27 November 2023

Despite El Niño, hurricane season activity ends up well above average

This post marks the end of my 28th year writing these updates on tropical Atlantic activity.  During that time, I have written approximately 1410 posts spanning 481 tropical cyclones, 212 hurricanes, 97 major hurricanes, and 53 retired storm names. I know some of you reading this have been following along the entire time, but whether you've been reading these posts for 28 days or 28 years, I truly appreciate your continued interest!

Tracks of all tropical and subtropical cyclones during 2023. Each storm's peak intensity, lowest central pressure, and total Accumulated Cyclone Energy is provided in the table on the right.

The Atlantic hurricane season officially ends on Thursday, and it was a busy one.  Heading into the season, we were looking at an odd combination of two significant competing factors: a developing strong El Niño and record-breaking warm ocean temperatures.

With a lack of historical analogs from which we could gain insight, it was not obvious if one factor would dominate or if they would essentially balance each other out.  It turned out that the super-charged ocean was the dominant influence, and the seasonal activity ended up well above average.

There were 20 tropical storms, 7 of which became hurricanes, and 3 of which became major hurricanes (Category 3+ on the Saffir-Simpson Scale).  Using a climatology from 1991-2020, the average numbers of each are 14, 7, and 3.  The Accumulated Cyclone Energy (ACE) ended up at about 119% of average... essentially identical to the 2008 and 2021 seasons by this metric.

Although there was a fairly long gap in activity as we headed into the heart of the season from July 26 to August 19, the Atlantic then had non-stop activity from August 20 through October 6... a remarkable 48 days spanning Emily through Philippe.

Next, I'll pick out some of the highlights from the season that stood out to me.

Bret and Cindy both formed in mid-June out near 40°W from African easterly waves.  That is quite unusual, and not something we would typically see until a couple months later.  In fact, having two simultaneous active "main development region" storms in June was unprecedented.  It seems to me that the exceptionally-warm ocean and lack of Saharan air layer plumes allowed these early MDR storms to form. [see "Bret and Cindy usher in a historic day for the tropical Atlantic"]

One of the three major hurricanes this season was Idalia.  It formed near the northeastern tip of the Yucatan Peninsula and then rapidly intensified to a Category 4 hurricane as it headed for Florida, then made landfall in the Big Bend region as a Category 3 hurricane just four days after it formed.  The area had not experienced a major hurricane landfall since 1896. [see "Idalia rapidly intensifies and makes landfall, Franklin weakening"]

Lee was the season's only Category 5 hurricane, and that intensity was fortunately achieved over the open ocean east of the Leeward Islands.  It intensified VERY rapidly... 85 mph in a day... going from a Category 1 hurricane to a Category 5 hurricane.  Over the past 50 years in the Atlantic, only about 2.7% of tropical cyclones reach Category 5 intensity, so they are noteworthy.

Tropical Storm Philippe, which many of you might not even remember, was a real headache for forecasters.  Model guidance was consistently showing a north turn, which it did not make until several days later, and the intensity was curiously steady although the model guidance indicated periods of strengthening and weakening.  The average five-day track forecast error for Philippe was 2.4 times the average error at that lead time... pretty brutal.

Remarkably, there were only two hurricane landfalls anywhere in the Atlantic basin all season long: Category 3 Hurricane Idalia in Florida and Category 1 Hurricane Tammy in Barbuda.  However, there is another that deserves a mention: Lee made landfall in Nova Scotia in mid-September at Category 1 hurricane intensity, though it had technically transitioned to an extratropical cyclone by then.  That's just an academic difference... for practical purposes, it was a hurricane.

Another aspect of the season that stands out when you look at the track map at the top of this post is that there were no hurricanes in the Caribbean Sea, and with the exception of Idalia for 1.5 days, no hurricanes anywhere west of 72°W.  A weaker-than-normal Azores High helped to allow storms to turn north well before reaching the U.S. or even the Caribbean.

The season presented forecast challenges, primarily in track.  I singled out Philippe above, but there were other troublesome ones in the mix.  The two charts below show the preliminary verification statistics for the 2023 Atlantic season -- track on top then intensity below.  At each lead time, the difference between the NHC forecast and the observed value are averaged together to create these.  I also include NHC's average errors over the previous five seasons for context (the black dashed line).  As you see, the NHC's track forecast errors were higher than their own average at every lead time, but the intensity errors were lower.  Progress is not always linear.

