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 content 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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