Hurricanes have been the cause of many maritime disasters and unfortunately, there is no single rule of thumb that can be used by mariners to ensure safe separation from a hurricane at sea.
Instead, constant monitoring of hurricane potential & continual risk analysis when used with some fundamental guidelines become the basic tools to minimize a hurricane's impact to vessels at sea or in port. Today, even as our understanding of hurricanes increases, there is still much error inherent in forecasting the movement & intensity of these systems. Through the use of a recurring risk analysis, mariners can minimize potential impacts of a hurricane encounter. Coincidental with the fact that NHC issues 4 Tropical Cyclone Forecast/Advisory Messages (TCM) per day when a system is active, the risk analysis needs to be done in conjunction with each new TCM to ensure that the sailor is evaluating the latest information to make navigation decisions.
This risk analysis includes a number of extremely important factors needed to make sound decisions & ultimately remain clear of hurricanes either at sea or in-port.
History of Regional Hurricane Tracks
There are climatologically favored regions/tracks for hurricane development/movement in the North Atlantic. Both are important to vessels at sea or in port in order to begin assessing risks involved during the hurricane season. Knowledge of hurricane climatology is the first significant aids in helping mariners avoid these systems in the North Atlantic.
Impact of Ocean Currents & Warm Water
Certain areas in the basin support rapid intensification of hurricanes. Understanding the contribution that warm water plays in the growth of a hurricane, it is easy to appreciate that ocean regions with high sea-surface temperatures (greater than 79° F or 26° C) are often dangerous locations for mariners to be found as a hurricane threatens. Knowing local sea-surface temperatures (SST) & location of ocean currents are also important factors for the mariner. The two most prominent areas to possess this danger are the Gulf of Mexico & the Gulf Stream.
A second impact that the Gulf Stream places on vessels is enhanced sea states resulting from the interaction of ocean current with hurricane wind field. Winds of tropical storm or hurricane force opposing any ocean current can quickly create very steep, short period waves making navigation through these areas extremely dangerous and difficult.
Knowledge of Inherent Hurricane Track/Intensity Errors
Generally speaking, the smallest errors associated with hurricane track forecasts occur while a system is moving in a general west to west-northwest track, south of the Atlantic subtropical ridge. Conversely, the largest errors involved in hurricane forecast tracks occur during recurvature & beyond as systems first slow when starting to recurve, then typically accelerate northeast after recurvature. Similarly, increased uncertainty in track forecasts often occurs when a system is in an area of little to no environmental steering, a situation tending to occur most often in the Western Caribbean Sea & Gulf of Mexico.
By themselves, intensity errors can be quite large through the 72 hour forecast of the TCM. These errors are accentuated when a poor intensity forecasts is combined with the average track forecast errors occasionally resulting in even worse forecasts of the radius of tropical storm force winds associated with hurricanes, particularly at the 2 to 3 day range.
Guidelines For Avoiding Hurricanes At Sea
In order to help account for the inherent errors in hurricane forecasting, a few guidelines should be used by the mariner in order to limit the potential of a close encounter between ship & storm.
34 KT Rule
For vessels at sea, avoiding the 34 KT wind field of a hurricane is paramount. 34 KT is chosen as the critical value because as wind speed increases to this speed, sea state development approaches critical levels resulting in rapidly decreasing limits to ship maneuverability. It also deserves mention that the state of the sea outside of the radius of 34 KT winds can also be significant enough as to limit course & speed options available to the mariner and must also be considered when avoiding hurricanes.
This is the single most important aid in accounting for hurricane forecast track errors (FTE). Understanding & use of this technique should be mandatory for any vessel operating near a hurricane. The rule is derived from the latest 10-year average FTE associated with hurricanes in the North Atlantic. Application of the rule requires information from the TCM and is extremely important to remaining clear of a hurricane at sea. See Marine Safety Rules of Thumb at right for details on applying this most important technique.
The 1-2-3 rule establishes a minimum recommended distance to maintain from a hurricane in the Atlantic. Larger buffer zones should be established in situations with higher forecast uncertainty, limited crew experience, decreased vessel handling, or other factors set by the vessel master. The rule does not account for sudden & rapid intensification of hurricanes that could result in an outward expansion of the 34 KT wind field. Also, the rule does not account for the typical expansion of the wind field as a system transitions from hurricane to extra tropical gale/storm.
Ship Versus Hurricane Track Analysis
In the dynamic state of moving ships & hurricanes, recurring comparison of hurricane forecast track versus planned ship movement is mandatory. The continual monitoring of the latest official NHC forecasts compared to current or planned evasion options can greatly increase a mariner's confidence regarding vessel safety.
