High-stakes interaction for Humberto and Imelda

In one of the more stunning and high-stakes cyclone interactions in memory, Hurricane Humberto and Tropical Storm Imelda appear poised to interact in a process commonly known as the Fujiwhara Effect. There have been quite a number of such interactions documented across the world, but this particular one will steer Imelda away from making landfall on the Carolinas as a hurricane and creating widespread flooding -- just in time.

This wasn't a complete surprise either... I eluded to it in a Bluesky post last Wednesday and my blog post last Thursday (the track that future-Imelda takes depends on how close it gets to Humberto, and even then, the ensembles were showing possible right-turn scenarios for future-Imelda).  However, it was a very challenging forecast packed with uncertainty because there can be a very small difference between not being close enough to each other to do anything and being barely close enough to tug on each other.

Both storms are moving generally northward now, and Humberto is forecast to make a turn to the northeast on Tuesday-Wednesday, while Imelda should begin making an abrupt turn to the northeast very soon.  As of 5pm EDT today, the centers of Humberto and Imelda are just 570 miles apart.  For reference, we start looking for potential Fujiwhara interactions when the storms are roughly 800 miles apart.

Hurricane Humberto is forecast to track west and then north of Bermuda, giving the island peripheral but still pretty hefty impacts. Imelda may not be so kind. It is forecast to track almost directly over the island on Wednesday evening as a hurricane. A hurricane watch has already been issued. The last few direct hits by hurricanes were Paulette 2020, Nicole 2016, and Gonzalo 2014.  I also have a long updating radar loop from the island at https://bmcnoldy.earth.miami.edu/tropics/radar/

As I mentioned in my previous post on Saturday, the season's Accumulated Cyclone Energy (ACE) is getting a huge boost from Humberto, and between Humberto and Imelda, the 2025 ACE could catch up to the climatological value somewhere around October 2. As of today, it's at 87% of average for the date. This is now more ACE that the recent 2024 and 2022 seasons had by this date.

Elsewhere the basin is quiet and we likely won't see any new storms form in the coming week.  But when the time comes, the next name is Jerry.

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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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