Humberto becomes second Category 5 of the season, TD9 triggers storm watches in Florida

Hurricane Humberto continues to overperform and rapidly intensify, reaching the extremely rare Category 5 intensity as of Saturday afternoon.  In the past century, only 3.8% of named storms reach Category 5 status (including 2025). Of the 44 Category 5 hurricanes in the past century, 12 of them occurred in just the past decade. But perhaps more impressive is that of the three hurricanes in this season so far, two of them were Category 5s!

Humberto is forecast to intensify further in the near future as it heads northwest, then gradually weaken and turn to the northeast somewhere west of Bermuda on Tuesday.  It also looks quite likely now that it will be close enough to TD9 (future Imelda) to influence its track in a process called the Fujiwhara effect.

This vortex interaction greatly complicates the forecast of TD9.  Since Humberto is much larger and stronger, the influence on its track will be barely noticeable, but the influence on TD9's track could be significant.  Or, they could stay just far enough apart that TD9 continues on and makes landfall in the Carolinas and drifts inland. Global model ensembles continue to show both as viable outcomes. It's hard to overstate how challenging this forecast is and what's at stake for impacts in the southeast U.S.

As of Saturday afternoon, there are tropical storm warnings for most of the Bahamas, and tropical storm watches for some of the Florida east coast.  TD9 is forecast to become Tropical Storm Imelda quite soon, and tropical storm conditions could reach South Carolina sometime early Tuesday morning. On the cone graphic below, it's important to note that the track forecast uncertainty is much larger than normal with this, but the cone size is fixed all year long for all storms, so it's under-representing the forecast uncertainty in this case.

One of the greatest concerns with future-Imelda is the rainfall and inland flooding.  The track it takes and the speed at which it moves both greatly affects the extent of flooding.  The latest outlook in the coming five days is shown below, and I also have a long radar loop of the entire region at https://bmcnoldy.earth.miami.edu/tropics/radar/ to monitor ongoing rainfall.  If this storm ends up tracking more inland rather than getting slingshot out to sea, it's easy to visualize those very high rainfall totals shifting onto the Carolinas rather than over the ocean.

Looking at where the Accumulated Cyclone Energy stands, Humberto has pushed it up to 74% of average for the date, and it's rising very quickly.  Based on forecasts for Humberto and future-Imelda, 2025 could easily catch up to climatology (100%) around October 1.  Still, this is the least amount of ACE for the date since 2016.

Through today, the 2025 season has had 8 named storms, 3 hurricanes, and 3 major hurricanes.  The average by this date is 10, 4, and 2.

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Humberto rapidly intensifies, soon-to-be Imelda heading for Bahamas

Friday afternoon satellite image of Potential Tropical Cyclone Nine (left) and Major Hurricane Humberto (right).

As forecast, Humberto rapidly intensified and is now the season's third major hurricane.  But incredibly, it's also only the season's third hurricane! It has been 90 years since the first three hurricanes all became major hurricanes.  For additional context, the average date of third major hurricane formation is October 28 -- this is over a month ahead of climatology.

And it's not finished yet... as of Friday afternoon the intensity is up to 100 kt (lower threshold of Category 3) but is forecast to reach 130 kt by Sunday morning (mid-range of Category 4).  Fortunately, the forecast track will keep it away from land. It will create some problems for Bermuda on Tuesday, but it will likely not be a direct hit.

Also on Friday afternoon, Invest 94L was designated as Potential Tropical Cyclone Nine.  It's still really just an Invest, but this designation allows NHC to issue watches and warnings -- tropical storm watches and warnings are now in effect for the northern and central Bahamas. It is forecast to become a tropical depression and tropical storm this weekend.  The next name on the list is Imelda.

But here's where things get extremely tricky. It's entirely possible that PTC9 (future Imelda) never even reaches the US coast. Quite a number of models indicate that it could interact with Hurricane Humberto and get sent back out to sea in a process called the "Fujiwhara effect"... I have some extra background info on this in a thread on Bluesky: https://bsky.app/profile/bmcnoldy.bsky.social/post/3lzlompr3vk2e

Below I show the most recent forecast tracks from the European model ensemble (left) and the American model ensemble (right).  There is a rather sizable percentage of members that show this Fujiwhara interaction (the tracks that make a right turn before or nearly reaching the US coast).

