After explosive intensification, Erin reaches rare Category 5 status

Friday night into Saturday morning was quite eventful for Hurricane Erin. In just over one day, from Friday morning at 8am to Saturday morning at 11am, the peak winds increased from 70 mph (tropical storm) to 160 mph (Category 5 hurricane).  It was in an environment with very low wind shear and warm water, and something just clicked for it.  Thankfully, it did this over open ocean and did not threaten land. This image is from its peak intensity on Saturday afternoon when the central pressure bottomed out at 915 millibars.

In the official records which date back to 1851, there have been just 42 Category 5 hurricanes in the Atlantic, so it is indeed a very rare status to reach. Erin is now the 43rd.  Michael Lowry pointed out on Bluesky that Erin is also the earliest Category 5 on record outside of the Caribbean Sea and Gulf of Mexico.

Since that peak, it has undergone a couple eyewall replacement cycles, a very normal process for intense hurricanes. During these replacements, the peak winds decrease but the size of the wind field expands. So Erin isn't a Category 5 hurricane anymore (presently Category 3 and re-intensifying as of Sunday evening), but it is larger. This satellite loop below is from Sunday evening. Outer rainbands are impacting Puerto Rico, Dominican Republic, Haiti, and the Turks & Caicos, but no land will experience the eyewall.

I have also had radar loops from Saint Maarten, Puerto Rico, and the Dominican Republic to cover Erin's progress at the usual spot: https://bmcnoldy.earth.miami.edu/tropics/radar/

Erin is forecast to track northwest then north, sliding between the U.S. east coast and Bermuda this week. It should maintain major hurricane status (Category 3+) for at least the next three days, then it will weaken slightly as it encounters cooler water and stronger wind shear north of Bermuda. This map shows the forecast probability of tropical storm force winds and their most likely arrival time.

In terms of Accumulated Cyclone Energy (ACE), the 2025 season just surpassed 100% of an average season today for the first time all year. As of the end of Sunday, it's now at 110% of average for the date.  The coming days will boost that even more.

Looking much further east, there's a broad African easterly wave located just west of Cabo Verde (the one I referred to at the end of Friday's post) that is gaining support among the global models for eventual development as it makes its way westward across the deep tropics. This is the next feature to watch closely in the coming days as there's a hint among the models that it might make it pretty far west. I suspect it will be tagged as an Invest in the coming couple of days, and it would be Invest 99L. That designation triggers the regional hurricane models to run on it.

The next name on this year's list is Fernand.

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Erin becomes first hurricane of the season near Leeward Islands

Since my previous post on Monday, Erin has been racing westward across the deep tropics and very gradually intensifying. On Friday morning it was upgraded to the season's first hurricane. It's centered about 400 miles east of the northern Leeward Islands and tracking toward the west-northwest at 18 mph.

Erin became a hurricane on August 15, which is nine days later than the median date over the past fifty years. There's only a slight trend over that time period in earlier first hurricane dates, but rarely does one form before July 1. Note that I don't count Alex 2016 on here... that became a hurricane in January (!), but I argue that it was more of a late addition to the 2015 season than a super-early addition to the 2016 season.

The forecast for Erin has been pretty consistent in the models. They all indicate continued intensification as it tracks over increasingly warmer water (likely becoming the season's first major hurricane this weekend)... and a turn to the north in 3-4 days taking it between the U.S. east coast and Bermuda.

Elsewhere, there's a low pressure system that's been brewing over the far western Gulf of Mexico that could very briefly sneak into tropical cyclone status today, but will move inland soon. There will be an aircraft reconnaissance flight into it today and that will determine if the structure supports being classified as a tropical depression (it would be TD6), and even if the peak winds support a tropical storm classification (it would be Fernand).

Finally, this one is quite a bit off, but has growing model support for significant development in a week or so. There's a strong easterly wave still over Africa that will exit into the eastern Atlantic in a couple of days and begin its trek across the deep tropics. Lots of time to watch and wait on this one.

(by the way, I wrote a little blurb about looking at tropical waves over Africa on satellite recently... check it out on Bluesky! A 🧵 on "limb cooling" and how it relates to tracking tropical waves across Africa)

The next couple of names on this year's list are Fernand and Gabrielle.

