29 November 2021

An incredible sixth consecutive active hurricane season ends

This post marks the end of my 26th year writing these updates on tropical Atlantic activity.  During that time, I have written approximately 1350 posts spanning 444 tropical cyclones, 197 hurricanes, 92 major hurricanes, and 50 retired storm names. I was honored to have been invited to write for the New York Times' hurricane blog from 2007-2010, and then for the Washington Post's Capital Weather Gang since 2012.  I truly appreciate your continued interest!

The 2021 Atlantic hurricane season is the sixth consecutive season with above-average activity: there were 21 named storms, 7 of which became hurricanes, and 4 of those became major hurricanes (Category 3+).  The average values of those quantities are 14, 7, and 3.

2021 was only the third season to ever exhaust the regular list of 21 names... the other two times being 2005 and 2020.  It was also the seventh consecutive season with a named storm formation prior to the official start of hurricane season on June 1, helping to pull the trendline earlier when we look back on the past fifty years of first named storm formation date. 
Timeline of the date of first named storm formation over the past fifty years. The cyan line marks the official start of hurricane season, the magenta line is the median date of first storm formation, and the dashed gray line is the linear trend.  I do not count Hurricane Alex in January 2016 here since that was meteorologically a late addition to the 2015 season.

The Accumulated Cyclone Energy (ACE) was above-average all season long, and blew past a full average season's total on September 28th -- it ended up at about 141% of average (using 1971-2020 as the baseline climatology).  Rather than counting the number of storms, ACE is a metric that accounts for the overall intensity and duration of whatever storms there are.

The ACE was above 129 units for the sixth consecutive year -- this has never happened before, not during the satellite era, not since records begin in 1851. This sustained level of tropical cyclone activity in the Atlantic is unprecedented even for four years, let alone six!

Hurricane Larry and Hurricane Sam were the heavy hitters of the year by this measure... Larry contributed 33% of the season's total and Sam contributed 37% of the total. The storm in third place is way down at 7%, and that was Ida.  It is worth mentioning that 9 of the 21 named storms were only around for two days or less, and they contributed just 4% of the season's ACE, combined.

As you can see on the chart above, the burst of activity in late September came to an abrupt end on October 5th.  The lone straggler was Tropical Storm Wanda which was around from October 31st to November 7th, and formed from a former Nor'easter.  It's extremely rare and peculiar to have an active season essentially shut down in the first week of October!

Although still preliminary, the 2021 season is the 4th costliest Atlantic hurricane season, behind 2017 (1st), 2005 (2nd), and 2012 (3rd).  The economic losses are expected to exceed $70 billion, bumping the fresh-in-our-memory 2020 season down to 6th place.

Drilling down to individual storms, Ana formed prior to the official start of the season, on May 22, as referenced above.  Bill formed off the southeast U.S. coast and headed out over the open ocean. Claudette, Danny, Elsa, and Fred all made landfall on the U.S. mainland.

Grace was the season's first major hurricane, reaching Category 3 intensity right as it made landfall in Mexico near Veracruz.

Enhanced infrared satellite animation of Category 3 Hurricane Grace making landfall on August 21st.

Hurricane Henri was quite impactful in the northeast U.S. when it made landfall in New England on August 22nd as a tropical storm.  It caused widespread power outages and produced record-breaking rainfall in New York City and flash flooding across several states.  This same area would be impacted with more heavy rain by Ida in just over a week.

A full-resolution version is available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

The most intense of the landfalling storms was Hurricane Ida, which made landfall near New Orleans on August 29th, on the 16th anniversary of Hurricane Katrina's landfall in the same location.  It was a Category 4 hurricane, reaching peak intensity right as it made landfall.  Its strong winds and storm surge caused extensive catastrophic damage across southern Louisiana.

A full-resolution version is available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

As usual, Ida's trail of destruction didn't end at the coastline. Three days after landfall, post-tropical cyclone Ida interacted with a mid-latitude trough and a very focused band of extreme rainfall was the result.  Although the event was remarkably well-forecast days in advance, rainfall totals in PA, NJ, and NY and then into southern New England were incredible, devastating, and deadly.

Ida was responsible for 115 fatalities and over $65 billion in damages from Venezuela and Colombia, then Jamaica and Cuba, and finally the United States.  It's always tempting to "write off" a hurricane after landfall, but in fact, there are often several days of severe impacts remaining as it moves inland... quite far from where it made landfall.  Ida is tied with 2012's Sandy as the 4th costliest hurricane in U.S. history.

Moving ahead, Larry was a long-track major hurricane, and a named storm for 10.5 days.  It clipped Newfoundland at the end of its journey on September 11th, causing fairly significant damage, and rip currents associated with it killed two people in Florida and South Carolina.

Like Grace and Ida, Nicholas reached its peak intensity right at landfall -- near Galveston TX on September 14th as a Category 1 hurricane.

The strongest storm of the season was Sam, which fortunately remained over water in the central Atlantic. It was a named storm for 12 days, nearly 8 of which were spent above Category 3 intensity! It also produced the fifth most ACE of any Atlantic hurricane on record (https://twitter.com/BMcNoldy/status/1445428509220892673)! 

Hurricane Sam on September 26th, when it was just a click away from reaching Category 5 intensity.

Keep in mind that all statistics presented here are preliminary; the National Hurricane Center will have the final post-season reanalysis of all storms complete in a few months.  We will also find out which names will be retired early next year.  But we can reasonably anticipate that Ida will be retired, so I will offer these following updated charts with that in mind (and not knowing if any other names will be retired from this season).  Ida's all-but-certain retirement puts "I" storms even more in the lead, as well as retired storms that peaked at Category 4 intensity.  (The colors on the bars are only scaled by value for the sake of visual interest.)

Next year's name list starts off with Alex, Bonnie, and Colin.  Two new names appear on the 2022 list:  Martin and Owen, which replace Matthew and Otto from 2016.

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.