21 September 2022

Tropical Atlantic springs to life with activity

There is no shortage of features to discuss, but for the sake of brevity and relevance, I will skim over some of them.  On the map above, Hurricane Fiona and Tropical Storm Gaston are marked, Invest 98L is the disturbance over Trinidad, Invest 99L is the disturbance in the central tropical Atlantic, and soon-to-be Invest 90L is still over Africa.  Within the next five days, NHC is giving them a 90%, 30%, and 50% chance of formation, respectively.

I will begin with the key player in the basin: Category 4 Hurricane Fiona. This has matured into a very strong hurricane and fortunately the eyewall will likely miss land until it reaches Canada on Saturday. 

But before reaching Nova Scotia and Newfoundland on Saturday as a very intense storm (perhaps setting new record low surface pressures), it will have a close encounter with Bermuda on Thursday night into Friday morning.  A hurricane watch is in effect for the island, and if the storm tracks on the eastern periphery of the model guidance, the eastern eyewall could actually pass over the island which would make conditions dramatically worse.  This has the potential to be the strongest hurricane to ever hit the island, so hopefully it misses.

This map shows all of the Category 4+ hurricanes that passed within 100 miles of Bermuda since 1851 -- it's a short list!  The most recent, Nicole (2016) weakened to a mid-range Category 3 hurricane by the time it passed over.  But before that, the previous was in 1961 and even that was way out on the edge of that 100-mile circle.

The American global model, GFS, has the following portrayal of Fiona just as it's about to make landfall in Nova Scotia early Saturday morning.  Although hard to see on there, that innermost isobar is 928 mb, which would be absolutely stunning if that verified.  For comparison, Fiona's central pressure on Wednesday afternoon is 937 mb.  Between the catastrophic flooding that Fiona caused in Puerto Rico and this all-but-certain historic landfall in eastern Canada, there's little doubt that the name Fiona will be retired after this season.

Moving on, Tropical Depression 8 was upgraded to the season's seventh named storm, Gaston, on Tuesday afternoon.  It's now only 800 miles west of the Azores, but is more or less in the middle of the north central Atlantic.  The forecast has not changed since yesterday: it will approach the Azores over the next couple of days, then stall and meander as it transitions to an extratropical cyclone.

Invest 98L is disorganized and has not developed coherent thunderstorm activity.  Furthermore, the center of the low pressure system is about to scrape VERY close to the South American coast which will likely restrict development even longer.

Given these obstacles, it could be at least another day or two until this has a chance of formation.  However, model guidance continues to indicate significant development once it's in the central to western Caribbean late this weekend.  Like yesterday, these two maps show the ensemble guidance from the European (left) and American (right) models.  The ensemble means are the thick black lines.

Clearly, there is a lot of interest in what this does anywhere from the Yucatan to the northern US Gulf coast to Cuba and Florida.  But among all 70 of these ensemble members, one thing you notice is that they all delay intensification until Sunday or so, then the vast majority of them become hurricanes rather quickly.

If we look at the ocean heat content (not just the sea surface temperature, but an integrated measure of temperature below the surface), we see a powder keg of energy sitting untapped in the western Caribbean.  This could be an important player in the potential for rapid intensification in the Monday-Tuesday timeframe.

I'm going to just briefly show the two areas of interest further east in the tropical Atlantic for completeness.  As I mentioned at the beginning, the western one is Invest 99L and has a 30% chance of formation within the next five days, and the one just about to leave Africa will likely be tagged as Invest 90L soon and that has a 50% chance of development within the next five days.  It could actually become a tropical cyclone EAST of Cabo Verde which is less rare than you might think... the last few to do so were Larry (2021), Rene (2020), TD10 (2020), TD15 (2019), Lorenzo (2019), etc, etc.

At this point, it's a race for names, but the next three are Hermine, Ian, and Julia.

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