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Cabo Verde season kicks off with Tropical Storm Erin

This time of year, mid-August or so, is when we typically start looking to Africa for waves that emerge off the west coast every few days. Most of them never become anything, but some do, and the large majority of Atlantic major hurricanes have African Easterly Wave pedigrees.

Today, tiny Tropical Storm Erin just formed very far east, just barely west of the Cabo Verde islands. It is forecast to become the season's first hurricane this week as it treks across the deep tropics. Over the past fifty years, the median date of first hurricane formation is August 4, so this year will be a bit late and probably push that median later by a couple days.

It will be in marginally-favorable environmental conditions for intensification this week and the initial advisory from NHC indicates that: a westward track and continued strengthening through the entire 5-day forecast period.  They forecast it reaching major hurricane (Category 3) status on Saturday.  It's a very small circulation that will have to contend with some dry air in its path but very low vertical wind shear.

Beyond five days, the global model ensembles have quite a lot of spread, but for the most part, they begin to turn it poleward around next Monday. Some ensemble members recurve it much sooner, while a handful maintain a west-northwest heading and could pose a threat to the U.S. on Tuesday-Wednesday next week -- but the uncertainty surrounding that scenario is enormous at this point.

The map below shows track density (and individual members tracks) from a multi-model ensemble. From this, it's easy to spot the outlier scenarios, and certainly the southern ones among them would be troublesome. From each new model cycle, we will be able to see if there are fewer members taking that southern track, more, or the same.

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Dexter intensifies, watching two other areas across the tropics

Tropical Storm Dexter formed on Sunday night but has been fairly weak and over the open ocean northeast of Bermuda. Additionally, there's an area of low pressure off the southeast U.S. coast that could form in the coming week, and an African easterly wave out near 35°W that could also form in the coming week.

Dexter's position as of Wednesday morning is marked and labeled, then the two areas of interest are marked with orange Xs and their area of potential formation in the coming seven days is denoted by the orange shapes.

The National Hurricane Center is giving both disturbances a 40% of development within a week.  Dexter is forecast to intensify somewhat, perhaps even reaching hurricane intensity on Thursday as it transitions to an extratropical cyclone. When that occurs, it would not be considered a hurricane, but rather just a very intense extratropical cyclone (hurricanes are tropical cyclones by definition). It's already interacting with a trough to its west and is quite sheared, so that transition is underway. It will not impact land as it tracks eastward across the far northern Atlantic. It's presently a tropical storm with 50 mph peak winds located south of Newfoundland.

Dexter is the fourth named storm of the season, and reached that mark 11 days ahead of climatology. There has not been a hurricane yet this season, and the average date of first hurricane formation is August 11. 

These two maps show the trackable low pressure centers over the next seven days from the European model ensemble (left) and the American model ensemble (right). They are in generally good agreement on the forecast of both areas of interest: the one off the southeast U.S. coast heads off to the northeast and remains fairly weak, and the easterly wave in the deep tropics should recurve to the north by the time it reaches ~60°W (roughly the longitude of the Lesser Antilles). The European ensemble is quite a bit more bullish on the development and intensification of the wave though.

There's another curious feature worth pointing out in these two maps: a few tracks popping up in the eastern Gulf of Mexico. The lows first appear this weekend west of south Florida and then head north toward the Gulf coast by Monday-Tuesday. There is not much support for this development among the models, and I had a hard time tracking it back to an origin; it appears to evolve out of a weak feature that's currently near Hispaniola, then moves westward.  It's worth noting that the water temperatures in the eastern Gulf are currently 1-2°C above average for the date -- which in an absolute sense, works out to a steamy 31-32°C!

The season has been abnormally quiet so far, but that can turn around quickly.  As of today, the Accumulated Cyclone Energy (ACE) is at just 28% of average for the date. 2022 actually had an even slower start, but before that, you have to go back to 2009 to find a quieter first 65 days of the season.

The next two names on this year's list are Erin and Fernand, and now that we're into August, expect activity to ramp up quickly, particularly by the end of the month (as illustrated in the climatological chart above).

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Storm threat increasing along northeast Gulf coast

An organizing low pressure system is centered about 100 miles east of Cape Canaveral and is bringing significant rain to the Florida peninsula on Monday afternoon -- it has been designated as Invest 93L. The National Hurricane Center is giving it a 30% probability of formation through the weekend... the proximity to land should keep the probability low of anything too strong developing, but that won't stop it from raining.

