31 October 2021

Trick or Treat? October ends with Wanda

What is now Subtropical Storm Wanda had its origins over Georgia six days ago, then was a potent Nor'easter a few days later, and then transitioned from an extratropical cyclone to a subtropical cyclone, earning a name in the process.  Wanda is the season's 21st named storm, and the last on the regular list of names.  Victor was the last named storm to form in the Atlantic (until now), and that was named way back on September 29th!

Wanda won't be affecting land in its future, but has peak sustained winds of 50 mph and is located about 1800 miles east of North Carolina and 1900 miles west of southern Portugal -- truly the middle of Atlantic!  It is not forecast to reach hurricane intensity, and should transition back to an extratropical cyclone by the end of the week.

The cumulative ACE (Accumulated Cyclone Energy) is at about 143% of average for the date, and Wanda won't contribute too much more to the overall tally.

With one month remaining in the official Atlantic hurricane season, there's a chance that Wanda won't be the last.  If anything should form, we'll switch over to the auxiliary list, shown below.  This list was chosen to replace the use of the Greek alphabet.  Only 2005 and 2020 ever exhausted the regular list of 21 names before, so to happen in two consecutive years now is extraordinary!

There is actually an easterly wave that just left the African coast, and NHC is giving it a 30% probability of becoming a tropical cyclone in the next five days.  It has been tagged as Invest 95L.  This would be a very late-season Cabo Verde storm should it form.

05 October 2021

Hurricane Sam clinches its place in history books

Hurricane Sam finally transitioned to a powerful extratropical cyclone on Tuesday morning, and no other tropical cyclones are active or brewing.  Through today, there have been 20 named storms in the Atlantic this season, including 7 hurricanes and 4 major hurricanes.  Of those four major hurricanes (Category 3+), two made landfall at their peak intensity: Grace and Ida.

Sam ended up producing an astounding 53.8 ACE (Accumulated Cyclone Energy) units, which singlehandedly accounts for 39% of the season's total so far!  It also clinched 5th place for most energetic storm in the Atlantic (during the reliable satellite era, and pending any post-season reanalysis tweaks to its intensity).  This table shows the new Top 10 list, using storms from 1966-2021.

Also during the satellite era, which is conventionally defined as 1966 onward, very few seasons have had as much ACE accrued by October 5th as the 2021 season: 1995, 2003, 2004, 2005, and 2017... all high-end infamous hurricane seasons.  ACE is a metric that doesn't depend on the number of named storms, only the overall intensity and duration of whatever storms form.

The only name remaining on the regular list is Wanda, then we would move into the new auxiliary list that replaced the Greek alphabet.  I shared this list of names in my post on September 28th, but here it is again as a refresher:

October can still be a potent month for hurricanes, so it's absolutely too soon to tune out the tropics.  Category 5 hurricanes such as Mitch '98, Wilma '05, Matthew '16, and Michael '18 occurred during October.  Sandy '12 was also born in the western Caribbean in late October and reached peak intensity as a Category 3 hurricane when it made landfall in eastern Cuba.

As we see in this map of climatological hurricane activity, south Florida is especially at risk during October due to the prevailing steering patterns and preferred formation zones. And unlike the long-track storms of African pedigree, storms that form in the western Caribbean Sea or Gulf of Mexico naturally give less lead-time for impacts.