Florida Keys Marine Weather Regions: Part IV – Straits of Florida

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1-june2014 This article is the fourth in a series exploring the spatial variations in marine weather covering various “regions” around the Florida Keys, including the southeastern Gulf of Mexico, Florida Bay and the shallow Gulf waters inside five fathoms, Hawk Channel, and the Straits of Florida.  The region to be explored this month is the relatively narrow oceanic waterway confined between the Florida Reef tract and the islands and banks of the Bahamas and Cuba.  This region is known as the “Straits of Florida” or “Florida Straits”.  The Straits of Florida are home to some of the busiest merchant shipping lanes on the planet.  In fact, the U.S. Department of Commerce has estimated that nearly 40 percent of the world’s maritime commerce passes through these shipping lanes each year.  In addition, the Straits are a popular fishing and boating destination for recreational and commercial angling interests based in the Florida Keys.  The underwater terrain beneath the Straits of Florida features a reverse L-shaped trough extending from south of the lower Florida Keys to east of the southeast coast of the Florida Peninsula.  The northern arm opens northward into the Atlantic Ocean, whereas the southern arm opens westward into the Gulf of Mexico.  Water depths slope southward from the shallow Florida Reef tract to about 500–700 feet, where they level off (this region is known as “Pourtalès Terrace”, named after Louis Francois de Pourtalès, the American naturalist who first surveyed the region in the 1850’s).  A rapid drop-off then ensues along the southern edge of Pourtalès Terrace.  This drop-off is known as “Floyd’s Wall” (off the upper Keys) or Wood’s Wall (off the lower Keys), and is easily seen on charts showing water depths.  Even deeper water is found farther south and southwest, with water depths greater than a mile in the lower Straits between Dry Tortugas and western Cuba. Weather across the Straits of Florida may differ significantly from the large land masses of the Florida Peninsula and Cuba to the immediate north and south. During the summer, intense thunderstorms will grow over the hot land masses as sea breezes penetrate inland.  However, such intense thunderstorms are less common over the relatively cooler Straits of Florida.  Therefore, summer lightning density across the Straits is much lower relative to surrounding land masses.  During the winter, however, the Straits typically are warmer than adjacent areas due to the warming influence of the Florida Current (known locally as the “Gulf Stream”).  In fact, the presence of the Gulf Stream in the Straits has significant implications for wind, waves, and weather year-round.  In particular, when a strong northeast or east wind opposes the eastward- or northeastward-moving Gulf Stream, both wave height and steepness are increased.  Wave heights may increase up to 40% when direct current-wind opposition is occurring.  Also, both sustained winds and gusts typically will be higher across the Gulf Stream during the numerous post-cold front blows of late autumn and winter.  This happens because the warmer near-surface air over the Gulf Stream creates a more rapid temperature decrease with increasing height above the sea surface, which, in turn results in stronger turbulence in the lower layer of the atmosphere, resulting in a more efficient downward transfer of the higher winds typically found a few hundred feet above the surface.  Finally, the Straits of Florida, containing a deep reservoir of warm ocean water thanks to the Florida Current/Gulf Stream, may under the right conditions, help to intensify some tropical storms and hurricanes moving through the area. Weather data buoys are nonexistent across the Straits of Florida, unfortunately.  Therefore, marine weather forecasters at the Florida Keys National Weather Service rely on the reports of wind, waves, and weather from voluntary observing ships, volunteer marine weather spotters, and members of the marine public.  If you work or play in the Straits of Florida, let us know what kind of wind, weather, or sea conditions you are experiencing.  Social media is a quick way to get a hold of us 24/7. Follow us on Facebook and Twitter at: www.facebook.com/us.nationalweatherservice.gov www.twitter.com/nwskeywest As always, remember to be weather-ready, and stay safe!


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