Should this develop into a tropical storm this week, the next name on the list is Dexter.  Dexter is a new name on this list's rotation, it replaces Dorian which was retired after the 2019 season.

It's worth noting that the water temperature in the central and northeast Gulf of Mexico is anomalously warm, so that provides a little extra nudge to development, especially if the center of circulation stays a little further offshore and gives it another day or two over water.  

As this low pressure system treks westward across Florida and then along the northern Gulf coast, it will dump plenty of rain, and the threat of flash flooding will ooze westward from Florida to Louisiana from today through Friday.  These five-day forecast rainfall totals will evolve, but as of Monday afternoon you can see where the highest totals could fall.

I didn't write posts on Barry or Chantal, but they formed and dissipated quickly, adding a combined total of just two named storm days. Barry formed on June 29 and Chantal on July 5.  The ACE (Accumulated Cyclone Energy) is at about 24% of average for the date -- you can barely see 2025's yellow line on this plot:

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First storm of the 2025 season forms east of Bermuda

Tropical Storm Andrea has formed over the north-central Atlantic, approximately 950 miles east-northeast of Bermuda.  The National Hurricane Center had been monitoring this area for possible development since Sunday morning, and it finally acquired tropical cyclone characteristics on Tuesday morning.

Over the past five decades, the median date of first named storm formation is June 15, so this year is slightly later.  In fact, it's the latest start to a season since 2014 (Arthur was named on July 1).

The storm appears quite anemic on satellite, and only has a day or so of marginally-favorable conditions before it encounters cooler water, drier air, and stronger wind shear on Wednesday.

The forecast map shows just how remote this location is:

This is the fourth time the name Andrea has been used for an Atlantic storm.  It was introduced in 2007 (replaced Allison after the 2001 season), then came around in 2013, 2019, and now 2025.  It will undoubtedly still be on the 2031 list.  Then, although there's nothing on the foreseeable horizon, the next few names on this year's list are Barry, Chantal, and Dexter.  Dexter is a new name this year, replacing Dorian which weas retired after the 2019 season.

If you're curious to learn more about this naming and retiring topic, check out my blog post from April 2: "3 Names Retired after the 2024 Atlantic Hurricane Season".

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3 Names Retired after the 2024 Atlantic Hurricane Season

During its annual meeting taking place this week, a hurricane committee within the World Meteorological Organization decided that 3 names would be retired following the 2024 Atlantic hurricane season: Beryl, Helene, and Milton.  They will be replaced by Brianna, Holly, and Miguel in the 2030 list.

So... how does this naming and retiring work??

Tropical cyclone tracks during the 2024 Atlantic hurricane season, color-coded by Saffir-Simpson category.  The tracks of Beryl, Helene, and Milton are highlighted with thicker lines.

The underlying reason for assigning names to tropical cyclones is that it makes communicating information about these huge and impactful storms simpler than referring to them solely by numbers or even by their coordinates on the globe.  A tropical cyclone is given a name once the sustained winds reach 39 mph (63 kph)... a tropical storm.

Tropical storms and hurricanes in the Atlantic Ocean started receiving human-sounding names 72 years ago.  After a few years of trying the World War 2-era Phonetic alphabet for storm names, the 1953 list featured all-female "human" names: Alice, Barbara, Carol, Dolly, Edna, etc.

That remained the practice (with a few modifications to the lists in 1955, 1960, and 1971) until 1979 when the modern naming system was introduced in which six lists of alternating male/female names are reused every six years.  In other words, the list of names from 1979 was reused in 1985, 1991, 1997, and will be again in 2027.  That is why the 2024 list will next be used in 2030.  Those six lists only contain 21 names, as it was felt that there were not enough common names that begin with Q, U, X, Y, or Z to be sustainable.

Trivia 

Q: What was the first "male" name assigned to an Atlantic tropical cyclone?

A: It was Bob on July 10, 1979.

It did not take long to realize that some storms' names were particularly infamous and traumatic because of the death and destruction they caused, and it became the practice to retire those names and replace them with a different name of the same gender and letter (for example, Andrew was replaced by Alex and Irma was replaced by Idalia).

In 1954, three terrible hurricanes put this concept into practice: Carol, Edna, and Hazel.  Since then, and including 2024, there have been 99 names retired.  Since 1979, the names and retirees have been chosen by a committee within the World Meteorological Organization.  The committee is comprised of representatives from 28 member nations that are affected by Atlantic hurricanes, so names tend to reflect a broad range of nationalities.  

