26 August 2022

Tropics slowly waking up from a long slumber

It has been 55 days since the last named storm, Colin, formed in the Atlantic.  Since then, there has barely been a whisper of activity anywhere across the basin.  Even now, there is nothing imminent, but there are a couple areas of interest that could develop in the coming week.

This map shows a recent infrared satellite image with areas of potential development highlighted (x is the current approximate position, the "blob" is the potential area of formation over the next five days).  NHC is giving the one to the west a 20% probability of formation and 30% for the one in the east.  Note that even at the current positions, the cloudiness is extremely scattered and disorganized.

I will start the with the western one.  It is anemic and strangled by dry air right now, but some model guidance indicates a chance for development in the western Caribbean and into the Gulf of Mexico next week.  Although the vertical wind shear is low and the water temperature is plenty warm, there is a LOT of dry air that will inhibit any development in the next 2-3 days.  From this satellite animation, it's evident that dry air is dominating the area and choking off cloud/thunderstorm development.

In terms of model guidance, the European global model and its ensemble do virtually nothing with it.  But the American model does, so it's worth paying attention to as a possibility.  This image shows tracks of low pressure centers in the American GFS ensemble -- note that the tracks don't start until it's in the western Caribbean, and at that point, they generally go toward the northwest.  I ended the tracks at 7 days from now, and even then, you can see the spread in intensity and forward speed.  So it's not anything to worry about AT ALL now, just something to watch in the coming days.

The eastern wave (waves?) looks more robust, but is also more complicated.  There are a couple areas with signs of rotation clustered together, and it's not clear which one will become dominant or if they will merge.  NHC places that yellow X in the top image somewhere between the two areas of thunderstorm activity west of Cabo Verde.  Messy.

As has been the case for the past couple of months, dry air is widespread in the area, especially to the north.  This map shows areas of dry dusty air from the Sahara in shades of yellow, orange, and red. This air can easily work its way into the tropical waves and hinder development.  But farther west (in a few days), there's a chance.

The European and American global models are more uniform in developing this.  They both bring it to hurricane intensity in 7-8 days (Sep 2-3) just north of the Lesser Antilles, so clearly, this is one to watch closely.  These are the most recent comparisons of model-predicted low-level wind and pressure fields next Saturday, Sep 3.  Of course lots can and will change, but it's unusual to see this level of agreement in an 8-day forecast.

So just how quiet has it been lately?  Using the common metric called ACE (Accumulated Cyclone Energy), the season is at just 13% of the average value for the date, using the past fifty years as the baseline.  ACE is a measure of the duration and intensity of storms, so the number of storms is not a factor.  This chart below puts that value into historical perspective.  The last seasons to have such low ACE by August 26 were 1988, 1984, 1977, 1967, 1962, etc.  So it's certainly not unprecedented, but it's rare. Of those five seasons that I mentioned, 1967 and 1988 ended up near or slightly above average by the end of the season.

Whenever the next storms do come along, Danielle and Earl on the names on deck. Again using the past fifty years as the baseline, the average dates of the fourth and fifth named storm formation are August 19 and August 28.

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