Written by Amanda Delaney
There is nothing worse than hearing the news that a tropical storm or hurricane could potentially threaten your location. Not only does it ruin your plans but it forces you to make critical decisions such as, can I get the vessel out of harm’s way in time and, if I can’t, where can I safely dock and wait out this system? Certain areas of the Caribbean Sea are more prone to tropical cyclone impacts than others. Having the general knowledge of how a tropical cyclone develops and where these systems impact areas across the Atlantic Ocean can help you make the appropriate preparation and plans ahead of the tropical cyclone season.
How does a Tropical Cyclone Evolve?
The majority of tropical cyclones originate from a tropical wave. Tropical waves, or also known as easterly waves, originate offshore the western African coast near Senegal where dry Saharan air meets the moist air over the Atlantic waters. The surface winds converge to the east of the tropical wave and generate a line of thunderstorms that can be viewed on satellite imagery. Most tropical waves will not produce a tropical cyclone for several reasons. The first is that the sea surface temperatures across the tropical Atlantic Ocean are not warm enough to sustain a tropical cyclone. Sea surface temperatures generally need to be at a minimum of 26.5ºC (or 80ºF). The other reason is that there are areas across the tropical Atlantic Ocean where winds aloft are too strong to sustain thunderstorms near the tropical wave. These winds will rip the thunderstorms away from the tropical wave and this disrupts any further organization along the tropical wave.
If the sea surface temperatures reach or exceed the 26.5ºC threshold and the winds aloft across the tropical Atlantic Ocean subside or become calm, then the thunderstorms associated with the tropical wave will continue to become more numerous and persist. This becomes known as a tropical disturbance where numerous thunderstorms develop but are not yet organized near the tropical wave and the surface winds have not begun to turn cyclonically. Once the surface winds begin to turn cyclonically and form a closed surface circulation, or low, then a tropical depression has formed. A tropical depression generally produces maximum sustained winds of less than 34 knots.
A tropical depression will evolve into a tropical storm when the thunderstorms around the center of circulation begin to develop a more cyclonic appearance on satellite imagery and the sustained winds near the center increase above 34 knots. Once the tropical depression is classified as a tropical storm it will be officially named by the National Hurricane Center.
A tropical storm can strengthen further in the right environment where the outer bands will further tighten near the center of circulation, the barometric pressure continues to fall and an “eye” develops at the low center. Once the sustained winds have reached 64 knots, the tropical storm is classified as a hurricane. Hurricane winds can range from 64 knots to greater than 135 knots.
The Saffir Simpson scale categorizes the strength of Atlantic Ocean hurricanes as follows:
Category 1: 64-82 knots
Category 2: 83-95 knots
Category 3: 96-113 knots
Category 4: 114-135 knots
Category 5: 135 knots or greater
Once a hurricane strengthens to a category 3 system, it is recognized as a major hurricane.
A hurricane can meet its demise in several ways. The first is if the system moves over a large landmass. The system will quickly weaken inland once it is cut off from its warm water energy source. The second is if the hurricane moves over cooler waters. The hurricane is unable to generate as many thunderstorms near the center of circulation and the system begins to transition from a tropical cyclone to an extratropical cyclone. In this case the tropical cyclone begins to look more like a large gale or storm that you would find in the mid-latitudes. Usually these tropical cyclones weaken once encountering stronger winds aloft again that will push the thunderstorms away from the center of circulation. Generally these extratropical cyclones merge with a cold front or become a gale or storm, depending upon the strength of the winds once the transition is complete.
Where do tropical cyclones form and where do these systems generally track?
Tropical cyclones generally originate in one of two ways: either along the tail end of a stalled cold front or, as we just discussed, from a tropical wave. During the first scenario, an area of low pressure can develop along a stalled cold front, typically over the northwestern Caribbean Sea, north of the Bahamas or along the U.S. Gulf or East Coast. These lows generally strengthen quickly over warm ocean waters and become their own separate identities from the cold front. When tropical cyclones form in the western Caribbean waters, these systems can either track northeastward over the Bahamas to the open Atlantic waters. However, a tropical cyclone can also turn west or northwestward and move inland over Central America or over the Gulf of Mexico and impact either the southern U.S. or Mexico. The tropical cyclones that develop along cold fronts can occur anytime of the year but it is more common to occur in the western Caribbean Sea during June through July and during October through November.
