The Late Autumn Transition

by Chip Kasper

The Late Autumn Transition

The notion of the meteorological “four seasons”, commonly waxed about by our northern, continental neighbors is more fantasy than reality in this part of the world. Yes, one still may identify the time and date of the astronomical equinoxes and solstices. And, sure, we have “seasons” in the Florida Keys: the hurricane season; the rainy season; the dry season; the snowbird season. The Florida Keys, however, as fabulous as they are, lack the dramatic shifts in weather and landscape common to our northern neighbors. Nevertheless, there is one time of the year in the Florida Keys during which the wind and weather seem to change more rapidly than the rest of the year—the final weeks of autumn.

The last two months of the year are noteworthy on the meteorological calendar in the Florida Keys for a number of reasons. First, the nearly six-month-long rainy season usually ends right around November 1st. Nearly 68% of the annual rainfall and 91% of the total lightning occurs during the months between May and October. In addition, the six-month hurricane season ends on November 30th, putting an end to a daily vigil involving routine checks of the “Tropical Weather Outlook”. Not surprisingly, the first cold fronts of the season sweep through the Florida Keys during this time as well, often bringing a welcome drop in both air temperature and humidity. Finally, the cold fronts are accompanied by wind, and lots of it. On average, November and December are two of the windiest months of the year in the Florida Keys.

Rainfall
The end of the Florida Keys rainy season varies from year to year, but typically occurs around November 1st. However, it has occurred as early as the first week in October, or as late as Thanksgiving. According to a long record of weather observations at the official station at Key West International Airport, the number of days per month with measurable rainfall (0.01 inch or more) averages between 11 and 16 during the peak rainy season months of June, July, August, September, and October. In November, that number is cut nearly in half, dropping to 6.6, with an average of 6.4 days in December. In addition, the average number of days with thunder reaches a peak of 13.5 during the month of August, and drops to 1.1 for the month of November. This reflects the sharp drop-off in lightning strike density around the Florida Keys during the late autumn. Despite November typically being the first month of the dry season, an unusual deluge occurred on a November day in 1980, as a tropical storm interacted with a stalled front. In total, nearly two feet of rain (23.38 inches) fell in Key West during a single 24-hour period! This remains an all-time record for rainfall in a 24-hour period at Key West, with precipitation records dating back to 1872.

Tropical Cyclones
The Atlantic Basin hurricane season officially ends on November 30th. Climatological tropical storm and hurricane frequencies plummet during the month of November across the Atlantic Basin. However, the Florida Keys have experienced significant impacts from November storms, including Hurricane Michelle in 2001, Tropical Storm Mitch in 1998, and Hurricane Kate in 1985, to name a few.

Temperatures
Cold fronts are likely to sweep through with increasing frequency and vigor during the months of November and December in the Florida Keys. The average high temperature at Key West on November 1st is still a warm 82 degrees Fahrenheit (F). By New Year’s Eve (December 31st), the average high temperature is only 74 degrees F, a drop of eight degrees Fahrenheit in two months. This may not seem like much compared with northern locations, but it is significant for a subtropical marine location such as the Florida Keys.

Winds
Winds increase significantly across the Florida Keys and surrounding waters during November and December. In fact, November is the windiest month, on average, in the Florida Keys, and December is not far behind. Averaging the hourly wind speeds at the four marine-exposed weather stations at Molasses Reef, Sombrero Key, Sand Key, and Dry Tortugas yields an average sustained wind (2-minute average) of nearly 14 knots, with an average peak hourly gust (5-second average) of about 17 knots. With the wind comes an increase in wave heights, Small Craft Advisories, and seasickness. 

As we transition to late autumn in the Florida Keys, remember to check the weather before heading out on the water, and, as always, be weather-ready and stay safe!


Chip Kasper
Chip Kasper

Author

Chip is a senior forecaster and marine program meteorologist at the NOAA/National Weather Service Forecast Office in Key West. The National Weather Service provides weather, water, and climate information for the protection of life and property on land and at sea. Email Chip at kennard.kasper@noaa.gov.



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