembles have indicated in the past several runs is that a stronger version of the storm is more likely to turn northward well before the Lesser Antilles, while a weaker version could cruise much closer to the Caribbean.  We would be wise to watch the progress of a strong wave in the deep tropics in mid-September very closely.


And finally, that area north of the Bahamas that I mentioned on Sunday now has a more focused low pressure center, and we could see some development of that in the coming days as it drifts north.  As of now, it's nothing to be concerned about, just worth keeping an eye on it.  The National Hurricane Center is giving it a 60% probability of becoming a tropical cyclone in the next five days somewhere in the shaded area on the map.