Florida Keys Marine Weather Regions:Hawk Channel

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Florida Keys Marine Weather Regions

Part 3

This article is the third 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 Hawk Channel.  Hawk Channel is a widely used waterway, nestled between the Florida Keys and the Florida Reef tract, from Ocean Reef all the way out to about Halfmoon Shoal.  Underwater terrain is diverse, ranging from intertidal flats, to patch reefs, to sandy bottoms, to barrier reef.  Interspersed within these regimes are numerous, primarily north-south channels which connect the Gulf of Mexico and Florida Bay waters with those south and east of the Florida Keys.  As a result, tidal currents can change rapidly from one location to the next.  Water depths range from a few inches to about 60 feet.  Nearly all of Hawk Channel lies within the Florida Keys National Marine Sanctuary, a fact which underscores the marine biodiversity of this waterway.  In addition, a large number of wrecks populate the sea floor of Hawk Channel.  Millions of people will visit Hawk Channel each year to boat, fish, swim, dive, and sight-see, many of whom will carry lifetime memories of their visit back to their homes up north, or abroad.  Hawk Channel also is the daily workspace for thousands of Florida Keys-based captains, guides, and mates from boating, fishing, diving, natural resource management, law enforcement, and other industries.  According to the Florida Fish and Wildlife Conservation Commission, most boating accidents in the Florida Keys have occurred in Hawk Channel and other areas relatively nearshore, rather than more remote areas offshore.  Indeed, marine weather in the zones comprising Hawk Channel may be of greater relevance to more people on a daily basis than all of the other zones around the Florida Keys. Wind, waves, and weather are important to all mariners.  However, wave behavior in Hawk Channel is highly variable, and strongly dependent upon wind direction.  A world of difference exists between “north at 20 knots” and “south at 20 knots” in most of Hawk Channel, and this is because of the typical corresponding sea states associated with those two wind velocities and the effect those sea states have upon boat handling and passenger and crew comfort.  Wave growth due to wind is influenced by the actual wind speed, as well as both the fetch and duration of the wind over open water.  The “fetch” is the length of open water over which the wind is blowing.  Greater fetches lead to higher waves in time.  This is one of the reasons why “south at 20 knots” across Hawk Channel leads to higher seas than “north at 20 knots”.  With a north wind, the land and vegetation of the Florida Keys will limit the “fetch” of open water across which that north wind blows, limiting the growth of waves.  However, a south or southeast wind blowing across a vast expanse of the Straits of Florida, Santaren Channel, or the Old Bahama Channel can lead to much higher seas.  Likewise, a southwest or west wind blowing across the Gulf of Mexico may build higher seas. When strong to near gale breezes blow parallel to Hawk Channel (e.g., from the east-northeast around 25 or 30 knots) for a couple of days (thereby maximizing both fetch and duration), then significant wave heights actually can reach 6–7 feet inside the reef!  However, most of the time significant wave heights generally will be in the range of 1–2 feet, and occasionally, during light wind regimes, the sea can be flat calm in Hawk Channel.  The highest seas no doubt will occur infrequently, during hurricanes, when hopefully, no one is there to witness them. Weather stations are scarce across the waters adjoining the Florida Keys.  Of the few stations which do operate, three can be found on Florida Reef lighthouses, along the southern border of Hawk Channel.  These stations are located at Molasses Reef Light, Sombrero Key Light, and Sand Key Light, and the corresponding wind observations can be found online at: http://www.srh.noaa.gov/?n=obs Next month, we will explore the marine weather across the Straits of Florida.  Until then, remember to be weather-ready, and stay safe! wc-may2014


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