The First Fronts of Autumn

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FirstFrontsOfAutumnIn the Florida Keys, many expressions exist to describe the onset of cooler, drier, breezier weather which typically increases in frequency during November, as shown by past weather observations. After all, not all cold fronts are alike. Indeed, some fronts are more adequately described as “cool” fronts. Others really “bring it” (“it” being the cold, dry, blustery air that is born over a frozen, continental landscape nearly 2,000 miles north of here). Many fronts move in from north to south, accompanied by a “northerly blow”. A handful of fronts may approach from the northwest or west during a winter season. Occasionally, a front will reach the Florida Keys by way of the Atlantic, approaching from the northeast (these are called “backdoor” cold fronts). By April, cold fronts are few and far between. In most years, the first few cold fronts of autumn have traveled southward off the Gulf Coast toward the subtropics, only to be utterly destroyed by the warm waters of the Gulf of Mexico and Atlantic Ocean. Simply put, the underlying warm ocean water can modify the oncoming cooler and drier air faster than that air can maintain its polar air mass properties during its equatorward migration. Consequently, you can expect the first autumn front in southern Florida to be more of a “cool front”, or a “dry front”. Occasionally, the temperature gradient which gave rise to a “front” in the first place has washed out somewhere between the Mississippi Delta and the Everglades, and all that remains is a slight “air mass change”, which can take the edge off the persistent heat and humidity. A true “cold air outbreak” in the Florida Keys requires an air trajectory rooted in an extremely cold air mass, with a path from the Arctic territories of northern Canada, nearly straight south over the continental United States, and then down the Florida Peninsula, and pointed directly into the Keys. Such a path will avoid the tempering effects of the Gulf of Mexico and Atlantic Ocean, conserving the thermal and moisture properties of a frigid air mass. In most years, you may expect such a polar path to set up only once or twice a season, and because the continental high pressure areas driving these air trajectories usually migrate fairly quickly toward the southeast, the incoming air trajectories veer quickly with time. On occasion, this polar path has been maintained for several days or even weeks. Such was the case in January 2010, when sea surface temperatures nearshore actually dropped into the 50s in the Florida Keys. In fact, the sea surface temperatures in northern Florida Bay fell into the 40s for a short time. Fish kills and turtle shocks were widespread, and the Snook population recovered only recently. Coral bleaching even occurred during that month in some areas due to the stress cold water imposed on the coral’s symbiotic algae. Usually, coral bleaching in the Florida Keys is associated with very warm ocean water, full sun, and light winds, as was the case for a portion of this past summer. Cold air outbreaks of this magnitude have been rare in the Florida Keys.FirstFrontsOfAutumn2 Local weather records reveal that first few cool fronts usually limp into the Keys during October, with the first “bring it” front often arriving during November. Often, the first significant cold front of the season follows a wet and windy tropical system. Examples in the Florida Keys include Hurricane Irene (October 1999), the “No-Name Storm” of October 2000, and Hurricane Wilma (October 2005). The amplified upper-air weather pattern that is intense enough to bring a significant cold front southward this far into the subtropics, is first able to draw northward rich moisture or disturbances from the deep Caribbean Tropics. The frequency of cold fronts increases through December, reaching a peak in late January and early February, and then decreases slowly through March. By April, sea surface temperatures once again will be on the rise across the waters surrounding the Florida peninsula, and any fronts that make it this far south are likely to be quite weak. Both tourists and cold fronts travel to the Florida Keys during the autumn and winter months. Fronts, like people, come in all different sizes, and from many different directions. Remember to check the weather before heading out on the water, and as always, be weather-ready, and stay safe!


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