The numbers aren’t all in yet, but we already know that one year does not necessarily make a trend (published Nov/Dec 2017)
For many, the hurricane results of 2017 have been horrific. That is undeniable and a statement which should add caution to our actions in the future. Frantic TV crews may have been selective in the sites they chose from which to feature their wind-swept, rain-soaked shrieking commentators, but the fact remains that much of the Caribbean, Florida and parts of Texas, Louisiana, Georgia and other states were decimated by Hurricanes Harvey, Irma, Jose and Maria and their aftermath. The pain suffered there was endured by many of our friends – yours and mine. In earlier years, similar pain has been encountered in New Orleans, New York, New Jersey and elsewhere, but somehow this year seemed different. Is it a trend or something to be tossed off as a one off? How do these things work, and how should we prepare and react going forward? It hasn’t been a joke this year, and it won’t be next year either.
Whether onshore or offshore, hurricanes are not to be taken lightly. It’s true enough that our weather forecasting techniques have improved significantly since some of the record-keeping started in 1850. Communications now enable us to spread the word far and wide. People are more aware of the importance of taking early precautions, especially following Hurricanes Katrina and Sandy, and those precautions may have played a significant role in saving lives this past September. Is the noticeable rise in hurricane activity a trend? A quick look at the number of named storms in the Atlantic gives us a quick insight into the numbers, even if it is inconclusive about any kind of slowly rising possible “trend”. It’s not inconceivable that those numbers in the 1800’s may have missed a few mid-ocean storms. In any case, it doesn’t currently look like 2017 will replicate 2005’s 28 named storms, 15 hurricanes, or 8 major hurricanes, although as I write this article (September 24th), we already have logged 12 named storms, many of which developed into hurricanes, and the year is far from over.
Getting bogged down in those kinds of numbers doesn’t quite do the weather systems justice however. Putting hurricane realities into perspective, it should be mentioned that the weakest level of hurricane has 74-95 mph winds. The strongest is in excess of 157 mph, and when Hurricane Irma hit Key West, it was reportedly as a Category 3 storm (out of 5 categories). Putting these things in practical terms, in 95 mph winds the wind strength prevents people from walking upright. Who would want to? With lawn furniture and BBQ grills flying through the air on shore, the upright person would present a larger target for a multitude of lethal missiles unleashed by the wind! Offshore, a person trying to walk upright could easily be blown overboard. Crawling becomes an option. It’s not a great one! A stunning fact, however, is that in many places, it’s the water that accompanies a hurricane which creates the most devastation. If “knowledge is power”, knowing a bit about hurricanes can help us avoid, or at least prepare for, the worst of it.
A hurricane is born
Weather is a complex operation with a wide range of variables. Tropical low pressure systems, as an example are quite different from extra-tropical lows–lows that are predominant in areas outside of the tropics. Hurricanes often start out as tropical waves, becoming tropical depressions, lows, and storms if conditions are suitable for development. A variety of criteria must be met in order to progress from tropical storm to hurricane strength.
During the initial stages of a tropical low pressure system, there will be a disturbed area with clusters of thunderstorms. In the Atlantic, as an example, the clusters could be along a tropical wave that originates in northwest Africa or along an old cold front that can occur anywhere from Cape Hatteras, North Carolina to the Bahamas into the Gulf of Mexico. As the tropical wave develops into a tropical depression, the thunderstorms organize, and an area of surface low pressure forms. The thunderstorms converge towards the center of lower pressure. In the Northern Hemisphere, the thunderstorms spiral in a counter-clockwise direction. In the Southern Hemisphere, the thunderstorms spiral in a clockwise direction. When sustained winds are less than 35 mph, the tropical low is classified as a tropical depression.
In order for the tropical depression to continue to develop and become a tropical storm, the inflow of thunderstorms and warm humid air must increase. The pressure continues to fall, and sustained wind speeds must exceed 35 mph. At that point, in the North Atlantic, a tropical storm is named. Although naming protocols vary in other parts of the world, in the North Atlantic and both the Eastern and Central North Pacific, each storm is named alphabetically beginning with “A” for the first storm of the new calendar year. In the Western North Pacific, the alphabet is not restarted at the beginning of the new year. Rather, the alphabetical process proceeds through “Z” and restarts at “A” without regard to calendar years. Additionally, some parts of the world refer to these types of storms as “tropical cyclones.”