Trivia 

Q: What names were retired upon their very first use in the modern naming era?

A: There have been nine, though four of them are from the initial use of the lists.  David (1979), Frederic (1979), Allen (1980), Alicia (1983), Michelle (2001), Ike (2008), Igor (2010), Irma (2017), and Milton (2024).

The process of retiring a name is surprisingly subjective; there are no minimum requirements for the number of fatalities or the economic losses to be met in order to be considered for retirement.  The country or countries affected simply nominate a storm name for retirement, make a case for it, and the committee votes.  It is tempting to construct some objective criteria, but consider the same hurricane hitting a rural stretch of the United States coastline versus passing over a small Caribbean island nation.  In the U.S., such a storm would likely not make a significant or lasting impact, but the small island could be effectively destroyed.

The following sequence of charts groups the retired names by various criteria.  The most retired letter is "I" (13 times) and it's not even close.  One might suspect that "I" storms tend to occur at the peak of the season when intense hurricanes are most common, but why wouldn't "H" and "J" be way up there too since they'd likely occur right around the same time?  Instead, "F" and "C" names are the most commonly retired after "I".  Climatology certainly plays some role, but luck takes care of the rest.

Since storms can affect multiple places over several days or even a week, it can sometimes be hard to assign a date to what event caused the storm name to be retired.  The next two charts simply assign the storm's month and Saffir-Simpson category based on when it achieved its peak intensity.  Not surprisingly, the peak month is September with nicely-distributed drop-offs in August and October, and then the storm that is most frequently retired reached Category 4 hurricane intensity.  Roughly 45% of all major hurricanes (Category 3-4-5) end up getting retired, and major hurricanes account for 86% of all retired names.

The next chart shows how many names were retired each year.  The most was 5 (in 2005), and 21 of the 72 years had no names retired (most recently in 2023).  However, I also added a trend line, which is decidedly upward.  Since there are no objective criteria for retirement, we can't logically blame that trend on evolving criteria.  One contributing factor could be that, on average, hurricanes are gradually getting stronger and wetter due to climate change.  But primarily, there are simply more people and there is more stuff in harm's way now compared to several decades ago, so the same hurricane is more likely to cause more problems today.

There is really no meaningful trend, up or down, in the number of fatalities caused by storms that end up getting retired, but the median number is 67 people (as few as 1 and as many as ~12,000).  The 2024 season was responsible for the most deaths since 2017.  In terms of inflation-adjusted economic incurred losses, there is definitely an upward trend.  The hurricane seasons of 2017 and 2005 dominate the data, but even without them the trend is upward.  The median value of economic losses caused by retired storms is $7.5 billion 2025 USD (as low as $10m and as high as $205b).

Since the practice of retiring storm names is so subjective, the statistics presented here are not simply a reflection of what's happening in nature -- there is a significant human component to it.  And even the human component has components to it: 

1. A storm can be less deadly if more people are able to evacuate or have a safe shelter
2. A storm can be less costly if construction is more resilient to hurricane hazards
3. With no objective guidelines, a couple dozen people nominate and vote on which names get retired.  There are several examples of past hurricanes that probably should have had their names retired, but didn't.  Two extreme cases are Gordon (1994) which was responsible for nearly 1,200 deaths and Sally (2020) which caused $7.3 billion in damage. Both names are still in use. 

 

Trivia 

Q: Were there any names retired in 2019, and if so, what will replace them in the 2025 list?

A: There was only one: Dorian.  It will be replaced by Dexter this coming season.

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2025 "Cone of Uncertainty" Update & Refresher

Anyone who lives on or near a hurricane-prone coast is undoubtedly familiar with the track forecast cone or "cone of uncertainty" which has been produced by the National Hurricane Center since 2002.  It begins as a point at the current position of a tropical cyclone and expands to show the potential position of the storm's center in the next five days. It is often called the "cone of uncertainty" because the further out in time you go, the more uncertain the forecast becomes... and it tends to look like a cone!

(By the way, "tropical cyclone" is a blanket term that refers to tropical depressions, tropical storms, and hurricanes.)

A "cone of uncertainty" for Hurricane Irma (left) and Hurricane Harvey (right). Both cones are from 2017 and are therefore identical to each other in their construction. 

