‘Tis the Season for Cold Fronts

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SourceRegions The average yearly air temperature in Key West, Florida is 77.8 degrees fahrenheit, a tenth of a degree higher than the average yearly air temperature in Honolulu, Hawaii.  This year-round warmth makes the official NOAA/National Weather Service station in Key West the warmest in the United States (excluding the tropical territories of Puerto Rico and the Virgin Islands, Guam, and American Samoa).  The all-time low temperature in Key West is 41 degrees, last observed on January 13, 1981.  The all-time high temperature is 97 degrees, last observed on August 26, 1956.  In weather, as in real estate; location, location, location is very important.  The Florida Keys are a subtropical, low-lying archipelago (group of islands) in the western portion of a major ocean basin, adjacent to a large continent.  A large area of shallow water adjoins this archipelago, with deeper north-south channels connecting the islands.  A fascinatingly rich natural region of wetlands, known as the Everglades, borders the eastern end of the island chain.  Meanwhile, the westernmost islands jut out into the extreme southeastern Gulf of Mexico.  Just offshore to the south is the Florida Reef tract, the third largest coral barrier reef system in the world, and just beyond the reef, in the Straits of Florida, is the Gulf Stream, one of the warmest, largest, and fastest major ocean currents on the planet.  Such a complex environment is one of the reasons for the large number of options available to anglers in the Florida Keys.  It also contributes to the unique climate and weather observed in the Keys. Summertime weather in the Florida Keys is hot, humid and wet.  One may expect the jet stream and polar air to stay far away to the north for most of the June–September period.  The expansive ocean areas surrounding the Florida peninsula will heat up, and hold the heat, maintaining days on end of warm, muggy nights, tempered only by rain or the occasional fresh breeze.  By early October, over three months of gradually eroding solar energy will allow the surrounding seas to cool, taking the edge off the heat.  At the same time, the jet stream will gain energy due to increasing thermal contrast between the North Pole and the Equator, as the Arctic cools faster than the Tropics.  Sometime between mid-October and early November, Florida Keys residents and visitors may anticipate the season’s first “cold” front.  A cold front simply is the edge of an advancing cooler air mass.  Each front is unique, with a pattern of wind, weather, and sea based on its movement, intensity, shape, depth, orientation, and other factors.  The determination of this pattern is one of the challenges associated with marine weather forecasting.  Some fronts will bring strong north breezes, while others may be associated with northwest or northeast breezes.  A few times per year, a front will be followed by a bona fide gale.  Occasionally, during the spring, a front will be preceded by a line of severe thunderstorms or squalls.  In late autumn, fronts will tend to stall over the Straits of Florida, or even back up as warm fronts!  Cold fronts can follow hurricanes (like the aftermath of Hurricane Wilma in 2005).  Rarely, a tropical system will “merge” with a front, unleashing torrential downpours (it was the combination of Tropical Storm Jeanne and a front in November 1980 which resulted in over 20 inches of rain in Key West in less than a day!).  About 40–50 cold fronts of varying intensities will pass through the Florida Keys in an average year.  The frequency of cold fronts in the Keys usually will spike abruptly during November, peak in January, and drop off gradually from February through April.  Cold fronts are rare in June and September, and almost unheard of in July and August. Any front marks a division between air masses of differing densities.  Specific air masses form beneath persistent areas of high pressure where air is able to take on the properties of the underlying surface.  For example, a “continental polar” air mass is cold and dry.  A “source region” for such an air mass might be the prairies of western Canada or Siberia.  A “maritime tropical” air mass is warm and moist.  One source region for maritime tropical air is the Caribbean Sea.  A “continental tropical” air mass is hot and dry.  Source regions for this type of air mass include many of the world’s great subtropical desert belts.  Finally, a “maritime polar” air mass is cold and moist (think Seattle, Washington during the winter).  In the Florida Keys, a typical front will separate maritime tropical air from continental polar air.  However, the continental polar air usually is highly “modified” through a long fetch over the Gulf of Mexico or Atlantic Ocean.  As a result, the air is much warmer and more humid when it arrives in the Keys than when it left the coast at Tampa, Panama City, or New Orleans.  This is one of the reasons it has never snowed in the Florida Keys, at least since official weather records have been kept (~ 1870).  However, occasionally, air temperatures will dip into the 40s or 50s behind a strong cold front.  This usually happens when a long trajectory of northerly winds develop in a nearly straight line from Canada to the Keys via the Florida Peninsula (such that the cold air is unable to warm up over the warmer waters of the Atlantic or Gulf).  For this pattern to develop, both a strong high pressure center and a strong low pressure center are needed in such an orientation so that the clockwise winds around the high pressure system augment the counterclockwise winds around the low pressure system, driving the cold, continental air mass due south to southern Florida with little modification over water.  Such was the case during an unusually prolonged spell of cold weather in January 2010. Wintertime cold fronts affect not only landlubbers in the Florida Keys.  The wind and weather associated with fronts influence sea surface temperature, currents, water levels, and underwater visibility.  Consequently, they affect the behavior of fish and other creatures of the sea. One way to get a quick narrative summary of the marine weather expected over the next five days is to access the forecaster-prepared “Coastal Waters Forecast Synopsis”, available online at http://forecast.weather.gov/shmrn.php?mz=gmz005. Remember to be weather-ready, and stay safe!


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