So, there is clearly an ENORMOUS amount of uncertainty after the weekend, and that has implications for what could be a terrible flooding event in the southeast US.  The current rainfall outlook over the coming week is shown below, and there are some significant totals possible in the Carolinas, including areas still recovering from Hurricane Helene-induced flooding last September.

Elsewhere across the basin, there are no areas of interest in the foreseeable future.

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Rare hurricane for the Azores, and two areas of interest in western Atlantic

Hurricane Gabrielle has continued its trek across the northern Atlantic and is about to pass over the Azores at hurricane intensity.  It is losing its tropical characteristics and will transition to an extratropical cyclone within the day -- though that will not diminish the significant impacts it will create in the Azores.

The islands have had fifteen hurricanes pass within fifty miles of them since records began, and the most recent was Lorenzo in 2019. Four of the fifteen were at Category 2 intensity, and the other eleven were Category 1: https://bsky.app/profile/bmcnoldy.bsky.social/post/3lzlq3kdb422e

I have my first-ever radar loop in the archive from the Azores, and you can find the latest version at https://bmcnoldy.earth.miami.edu/tropics/radar/: 

In the western Atlantic, Invest 93L was upgraded to Tropical Storm Humberto on Wednesday afternoon, and Invest 94L is now centered just north of Hispaniola and is close to becoming a tropical cyclone -- the next name is Imelda.

Humberto is the season's 8th named storm, and the average date of 8th named storm formation was September 9. However, Humberto could also become the season's 3rd major hurricane in a few days, and the average date of 3rd major hurricane formation is October 28!

Conditions ahead of Humberto appear very favorable for intensification, and the 11am EDT advisory from NHC explicitly includes a period of rapid intensification on Saturday into Sunday.  Beyond this 5-day forecast, it looks likely that it will make a turn to the northeast around the time it crosses 30°N. It does not appear that it will impact land along its journey. 

Invest 94L, on the other hand, could create problems in the southeast U.S.  It's currently fairly disorganized and centered north of Hispaniola and west of Humberto, but it looks likely that it will become a tropical depression or tropical storm sometime on Friday (the next name is Imelda).

Satellite loop of Invest 94L (left) and Tropical Storm Humberto (right).

It is expected to intensify as it passes over the Bahamas, then the exact track is extremely challenging to predict because of the uncertainty in the intensity AND because of its proximity to Humberto -- it's possible the two cyclones will influence each others' tracks in a process commonly called the "Fujiwhara effect" (I have a mini-thread about this at https://bsky.app/profile/bmcnoldy.bsky.social/post/3lzlompr3vk2e).

A similar plot to what I showed above for Humberto is shown below for Invest 94L.  The track density for the multi-model "super ensemble" indicates the highest probability of a track into the Carolinas, though south Florida and northward should also be on alert and paying close attention. If it does make landfall, it could be as a hurricane.  This could also be a significant rainmaker for eastern South Carolina, North Carolina, and Virginia.

We will have to wait and see how long it takes to develop, where the center ends up forming, and how close it gets to Humberto.  It's a complex setup, but has the potential to have big impacts in the southeast U.S. in the next 4-6 days.

Looking at ACE again, the season is now up to 67% of average for the date, and although Gabrielle will stop contributing once it becomes an extratropical cyclone, Humberto (and possibly Imelda) will pick up and continue to accrue ACE in the coming days. As of today, this is the least ACE accrued through September 25 since 2016.

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Surge of activity includes Major Hurricane Gabrielle near Bermuda

First off, apologies for the lack of posts lately!  But it will be quick to catch up because the Atlantic had an incredibly inactive three weeks (last week of August into the first two weeks of September).  Then, Tropical Depression 7 formed on the 17th in the deep tropical Atlantic, and that was upgraded to Tropical Storm Gabrielle shortly after.  It just reached hurricane intensity on the afternoon of the 21st, making it only the second hurricane of the season so far (Erin was the other) -- then on Monday morning it rapidly intensified to become the season's second major hurricane (Erin was the other).