The size of the cone is fixed for every forecast of every storm during an entire hurricane season, but the size slowly evolves from year to year. If the storm is moving quickly, the cone will appear more elongated and if the storm is moving slowly, the cone will appear more compact... but it's the exact same cone.  The examples shown above are from Irma (left) and Harvey (right); both storms occurred in 2017, so both cones are identical in their construction.

The cone is updated each year prior to the start of hurricane season, and it usually shrinks each year.  Overall, hurricane track forecasts are gradually improving, meaning that in general, there is less uncertainty where a storm will track now than there was a couple decades ago.

The map below shows a sample satellite image with the new 2025 cone overlaid on the 2015, and 2005 cones for comparison.  This year, the cone is slightly smaller at most lead times compared to 2024.

Improvements are getting increasingly challenging to achieve because there can never be a perfect forecast of a chaotic system like the atmosphere. We call this a "limit of predictability", and there will come a time when we reach it and meaningful improvements can no longer be made.  Some would argue we are very close to -- if not already at -- that limit.  It's critical to understand that forecasts evolve and there is always some amount of uncertainty in them.

So just how is the size updated each year?  The National Hurricane Center uses its own track forecast errors over the previous five years to calculate a circle at each "lead time" (1 day, 2 days, ... 5 days).  The size of that circle is designed to enclose the position of the storm's center with 2/3 probability, meaning that there's historically a 1/3 chance the storm will track outside the circle at that time.  Lines connecting the various circles complete the shape of the cone. [Note that the 2025 cone size is thus determined from track errors during the 2020-2024 seasons.]

This next chart shows the evolution of the cone's size at each forecast day -- it's clear that progress is slowing and perhaps has reached a limit at some (most?) forecast lead times.

Since the cone is so widely used yet sometimes misunderstood, here are some key refreshers:

• The cone does not tell you anything about where impacts will be experienced.  It is NOT a "cone of concern"! Even for a perfect down-the-middle track forecast, impacts such as strong wind, heavy rain, storm surge, and tornadoes will extend beyond the cone. Cone graphics on the NHC website include some of the relevant watches and warnings, as shown in the examples at the top of the post.  Starting experimentally this year, tropical storm and hurricane watches and warnings will be displayed inland too, not just on the coast. 

• The cone does not tell you anything about the size of the storm.  Regardless of how strong they are, hurricanes come in a wide range of sizes.  Back in 2017, NHC added the latest observed size of the wind field to its cone graphics to help illustrate this (see the Irma and Harvey examples above... the orange and red shading indicates the extent of tropical storm and hurricane-force winds at the time the forecast was issued).

• The cone does not tell you anything about the actual uncertainty associated with the forecast. Since the size of the cone is fixed, it cannot become more narrow or broad to accommodate a more or less predictable environment.

• Nothing magically happens at the edge of the cone. If a hurricane is approaching and you are scrutinizing each new forecast to see if you are inside the cone or not, you are missing the point of it.  It is arbitrarily chosen to be the 67% historical probability threshold... a 75% probability cone would be larger, and a 50% probability cone would be smaller.  Again, it is NOT a "cone of concern".

• If you use the cone graphics from NHC, there is some information about intensity provided. At each forecast point, there is a letter written inside the black dot corresponding to a general intensity range: D (tropical depression), S (tropical storm), H (hurricane (Category 1-2)), and M (major hurricane (Category 3+)).  But keep in mind that there is uncertainty associated with the intensity forecasts too!

So, let's imagine what a cone of uncertainty for intensity might look like. Consider this: averaged over the past five years (2020-2024), the mean error in a 1-day forecast is +/- 8.1 mph, the error in a 3-day forecast is +/- 12.6 mph, and the error in a 5-day forecast is +/- 16.5 mph.  But there is also a wide range of values that go into those averages, meaning that there is a small probability of a very large error and a small probability of near-zero error.

To create the next figure, I simply averaged five years of intensity errors together such that the values listed for 2025 are based on the NHC's intensity errors during the 2020-2024 seasons (and 2024 used 2019-2023, and so on).  This five-year averaging helps to mimic the smooth trends of the track forecast cone, but it is not a 2/3 probability like the track cone.

Next, using a made-up five-day intensity forecast and the average error values for 2025, I created the following chart.  This is what a cone of uncertainty for intensity could look like; a hypothetical intensity forecast is shown with the red line and the cone of uncertainty is the light red shading surrounding the forecast.