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Atlantic Crossings - Knowing the Dangers That Lie Within

I came across an interesting article and though that I might share it with you all. It is really important that when you do decide to do a crossing, to take notice of the weather. Now mother earth is really an unpredictable lady. Things can go from good to bad in a matter of seconds. 

I found this article on the following website : http://yachtpals.com/atlantic-crossings-9439 

I am only posting some of it, so please follow the link for the rest of the information. It is really interesting.


It happens every year. Autumn in the Northern Hemisphere brings an end to the summer boating season across the Mediterranean assailing yachts and power boats begin their exodus across the Atlantic to their winter ports of call farther west. And of course with the change in season comes changes in the weather - it’s just a fact of life.


As with any transit, weather is one of the most if not the single most important factor in planning when making such a lengthy transit. There’s just so many weather features to consider. Everything from cold fronts, gales, and storms, to high pressure ridges… and oh yes, the tropics. One must always be mindful and have an awareness of the dangers that lie ahead. As everyone knows, knowledge is power and with this in mind we will take an in-depth look at typical weather patterns and features to consider. Going beyond that, we will also provide some helpful hints and tactics that should be considered in avoiding the adverse weather that often accompanies the weather features involved…


So without further ado, let’s have a look! 



Typical Atlantic Weather Patterns:

The weather pattern in the North Atlantic this time of year is said to be in transition. As time goes on, we’re talking less about the tropics - then again, the tropics are not completely gone from our thoughts yet. However, the weather becomes increasingly active as gales and storms become increasingly frequent, larger, and stronger over time. Further, the cold fronts that are associated with these systems are more frequent and intense, covering a greater area, and slower to weaken.


Gales and storms during mid to late fall as a general rule track from the Canadian Maritimes and Gulf of St. Lawrence eastward through the Grand Banks, and farther east across the North Atlantic to the British Isles. Associated cold fronts extend southward along and near the U.S. East Coast to the eastern coast of Florida, reaching the coast about every three or four days. These fronts then move toward the east and southeast, commonly sweeping across much of the Bahamas during November, with fronts typically not weakening until they are well offshore, beginning around 55W-60W (Figure 1).



The approach of cold fronts tends to bring increasingly active weather, particularly across more northern waters, north of 35N. As fronts near, winds become S-SW-W and increase, reaching as high as force 6-7, with SW-W seas developing and building, often reaching as high as 10ft with localized higher sets.


The passage of cold fronts brings about a shift or rapid veering of winds, with winds becoming W-NW-N. Wind speeds can approach, even reach gale force, especially near and north of 37N-38N, or closer to the main gale/storm track. Seas in turn will become W-NW-N and build, with sea heights commonly as high as 10-12 feet (localized higher sets, as high as 15 feet, often occur as well). These seas tend to cover a wide area, extending well into the Tropical Atlantic due to the very large size of the weather features that generate these seas and the very large “fetch” (or distance over which seas can propagate) involved.


On cold fronts, secondary lows become more commonplace, especially later in the period (in November). These are lows that form near the tail (or southern) ends of cold fronts, forming off the southeast coast of the U.S. Once they form, they will move in a north to northeastward motion, more or less paralleling the Mid-Atlantic U.S. coast toward New England and the Canadian Maritimes. Thereafter, these systems will turn more toward the east, as they become part of the primary (or main) gale/storm track mentioned above. Significant deepening (or intensification) of secondary systems will occur, primarily due to beneficial gulf stream influences and as “energy” aloft gets infused into these systems.


The development and deepening of secondary lows only further enhances pre and post frontal passage winds, bringing frequent and/or more prolonged periods of gale and near gale force winds near and west of 65W. Slower-moving secondary systems will often generate larger seas as well, often bringing seas as high as 12-15 feet, with larger NW-N seas again propagating well to the south, often reaching the Lesser Antilles (though of course these seas will tend to be slightly lower and will tend to be longer-period as they approach the islands).


Currents also have a bearing on seas, especially in post-frontal passage seas across the Gulfstream. In the Gulfstream, the northerly seas found in the wake of cold fronts will run up against the faster-moving north to northeastward moving current to bring northerly seas near and in excess of 10 feet, especially in the wake of stronger fronts and/or as secondary lows develop and move northeastward and deepen.


Otherwise, high pressure is dominant across much of the North Atlantic, though tends to be weaker and have less areal coverage than during spring and summer. Ridging from high pressure will generally cover the Atlantic south of 35N-40N and east of 60W, extending well into the far Eastern Atlantic. This feature is said to be “semi-permanent”, showing little overall motion and little overall change in strength over a given shorter-term time period (over a period of days).


