In early June, the northern hemisphere becomes sufficiently hot to bring about tropical storms and hurricanes, which move huge quantities of heat from tropical to northern waters. By September the sun has heated Atlantic Ocean to its maximum, explaining the September 10 peak in activity.
After September 10, the northern hemisphere begins to cool and we would expect a continued decrease in tropical activity up to November 30, when the lack of heat makes the possibility of hurricanes remote. This decrease begins as expected, going down from 90 storms per 100 years on September 10 to 40 storms per 100 years by October 1. But then from October 1st to the 20th, storms increase, going back up to 50 in that time over a 100-year average. Then after October 20th, a steady decrease re-commences until November 30th when the number of storms flatlines to near zero.
You might well ask why is there such an increase in storm activity in early October when all indicators tell us the Atlantic region is cooling? The answer lies with the increase in frequency and strength of mid-latitude, low-pressure systems. These systems are triggered by the intrusion of cold air from Canada, replacing the retreating warm air. As low-pressure systems sweep across the US, their cold fronts move south and in doing so displace and push upward the remaining warm air.
Why does this cause an upsurge in tropical activity? Because when warm moist tropical air is pushed aloft by powerful fall cold fronts, the warm air cools, condensing moisture into clouds which release heat encouraging air to rise further. This rising effect causes a lowering of pressure on the earths surface, often just sufficient to bring about the pressure differential (gradient) needed to trigger formation of tropical storms or hurricanes.
Only between October 1 and 20 do the two necessary ingredients of residual tropical heat and intruding cold air exist to spark the formation of tropical weather. Prior to October 1 there is only heat and after October 20 there is only cold. But in the transition period there exists both, which allows resurgence in tropical storms and hurricanes.
How do we use this information? If you are heading south in the fall, lets say from New England to the Caribbean, it is best to cast off after October 20, while still being wary of tropical systems since they can occur up until November 30. After November 30 though, severe low-pressure systems bring untenable weather for heading south in the Atlantic, particularly for crossing the Gulf Stream.
Lets say youre standing to start your trip from Newport, RI or Norfolk, VA. You should be ready to expedite your departure and cross the Gulf Stream right after October 20th. Prudent mariners in this position will always be looking over their shoulders to the west and using the 500-mb chart to track formation and development of low-pressure systems.
An ideal departure scenario would occur after the passing of an upper air trough, which occurs near the time of a cold front passage on the surface. Additionally, there would be no tropical disturbances or depressions south of your proposed track. It sounds a little too obvious, but a good rule of thumb is never head south when there is a developed tropical system (disturbance, depression, storm, or hurricane) to your south.
To assist with making departure decisions the Tropical Prediction Center, which houses the National Hurricane Center, produces a multitude of tropical weather products. These can all be found at the National Hurricane Centers website, www.nhc.noaa.gov. A new product introduced this year is the Tropical Cyclone Avoidance Area graphic that shows out to 72 hours the combined forecast track area and radius of gale-force winds (Beaufort Force 8, 34 to 40 knots). Sailors can use this graphic to precisely plan which ocean areas to avoid. If you allow yourself to enter, or even closely approach, a tropical systems avoidance area you will quickly experience significant 12-foot and greater seas as well as sustained winds of 34 knots with higher gusts, up to 50 percent more than the sustained wind speed. That should be reason enough to keep your vessel out of these areas.
A detailed explanation of hurricanes, including links to products and guidance for hurricane evasion in the North Atlantic Ocean is available in the Tropical Prediction Centers (TPC) publication "Mariners Guide For Hurricane Awareness in The North Atlantic Basin." This booklet can be downloaded in Adobe Acrobat Reader directly from the TPC homepage: www.nhc.noaa.gov/marinersguide.pdf.
Additionally, an Atlantic hurricane-tracking chart is also available for downloading. Its specific URL is: www.nhc.noaa.gov/AT_Track_chart.pdf.
Remember the four critical hurricane dates; June 1, September 10, October 20, and November 30, and what each date historically represents. These date markers will assist you in navigating around and avoiding tropical storms and hurricanes.
The Science of Hurricanes by Michael Carr
Timing Caribbean Arrival by Beth Leonard
An Encounter with Hurricane Mitch by John Kretschmer
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