As previously mentioned, tropical waves originate offshore the western coast of Africa and will track westward along approximately 06N to 20N. These waves can move anywhere between 5 knots to 20 knots and will move through the eastern Caribbean Sea every 3-4 days from May through November. A tropical wave can be tracked on a surface map and is represented by an “inverted” trough, or a line that appears like an archer’s bow pointing west. Tropical cyclone development generally occurs along these tropical waves during late June through early October when the sea surface temperatures across the tropical Atlantic Ocean are optimal and winds aloft are generally calm across this region.
Once a tropical cyclone is spawned by a tropical wave, these systems generally track west or west northwestward from western Africa to near approximately 50W-55W, transiting along the southern periphery of a strong high centered near the Azores. From there the tropical cyclone’s track can diverge and is heavily influenced by the weather features to the north. If a high is centered near Bermuda, then the tropical cyclone can continue on a more westward track that will generally track through the eastern Caribbean Sea or impact the northern islands, such as the Leeward Islands, the Virgin Islands and Puerto Rico. If the high to the north is far enough west, the tropical cyclone will continue westward until impacting Central America or turning into the Gulf of Mexico and impact the southern U.S. or Mexico.
If the high near Bermuda weakens and a cold front tracks offshore the eastern U.S., then the tropical cyclone can turn more northwestward out ahead of the front. Depending on the position of the front, a tropical cyclone can track through the Bahamas and impact the eastern U.S. coast. Other times the tropical cyclone can recurve completely toward the northwest to north and eventually northeastward, therefore remaining out in the open Atlantic waters.
As we progress into later October and November, the tropical waves that move offshore the western African coast begin to weaken due to the sea surface temperatures becoming cooler in the eastern Atlantic Ocean. Any tropical cyclones that develop are more likely to track more northwestward and eventually northward ahead of cold fronts that are farther south during this time of year.
What to Look for Regarding Tropical Cyclones and Evasive Action to be Taken
So what conditions should you look for when a tropical wave approaches your location? There are several signs to observe prior to the arrival of the tropical wave:
- – The barometric pressure will drop 2-4 millibars.
- – Northeast winds will rapidly increase 24 hours prior to its arrival.
- – Stronger tropical waves will produce showers and thunderstorms. The stronger thunderstorms will produce locally enhanced winds up to gale force and building seas.
Once the tropical wave has passed, the winds will shift and ease out of the southeast and the barometric pressure will gradually rise.
A decision should be made 2-3 days prior to a tropical cyclone impacting a region on whether it is safe to escape well ahead of the system or find a safe port that will provide protection from the cyclone. When a tropical storm or hurricane watch is issued for an area, tropical storm or hurricane force are possible region within 48 hours. When a tropical storm or hurricane warning is issued then tropical storm or hurricane force winds are more likely to occur, impacting that region within 36 hours.
If you decide to seek shelter in a port, make sure you make all of the necessary preparations of tying down the vessel, securing everything inside and sealing any hatches. Here are the conditions that you will encounter while sheltering through a hurricane:
- – Swells will gradually build 2-4 days before the arrival of the hurricane.
- – The barometric pressure will slowly drop 2-3 days prior to the hurricane’s arrival. A rapid drop will occur within 24 hours before landfall.
- – The winds will gradually increase.
- – Clouds will increase and the outer bands of the hurricane will produce scattered showers and thunderstorms within 24 hours prior to landfall. The thunderstorms associated with the outer bands need to be monitored closely for the potential of any waterspouts or tornadoes associated with them.
- – If the eyewall reaches your area, expect the strongest winds and numerous thunderstorms to occur at this time. The barometric pressure will continue to decline.
If the eye (or center) of the hurricane tracks over the area expect:
- – The winds to ease for approximately a few minutes to an hour.
- – Thunderstorms to diminish.
- – The barometric pressure to bottom out.
- – The skies may clear briefly but this will only last a few minutes to an hour at most.
Once the eye of the hurricane moves through, the region will be impacted by the eyewall again which will produce numerous thunderstorms and winds coming from the opposite direction. The barometric pressure will steadily increase while winds and thunderstorms diminish and swells slowly (usually after 1-2 days) abate as the tropical cyclone moves away from the area.
During the Atlantic tropical season, always stay up to date with the latest tropical activities and consult a professional meteorologist in case you need to alter your plans or take evasive action due to a tropical cyclone threat. If you are going to be transiting in a tropical cyclone prone area, always have a hurricane port in mind that you can shelter in case a hurricane was to threaten and you are unable to get safely away from the system in time. Keeping these plans in mind will allow you to avoid a potentially dangerous situation during the tropical season.
Amanda Delaney is a Senior Meteorologist at Weather Routing Inc., which provides routing/forecast assistance and Meteorological Consultation for yacht/cargo ships, in business since 1961.