Never Cross The "T"
Never plan to cross the track (cross the "T") of a hurricane. Done out of respect for the negative effects that heavy weather places on vessel speed/handling, sudden accelerations in hurricane motion can ultimately place a vessel in conditions not originally expected thereby resulting in disaster. Adjustments to course & speed in order to remain clear of the danger area in a hurricane are the most prudent navigation decisions a mariner can make in these instances.
Forecast Track Tendencies
Comparison of the most recent NHC forecast track with forecast tracks from the past 24 hours can sometimes prove useful for determining a trend in the forecast motion of a hurricane. For instance, a comparison of forecast tracks issued every 6 hours over the last 24 hours, may show a noticeable shift right or left (with respect to storm motion) in the forecast track of a hurricane. This information may provide some indication as to how the forecast & actual hurricane track are tending and provide more guidance in navigation planning for avoidance, particularly in the 2-3 day forecast range & beyond.
Calculating Closest Point of Approach (CPA)
The last item to complete in the at-sea risk analysis is comparison of CPA between current & possible evasion options. Over time, increases in CPA between vessel & hurricane based on current navigation decisions should increase the mariner's confidence in current avoidance plans. However, decreases in CPA should be dealt with using the utmost urgency. An immediate review of all evasion options combined with a detailed look into the latest official forecasts/discussions needs to be accomplished with a goal of establishing a new evasion course/speed option to once again increase CPA from the hurricane.
Mariners must be cautioned never to leave themselves with only a single navigation option when attempting to avoid a hurricane. Sea room to maneuver is not a significant factor when operating in the open waters of the North Atlantic, but becomes extremely important in the confined waters of the Western Caribbean Sea/Gulf of Mexico. More often than not, early decisions to leave restricted maneuver areas are the most sensible choice.
Port Specific Risk Analysis Considerations
Vessels seeking shelter in port or considering movement toward or away from port need to consider all the factors discussed above while acknowledging some other factors in order to finalize their risk analysis.
Hurricane Approach To Port
In general, hurricanes forecast to make a perpendicular landfall tend to have the smallest amount of FTE. Conversely, systems that are forecast to parallel the coast, as is often noted in the Mid-Atlantic region of the United States, tend to have larger track errors. Additionally, hurricanes that make landfall within 50-100 NM of a particular port tend to be more destructive than those that approach the port from over land or parallel the coast in the vicinity of the port. Also, ports located in the right front quadrant, based on direction of movement of hurricanes during landfall tend to have higher winds, seas, and storm surge.
Go & No Go Decisions To Leave Port
The decision to leave port for hurricane avoidance must be made very early. Consideration to the latest safe departure time & likely avoidance routes must be balanced with a number of other factors. Most important of these is time versus distance. The risk of damage to a vessel at sea increases as the motion of the hurricane increases towards the maximum safe speed of the vessel attempting to leave port in advance of that system. When reviewing these time/distance considerations, mariners must include the effects "squally weather" associated with the rainbands in a hurricane will have on underway preparations & movement from port. Similarly, building wind & sea conditions found at sea, ahead of the hurricane, can also hamper vessel speed & maneuverability. Recognizing these time/distance problems, it cannot be emphasized enough that early decisions to leave port in attempt to avoid hurricanes are crucial. There have been a number of recorded instances where vessels have made the right decision to leave port in attempts to avoid hurricanes, yet were still either damaged/lost because that decision came too late.
Berthing & Shelter Requirements
Considerations to remain in port during hurricane passage must include an evaluation of the amount of protection afforded by the port. The direction from which the strongest winds are forecast to blow along with the potential for storm surge must be factored in when deciding whether to seek haven pier side, at anchorage, or further inland to more protected anchorage. For instance, storm surge can pose significant problems to vessels tied pier side. Substantial rises in water level may place a vessel, previously in a protected wind/wave regime, into an area exposed to significantly greater winds & waves. Similarly, many port & dock facilities, particularly in the Caribbean region are fixed. Although sufficient to support the normally small tidal range of the region, they can quickly become submerged when exposed to even minimal hurricane related surge. Additionally, attention to the tying of lines is also of considerable importance. This is because the force on a moored vessel will nearly double for every 15 knots of wind from tropical storm force (34 KT) to hurricane force (64 KT). Therefore, a vessel tied to the pier under normal situations can quickly break from the pier in periods of higher winds causing substantial damage to itself or other vessels.