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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