A Three Hour Gale
If you happened to be out on Florida Keys coastal waters during the late afternoon of January 11th of this year, you probably observed an abrupt rise in both wind and sea, with a crescendo into a bona fide gale. A “gale” is a sustained breeze of at least 34 knots, or Force 8 on the Beaufort Scale. The wind on this January afternoon and evening rose to a steady 30 to 40 knots for over three hours, with gusts peaking near 50 knots. This is on par with a weak tropical storm! Wind gusts carried sufficient force to topple a few trees and cause many a vessel to drag anchor within nearshore waters. Some of the bay-side canals in the upper Keys even were reported to empty as the southerly blow evacuated volumes of water, leaving vessels aground which were afloat just hours earlier. The men and women of the United States Coast Guard responded with speed and diligence to at least nine distress calls that day, with personnel from commercial salvage companies providing able assistance as well. The intriguing aspects about this particular blow were its short duration and its occurrence in otherwise fair weather. A rather nondescript line of showers with a few thunderstorms passed through the Florida Keys hours earlier and well ahead of a cold front. In fact, it was not raining anywhere in the Keys during the blow which, unusually, came from the south.
The cause of the rapid rise, short duration, and unusual direction of the wind in this case is what meteorologists call a “wake low.” A wake low refers to a small-scale low pressure system which forms behind departing squall lines. The term “small-scale” is important here because scale refers to both the spatial and temporal domains, and a “small-scale” weather system covers a smaller area and a shorter time period than a “large-scale” weather system. Compare and contrast a summer shower (very small scale) with the Bermuda high pressure system (large scale). You likely have noticed, living in Florida for any length of time, that a summer rain shower may be extremely localized, even to the extent of wetting the pavement on the street in front of your house while leaving your neighbor’s driveway bone dry a block or two away. Such a shower may last only a minute or two. Contrast the “small-scale” weather system we call a summer shower with the sprawling high pressure system which centers near the island of Bermuda in the western North Atlantic Ocean, extending both south and west for thousands of miles. The Bermuda highs are semi-permanent features of the weather map during certain times of the year, and they can drive very persistent weather regimes. A strong Bermuda high setting up shop may result in moderate east breezes in the Florida Keys which last for weeks. Time and space often are coupled in the atmosphere. Big things happen slowly; smaller things happen faster.
It turns out that in the realm of weather prediction, greater skill is associated with large-scale systems like the Bermuda high. Considerably less skill is demonstrated in the prediction of smaller features like individual summer showers, local shifts and surges in wind, and so on.
Wake lows are rare in the Keys, occurring roughly a few times per decade. The famous University of Chicago tornado researcher, Dr. Ted Fujita first described wake lows in a paper written during the 1950s. Wake lows apparently arise from a complex interplay among a unique juxtaposition of squall line dynamics, precipitation physics, and surface airflow response. They, subsequently, are nearly impossible to predict, owing to their small scale and complexity. Such events underscore the importance of routine weather checks. Florida Keys National Weather Service meteorologists directly program the output heard on the marine VHF “weather bands” (channel 2 in the lower/middle Keys; channel 5 in the middle/upper Keys; channel 4 near Ocean Reef). We call this “NOAA All Hazards Weather Radio,” and it includes the latest local marine weather information. Be sure to tune in, and remember to be weather-ready, and stay safe!
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