Cold fronts and associated gales and storms advancing eastward into the Western Atlantic will induce a weakening and a southward suppression of the ridge. This results in a disruption in the trade wind regime typically found on the south side of the ridge (generally found south of 32N-33N) for several hundred miles ahead of cold fronts, as NE-E winds give way to lighter E-SE-SW winds, along with longer-period E’ly swells across this same area.

We also find large high pressure ridges moving out of south central Canada in the wake of cold fronts, which then move off the east coast of the U.S. and eventually merge with the aforementioned Semi-Permanent Ridge farther east. This results in a reinforcement and strengthening of the ridge and surges of higher NE-E trades and larger NE-E swells across much of the tropical and subtropical Atlantic (south of 32N-33N), with winds as high as force 6-7 and NE-E seas often as high as 10-12ft during these trade surges. Periods of gale force winds are often found as well though are mainly found across localized areas where localized channeling of winds occurs between islands such as the Bahamas, the Lesser Antilles, and the Canaries.

The Semi-Permanent Ridge also has a significant bearing on the weather in the Eastern Atlantic. Interaction between the ridge and a thermal trough (low pressure) across the area from Iberian southward into Northwest Africa will bring prolonged enhancement of NW-N-NE winds, mainly south of 35N and east of 20W. Winds here will approach gale force, with NW-N seas as high as 9-11 feet. These winds and seas will ease and abate respectively as weakening cold fronts approach from the west, bringing a backing of winds to more southwest to westerlies, speeds generally of no more than a force 4-5, with abating westerly seas generally no more than 5-7 feet. Larger, longer-period W-NW swells also occur at times, generally up to 8-9ft with periods of near or over 10 seconds (longer-periods farther south), mainly occurring as larger gales and storms pass well to the north.

Of course in the Bay of Biscay and near the English Channel and British Isles it’s a different story. Cold fronts often bring frequent periods of gale to even storm force winds, generally lasting as long as 2-3 days in SW-W-NW winds, with large W-NW swells, reaching as high as 13-15ft with higher sets in the Bay of Biscay (and in similarly more open waters) a common occurrence.


Tropical Cyclone Patterns During Autumn in the North Atlantic:

The Atlantic tropical cyclone season reaches its peak during the month of September and gradually declines through the end of November. Tropical cyclones are more likely to form offshore the western African coast near the Cape Verde Islands and move westward during September. Based on where these tropical cyclones form, this time of year is known as the Cape Verde Season and it lasts through approximately mid-October.

Tropical cyclones generally track westward along the southern periphery of the broad high pressure ridge over the central Atlantic Ocean. Upon reaching approximately 60W, tropical cyclones can take one of two tracks (see figure 2). The first would be to continue west or west-northwestward over the Caribbean Sea and continue towards Central America or Mexico or turn northwestward towards the Gulf of Mexico or the U.S. East Coast. This likely scenario occurs if high pressure remains over the eastern U.S. or western Atlantic Ocean and blocks the tropical cyclone from heading north.



The second track would be to the northwest and continue towards the U.S. east coast, Canadian Maritimes, or continue to turn north and northeastward while remaining over the open waters of the western Atlantic Ocean. This track occurs when the mid-Atlantic high weakens over the western Atlantic Ocean and the tropical cyclone continues along the western periphery of the high. If a cold front moves offshore the U.S. east coast, the tropical cyclone will likely turn and ride up along or ahead of the cold front to the north and northeast over the western Atlantic Ocean.

Once the tropical cyclone moves into colder waters, the system will transition into a strong gale or storm (depending on its strength during its transition). Although the system loses its tropical characteristics, these systems are still dangerous. The remnants of these tropical cyclones accelerate much faster in the northern latitudes by as much as 20 to 40 knots and gale force winds will expand several hundred nautical miles from the center of circulation.


During mid-October through November, the Cape Verde season ends as the waters of the eastern Atlantic Ocean cool and become less favorable to support tropical cyclone development. However, the waters over the western Caribbean Sea and Gulf of Mexico retain higher sea surface temperatures. Tropical cyclones will develop over the western Caribbean Sea and can track westward over Central America or eastern Mexico. However, if a cold front slowly tracks through the southeastern U.S., the tropical cyclone can be drawn northward into the Gulf of Mexico or northeastward towards Florida and the Bahamas and out over the western Atlantic Ocean. These tracks are illustrated in figure 3.


By the end of November, the sea surface temperatures cool over much of the Caribbean Sea and the tropical Atlantic Ocean. Also, the gale/storm track shifts farther south and generates winds aloft that will not support tropical cyclone development.  Therefore, the end of November marks the end of the Atlantic tropical cyclone season.

Continue on website http://yachtpals.com/atlantic-crossings-9439

By David Cannon and Amanda Delaney of Weather Routing Inc. exclusively for YachtPals.com






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