Every year a certain percentage of tropical hurricanes work their way into higher latitudes. In the process, these storm systems change their fundamental structure. They frequently end up covering huge areas of ocean, much larger than their tropical antecedents, moving at speeds of as much as 1000 miles per day in the process. Two recent examples of this process on the Eastern Seaboard of the U.S. are Danielle and Mitch.
Because of their size and speed of movement , systems which have the potential to make this transition bear close watch by all passagemakers. Just because a tropical storm system is a thousand or more miles away does not necessarily mean it won’t have an impact on your future.
They key to analyzing the risk factors inherent when one of these systems is lurking is understanding the mechanics of the transition process, and then watching both surface and upper level wind charts for telltale signs.
Weather systems depend on differences in temperature and humidity for their energy. The greater the differential, the more energy available for surface level winds. Within a tropical storm an area of low pressure develops and surface winds rush in to fill the partial vacuum. Warm, moist air rises and releases energy into the cooler, drier upper atmosphere. If there is no venting mechanism at the top of the system, the central area of low pressure fills and the winds drop off. On the other hand, if a venting mechanism is provided in the upper altitudes (by a weak high pressure dome), and the storm center vents faster than the low level winds can fill the depression, the system will continue to grow.
A key ingredient necessary for this process to start, and then continue is a minimum of wind speed differential between the surface and the upper altitudes. If this wind “shear” is too strong, the venting process is disrupted.
Higher latitude depressions, referred to as extra-tropicals have a different structure. They typically start with upper level troughs (usually called short-wave troughs). These upper altitude short-wave troughs go through a life cycle where they eventually create an area of low pressure on the surface. The downwind side of the upper level trough (typically the eastern side) is made up of a region of faster moving winds which vent the atmosphere up from the surface. In this case, there is a considerable amount of wind shear from the surface to the upper level trough.
Certain atmospheric conditions are necessary for a successful tropical to extra tropical storm structure transition. The first stage is for the circulation of the tropical storm center to become embedded in the flow around the subtropical high pressure as the storm edges its way out of the tropics.
In order for the storm to continue it must maintain the ability to vent at higher altitudes. Initially, the venting process is typically disrupted as the storm moves out of the tropics. In fact, it is not uncommon to see a series of surface fax charts which indicate that a tropical storm is winding down and dissipating, even while the potential for the extra tropical transition exists.
A new way for the storm system to vent has to be provided by an upper level low pressure trough. The tropical storm is looking for a divergence (an area where it can spread out) aloft which the surface low pressure can rise into and deepen.
During this transition process you have a combination of warm moist tropical or semi-tropical air combining with cooler, drier air from the higher latitudes: a potentially explosive mixture. In addition, the transformed weather system is now hooked onto a high speed upper level wind jet, one which moves at multiples of the surface speed of the tropical storm system. The increased energy from the combined atmospheres, and strong venting provided by the upper level trough create the potential for a rapidly expanding storm system.
Even when conditions seem prime for a transition they don’t always occur. Still, these systems can develop so rapidly that it behooves the cautious mariner to keep a wary eye peeled. The best warning signs are found on the 500mllibar charts.
The favorite place for the tropical storm to hook up to the subtropical high pressure flow is ahead of an upper altitude short wave trough. When you are watching a tropical storm and keeping an eye on the 500 millibar charts, you will want to watch these short wave features closely. There is a pattern to the 500 millibar troughs, so it is best to begin to watch how they are working several weeks before departure. Even better, practice interpreting these charts by downloading them from the internet (http://weather.noaa.gov/fax/marine.shtml).
Because of the potential for rapid movement, the wind speed differential around these storms can be significantly greater than with other tropical systems. If you have a surface storm movement of ten knots, and a central wind speed of 100 knots, you will have a maximum of 110 knots on the dangerous side of the storm center and a minimum of 90 knots on the navigable side. If the storm system is moving at 50 knots, with circulation winds of 60 knots, the dangerous side of the storm will be at 110 knots while moderate breezes will prevail in the navigable side.
This large disparity in surface wind field maxima means that a much greater (and earlier) effort must be made to avoid the dangerous quadrant of the storm.
If you are voyaging at the time of year where these risks exist, you should shoot for a three to five day weather window in which to complete your voyage.
Seasonal issues vary with locale. On a historical basis, the East Coast of the U.S. is typically at greatest risk for tropical to extratropical transition during late summer and early fall-August through October. In the Western Pacific the range is much greater-typically August through December.
(excerpted from the May 1999 issue of Bluewater Sailing magazine)