In October, we’re distracted by all the trappings of fall – football and the race for the pennant, pumpkin spice everything, and maybe that first real cold front to kick off dry season. Anything but hurricanes. But like baseball and politics, October is a month of surprises when it comes to hurricane season.
Some of the strongest hurricanes on record (think Mitch in 1998 or Wilma in 2005) occurred in October, and October is South Florida’s most likely month for a hurricane strike. It’s the month the season begins to wind down, but it’s also the month that might matter most for us.
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Historically in October, about a quarter of storms have formed in the deep Atlantic east of the islands, about a quarter have formed in the subtropics (the Atlantic north of about 25°N latitude), and another third have formed over the western Caribbean. It’s this latter bunch in the western Caribbean that we’re most concerned with. Even though storms that originate in the western Caribbean make up only about a third of October formations, they comprise an astounding 90% of historic South Florida landfalls in October.
The western Caribbean is a breeding ground for strong hurricanes in October. Hurricane Wilma, the last October hurricane to hit South Florida, bottomed out with a minimum pressure of 882 mb on October 19, 2005, while in the western Caribbean, making it the most intense Atlantic hurricane ever observed to this day. The big driver behind these big October hurricanes in the western Caribbean is the deep reservoir of extremely warm waters. Compared to the rest of the Atlantic, waters here are warmer than in any other part of the basin.
This year in particular, the waters of the western Caribbean – like the Atlantic writ large – are the warmest they’ve ever been.
This week, for the first time on record, the average sea surface temperature across the western Caribbean topped an astonishing 87 degrees Fahrenheit (30.6°C).
Of course, sizzling sea surface temperatures alone don’t guarantee hurricane formation. We need a trigger for storminess and conducive upper-level winds. In October, the trigger can be either from westward moving tropical waves, cold fronts that drape southward into the southern Gulf, or from a broad area of low pressure and spin that sometimes sneaks over from the eastern Pacific. There are plenty of possibilities, and they tend to converge over the western Caribbean.
As we look at the upper-level wind pattern into October, the area with the lightest wind shear happens to also coincide with the region of warmest waters in the western Caribbean.
For now, we don’t see any looming threats in the models, but given the favorable environment and the tendency for storms that form in the western Caribbean to get pulled northward toward South Florida, we’ll keep a wary eye to the area in the weeks ahead.
Philippe falters while Rina takes shape
The two systems we’re tracking this week are far away over the open Atlantic.
Philippe remains a disorganized tropical storm and hasn’t been able to find its footing thanks to persistent wind shear. The forecast is for Philippe to struggle over the next few days before steadily weakening into the weekend. The system will continue generally westward but poses no threat to land.
Behind Philippe, Invest 91L continues to gradually organize about 1,000 miles west of Africa. It’s expected to strengthen into Rina, the next named Atlantic storm, by the weekend, but like its counterpart to the west, poses no immediate concerns for land.