With clouds and warm, humid air inflows continuing to build, the center barometric pressure continues to drop. An “eye” is likely to form at the center of the low pressure system. In order to continue storm development into hurricane strength, sea temperatures need to be at least 81 degrees Fahrenheit. Tropical systems become classified as hurricanes when sustained winds reach 74 mph. Warm water provides the energy that fuels the convective system. The warmer the water, the more energy is available for hurricane strength to develop. The moist air and clouds in the vicinity of the system allow for efficient transmission of that energy. And the lack of wind shear aloft, allow the convective system to develop fully, reaching higher altitudes and colder temperatures aloft.
Because there are various elements that go into building the strength of a tropical depression, storm or hurricane, if any of these critical elements are removed, the systems can lose some or all of their strength. As a hurricane goes over land, as an example, it loses its primary source of energy–the warm ocean. Warm air is often replaced by cooler air at the system’s lower altitudes. Additionally, the increased friction over the surface of land slows the inflow of energy into the convective system. Dry air is introduced, instead of the more moist air of the ocean, so fewer clouds are created. And strong wind shear aloft from wind currents such as the jet stream will tear the vertical structure of the storm apart.
Although tropical storms have been recorded in the North Atlantic in every month of the year, and hurricanes have only been absent during the month of April, the primary months for tropicals are during the late summer. That is when the water is the warmest and wind shear at its minimum.
Where hurricanes tend to form in the North Atlantic and how they tend to track, however, vary somewhat with the month. These are general trends, however, and not at all forecast routes. Each tropical will take on its own characteristics of development and track.
Hurricane tracks in the North Atlantic: June
Considered early in the North Atlantic hurricane season, June tropical storms and hurricanes frequently form in the Caribbean Sea or Gulf of Mexico. Their tracks generally take them into the Gulf States or across Florida and up the North American East Coast as they recurve towards Europe.
Hurricane tracks in the North Atlantic: July
Still somewhat early for the North Atlantic hurricane season, July tropical storms more frequently begin their formation a bit further to the east than the June storms. Tracks often take the tropicals into the Gulf States or northerly along the North American East Coast before they recurve toward Europe.
Hurricane tracks in the North Atlantic: August
With the peak of hurricane season approaching and the water temperatures of the tropical North Atlantic continuing to rise, August tropical storms and hurricanes form further to the east. They can form as far east as 40 degrees W. latitude or more. With the eastern formation, the hurricanes have more time to recurve back toward Europe without making landfall in North America.
Hurricane tracks in the North Atlantic: September
September is the peak month for hurricane formation in the North Atlantic. Water temperatures are among the warmest of the year. There is relatively little wind shear present compared to other times of year, and clouds are plentiful. The hurricanes often begin as tropical waves undulating their way off the northwestern coast of Africa. Formation can take place as far east as 30 degrees W. latitude, giving the tropicals a great deal of distance to develop and/or begin a recurve toward Europe prior to reaching the Islands of the Caribbean or North American continent.
Hurricane tracks in the North Atlantic: October
As temperatures begin to cool the water in the tropical North Atlantic, hurricanes and tropical storms tend to again form somewhat further to the west or in the southern parts of the Caribbean. This is not meant to imply that they are less dangerous or potentially less destructive. Merely it is meant to say that climatologically, their main areas of formation and frequency are changing.
Hurricane tracks in the North Atlantic: November
Early November is often considered the “end” of hurricane season. Offshore, vessels and captains departing the North Atlantic bound for the Caribbean often feel that the danger of tropicals or hurricanes is past. It isn’t. Remember that the only month in recorded history that doesn’t have a hurricane in the North Atlantic is April – and that month has had a tropical storm recorded as recently as 2003! With North Atlantic water temperatures continuing to drop in November, storm formation occurs more often in the Caribbean. Late season hurricanes tend to have somewhat erratic paths. Or, perhaps, another way of looking at it is that, with relatively little data from past late season tropical storms, their paths tend to be somewhat more difficult to predict. The hurricane that formed near Jamaica and went “backwards” into the trade winds almost 20 years ago, going over the British Virgin Islands, St.Maarten’s, and Antigua from the west towards the east, was a late season hurricane. Their tracks are difficult to predict and planning should take that into account.
