Florida Keys Marine Weather Regions: Part II – Florida Bay and Gulf Flats

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This article is the second 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 Florida Bay and the extreme southeastern Gulf of Mexico inside five fathoms.  This area comprises the shallow flats, basins, and channels nestled between the Florida Keys and the southern Florida Peninsula.  Water depths can range from a few inches to 30 feet, with some areas exposed at low tide.  The time of high and low tides varies significantly across this region due to the shallow water depths and complex terrain both above and below the water line.  These waters typically are much less saline than those in the Straits of Florida or open Gulf of Mexico.  This is because the Everglades (known as the “River of Grass”) continually infuses Florida Bay and adjacent water bodies with fresh water.  In fact, most of Florida Bay lies within Everglades National Park.  Farther to the west lie the Gulf-side flats adjacent to the lower Florida Keys.  Finally, a trip from Key West to Halfmoon Shoal will reveal the Lakes Passage, the Marquesas Keys, and the Quicksands.  The Lakes Passage (or “the Lakes” is a path of very shallow water (less than five feet) between Key West and Boca Grande Key.  The Marquesas Keys consist of a group of mangrove islands and an inner lagoon.  The Quicksands region is a large, shallow area with a sandy bottom.  Northwest Channel (between Key West and the Lakes Passage) and Boca Grande Channel (between Boca Grande Key and the Marquesas Keys) are the two primary north-south channels in this area.  Also, much of the area between the Content Keys and Halfmoon Shoal are within at least one of three National Wildlife Refuges (National Key Deer Refuge, Key West National Wildlife Refuge, and Great White Heron National Wildlife Refuge). The marine weather of this region is unique relative to the other zones due to its high frequency of lightning strikes and waterspouts. Analysis of lightning strike density across the Florida Keys and adjacent coastal waters by meteorologist Matthew Bloemer of the Florida Keys National Weather Service indicates that lightning strike density is much greater over and near land areas relative to the open ocean.  This finding is consistent with similar studies conducted around the world.  In particular, much greater lightning strike density is found across the southern Florida Peninsula than over the Florida Keys, and strike density is greater over the Keys than it is over the Florida Reefs and Straits to the immediate south.  A local “hot spot” in lightning activity is revealed over the lower Keys and adjacent shallow waters, roughly bounded by a triangle with vertices near the Spanish Banks (northeast of Big Pine Key), Marvin Keys (just east of Snipe Point), and Key Lois (Hawk Channel just south of Cudjoe Bay).  This “hot spot” of lightning activity likely exists due to the greater frequency of thunderstorms initiated by island-scale “differential heating”, or mini sea breezes.  During the summertime, the large island of Big Pine Key and the expansive region of shallow, hot water adjoining the lower Keys will produce a local heat maximum in the lower atmosphere.  In a typical summer easterly wind pattern, winds will turn northeast over the northern fringes of the lower Keys, while winds along the southern fringes bordering Hawk Channel will tend to blow from the southeast.  The converging air streams will transport cooler, denser air into the lower Keys as hot, moist air rises, eventually building towering cumulus and cumulonimbus clouds which line up in rows parallel to the mean low-level wind.  The taller cumulonimbus clouds are the thunderstorm clouds from which lightning strikes are born, and these are much more frequent in regions where sea breezes converge or collide. Another local marine weather implication of the routine generation of cumulus and cumulonimbus cloud rows in this region is the high frequency of waterspouts.  A waterspout is a tornado over water.  The waterspouts which so often are spawned beneath the dark and flat bases of towering cloud lines near Florida Keys derive their energy and spin from the hot, moist air which originates within the various mangrove and pine forecasts, mud flats, basins, and salt ponds of the Florida Keys and nearshore waters.  In fact, retired NOAA scientist, Dr. Joseph Golden, who studied waterspouts in the Florida Keys for decades, beginning in the late 1960s, has called this area “the greatest natural vortex lab in the world”.  He added, “Waterspouts probably occur more frequently in the Florida Keys than anywhere else in the world”! Next month, we will explore the marine weather across Hawk Channel.  Until then, remember to be weather-ready, and stay safe!


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