As of Monday morning, Category 3 Hurricane Gabrielle is approaching Bermuda but will pass well to the east of the island.  It could still intensify some more in the next 24 hours; afterward, colder water will cause it to gradually weaken though still be a potent hurricane as it approaches the Azores on Thursday night.

I have a long radar loop of Gabrielle from Bermuda available at https://bmcnoldy.earth.miami.edu/tropics/radar/

Turning our attention to the two tropical waves in the tropical east Atlantic, NHC is giving a 40% chance of development to the western one, and 70% to the eastern one within a week... neither have been tagged as Invests yet, but the next two slots are 93L and 94L. The current positions of the waves are marked by X's on the map below, and the potential areas of formation within the next 7 days are the shaded orange and red blobs.

As of now, neither of them appear to pose a serious threat to land in the immediate future.  Global model ensembles are a bit scattered on their long-term futures, so there's no reason for concern now, and there is time to watch and wait.  Even the western one, which will be close to the Leeward Islands, Puerto Rico, and Hispaniola, should just bring some rainy and unsettled weather to those areas on Wednesday-Thursday, but nothing too impactful in terms of wind. The Bahamas should be paying close attention for later in the week though as some models do develop it into a tropical cyclone by then.

By the end of today, the Accumulated Cyclone Energy (ACE) will be at about 56% of average for the date. At this point in the season, it would take some long-lived significant hurricanes to bring the ACE back up to average.  It could happen though, and the season is far from over.  The last few seasons with lower ACE by this date were 2015, 2014, 2013, 2009, and 2002.

As we watch the next couple of disturbances, the next two names on this year's list are Humberto and Imelda.

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Erin is a large and intense hurricane, producing coastal flooding along U.S. east coast

Hurricane Erin has made the anticipated turn to the north and it is now centered roughly between South Carolina and Bermuda. It is an upper-end Category 2 hurricane, and a large one at that.  Tropical storm force winds extend an average of 230 miles from the center, and hurricane force winds extend an average of 80 miles from the center.

To put those sizes on a map, I superimposed the latest sizes of the tropical storm (orange) and hurricane (red) wind fields on a satellite image from 5pm EDT on Wednesday:

And because of its size, tropical storm warnings are in effect for the Outer Banks of North Carolina up into the DelMarVa peninsula and there will also be noticeable and destructive storm surge and coastal flooding from South Carolina up to New York, particularly in the Outer Banks.

Erin could still briefly re-intensify to Category 3 status later today and into Thursday, then will gradually weaken over cooler water as it races off to the northeast, avoiding any direct landfalls. It will still be a formidable low pressure system when it reaches the UK in 6 days or so.

Through today, the 2025 hurricane season's ACE (Accumulated Cyclone Energy) is at 159% of an average season's value on August 20. The last years that had higher values by this date were 2024, 2008, and 2005. So not only is the ACE significantly higher than normal for the date, the season's first major hurricane formed 16 days ahead of climatology.

Erin is still an original name from the 1983 list, and it's been used every six years since then, so this is its 8th time around. It's very unlikely that the name will get retired after this incarnation, so it will be back on the list in 2031. Some of you may recall Erin 2001, which was a large intense hurricane, also sitting off the U.S. east coast. A weather satellite caught this stunning and haunting image of Hurricane Erin on the morning of September 11, 2001.

I co-authored a paper that featured this image, and it was the "Picture of the Month" in the American Meteorological Society's Monthly Weather Review journal, available here: Vortical Swirls in Hurricane Eye Clouds.

Elsewhere across the Atlantic, there are two tropical waves that could develop in the coming week. At this point, neither one appears to be a threat to land, but NHC is giving the western one (~50°W) a 60% chance and the eastern one (~30°W) a 40% chance of development. This map below shows the current infrared satellite image with tracks from the most recent American model ensemble run overlaid. It's messy, and only the western one looks like it has a shot as lasting more than a few days, although it's extremely broad and disorganized now.

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

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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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