Looking ahead to next season, the list of names is "List 4" and begins with Alberto, Beryl, and Chris.  This is the list that was first used in 1982 and repeated every six years since then.  The list has had six names retired from it over the years: Gilbert and Joan in 1988, Keith in 2000, Sandy in 2012, then Florence and Michael in 2018.  The 2024 version of the list features two new names: Francine and Milton which replace Florence and Michael.

01 November 2023

Formation chances drop, keeping the Atlantic quiet for a while longer

Of the two disturbances I mentioned in Monday's update, only the one in the Caribbean (Invest 97L) is still of interest. Today it's centered south of Haiti in the central Caribbean Sea and is expected to continue moving westward.

The National Hurricane Center is giving it a 20% probability of formation within the next two days and 40% within the next seven days.  However, on its current trajectory it will run into central America this weekend.

Models have become much less bullish on its development and intensification.  The European model ensemble barely finds anything trackable in the Caribbean, and the American model ensemble is generally very weak with just a small handful of stronger outliers.  With a much weaker system moving through the Caribbean, it's less likely to turn northward, so the tracks into Nicaragua look more reasonable now.

Strangely, it's not clear WHY this is failing to develop faster.  There are not obvious negative environmental factors in its way such as dry air or vertical shear, and the ocean under it is record-breakingly hot.  But the ample thunderstorm activity is just not consolidating around a center, so the pressure isn't falling, and the winds aren't increasing.  That can change quickly so we still have to be paying close attention, but it's at least off to a slow start.  Should this form and become a tropical storm, the next name on the list is Vince.

You may recall Hurricanes Eta and Iota in November 2020.  Both formed in the central Caribbean, and both made landfall as Category 4 hurricanes in Nicaragua just two weeks apart and a few miles apart.  Their tracks are shown below for reference, and as of now, there is absolutely no indication that this current system will intensify like they did.  But it's good to be reminded of what CAN happen in the western Caribbean in November and to watch things closely.

I will not have an opportunity to write another update until the 13th, so please keep an eye on NHC's website for the latest on any potential and active storms.

30 October 2023

Monitoring two areas for formation in the coming week

There are two areas of interest in the Atlantic that could become tropical cyclones in the coming week.  The first is near the northern Bahamas and the second is just south of Puerto Rico.  The one near the Bahamas (Invest 96L) has a hostile environment ahead of it, but the one in the eastern Caribbean (should soon be tagged as Invest 97L) is one to keep a close eye on.  The next -- and final -- two names on this year's regular list are Vince and Whitney.   

The discussion of Invest 96L will be very brief.  It's already encountering drier air and stronger vertical wind shear and both are expected to only become less favorable for development in the coming days.  Model guidance is in good agreement on it remaining very weak... either never form or perhaps briefly become a low-end tropical storm.  It will zip off to the northeast away from land and dissipate.

The feature in the eastern Caribbean is very poorly defined right now, but should gradually consolidate in the next couple of days.  There's a large difference between how the American and European model ensembles handle its future.  This first animation is a 10-day forecast from the American model's ensemble system.  The swarm of low pressures starts appearing on Wednesday and they track westward across the Caribbean.  The majority run into central America and subsequently dissipate, and about six of the members turn north and hit Cuba as a hurricane.  After Cuba, the Bahamas or even south Florida would be potential areas of concern.  The timing of the potential encounter in south Florida would be next Tuesday-Wednesday, but there is a LOT of uncertainty in this right now.

Switching over to the same animation from the European model ensemble, very few of the members do anything with it, and the ones that do are decidedly weaker than in the American ensemble.

But one thing is for certain: the Caribbean Sea is still extremely warm (1-2°C warmer than average for the date) and that warm water is deep, resulting in huge values of ocean heat content.  These factors will give a nudge to development and intensification, and be able to sustain a storm of any intensity.

Without a named storm out there, the Accumulated Cyclone Energy (ACE) is no longer accruing, and as of today, the 2023 tally is about 126% of average for the date.  But, as described in this post, we probably aren't done for the year yet.  Hurricane season officially ends on November 30.

Historically, the 2023 season sits rather high in the ranking for the date (in the top 12%):