Tracks are not the only aspect to a hurricane that is difficult to predict. Storm surges–the water that is pushed in front of the storm–are also difficult. The storm surge during the hurricane that hit Galveston in 1900 was 15 ½ feet high. The storm surge that hit New England in 1938 was quite a bit higher. They can reach 15 to 20 feet or more. And, if the tide is high at the time, the additional height can overwhelm bridges, docks and other coastal structures, including houses, a fact that was not lost on western Florida this year. Following the storm surge, which may only last 6 hours more or less, depending on the speed and timing of the system, rains will penetrate far inland, causing severe flooding and adding further reaching damage. Like the wind blowing BBQ appliances or shutters through the air, the storm surge and subsequent flooding can push boats, vehicles and other larger objects into bridge abutments or other structures. Planning needs to include the removal of all objects that can be carried by the severe winds, and anything portable should be moved to high ground. Of course, storm procedures are far more extensive than that. Vessels caught offshore need to take evasive action, getting out of the leading right-hand quadrant, the most destructive sector of the approaching hurricane. All loose gear and anything that provides unnecessary windage must be taken off the deck. All gear below must be stowed properly, keeping in mind that the vessel may be rolled upside down in extreme seas. Batteries should be fully charged. Crew must be fed and rested, and food, tools and emergency equipment must be both secure and immediately at hand. Knowing in advance when these preparations are required and when they aren’t provides a significant ability to manage both risk and costs resulting from potential damage.
Weather forecasting became significantly more accurate in the mid- to late-1990’s with the increased availability of remotely sensed weather data combined with increasingly accurate weather forecast models. Satellites now circle the globe, providing relatively accurate wind speeds and directions and other observations in near-real-time. Utilizing a number of models and a wide variety of data resources, the NWS’ National Hurricane Center provides a good baseline of current hurricane information on its site.
Weather forecast models have become increasingly accurate. However, having said that, they are far from perfect, hence we have disagreement between the various models. Still the best sources of information are provided by a “man-machine-mix”, combining the far sighted analytical potential of super computers with the critical, subjective perspective of trained and experienced meteorologists. There is a wide range of computer-generated weather models from which to choose, and comparing off-the-shelf models with each other, one can quickly see vast discrepancies. Far better accuracy is generally achieved in the near term when a trained meteorologist “adjusts” the modeling results tempered by experience. The NWS Ocean Prediction Center provides man-machine analyses and forecasts for the North Atlantic and North Pacific. Alternatively, private weather forecasting services such as Commanders’ Weather provide high quality, site specific forecasts anywhere in the world.
But what really made 2017 different from other years, at least for the Caribbean and North America? The past few years have had strikes but nothing like 2017. Discussing this situation with George Caras, one of the principals of Commanders’ Weather in Nashua, NH, he mentioned that this year seems to be in a somewhat neutral weather pattern, favoring neither a La Nina nor an El Nino pattern. “The upper level winds are generally calm, providing ripe conditions for extreme storm formation,” mentioned Caras. Earlier years have also had warm enough water (over 81 degrees Fahrenheit), but “the calm upper level winds allow warm, moist air to be entrained into the system at higher levels,” said Caras.
You should be aware that GRIB files tend to smooth the data, severely under-estimating wind speeds. I’ve seen GRIB files indicate maximum wind speeds in a particular hurricane estimated to be 45 knots! By definition, I knew that winds would greatly exceed that figure. Meanwhile, buoy reports indicated 120 knot sustained wind speeds. GRIB files will NOT provide accurate wind speeds for an approaching tropical storm.
On the “positive” side of the explanation, hurricanes are relatively small weather systems. The potentially destructive areas of a hurricane seldom measure more than 400 miles across. Given enough warning and reasonable mobility, vessels offshore can often sail around them – if they go the right way, and the predicted storm track is accurate. While offshore, I’ve seen them form overhead. I’ve watched them pass, several hundred miles away. And I’ve had one pass directly over the top of me – fortunately, from the cozy confines of home. Do yourself a favor: remain aware of the weather and avoid hurricanes entirely. They’re bad news when you’re caught inside their grip, and there’s no way to know how next year will develop.
Recommended hurricane websites
Tropical Cyclone Climatology
Hurricanes and tropical storms throughout the year
National Hurricane Center, provides a good baseline of current hurricane information
Intellicast, provides expert weather analysis utilizing a variety of computer generated models
Ocean Prediction Center, provides man/machine analyses and forecasts
Commanders Weather, provides specialized weather support for sailing and cruising yachts worldwide