(U) The classical doctrine held by most mariners is that ocean-going ships should leave ports which are threatened by a hurricane. Despite this natural caution, ships continue to be damaged in port or after leaving port, as a result of encounters with tropical cyclones. This stems mainly from the relative unpredictability of tropical cyclone movement. For example, the average 1970-1979, 24-hour tropical cyclone movement forecast error (Neumann and Pelissier, 1981) represents more than half of the average actual movement of these storms during the 24-hour period. In these circumstances, it is necessary to provide a means for the mariner to come to terms with large errors in the tropical cyclone forecast and to assess the relative risks of remaining in port or putting to sea according to the circumstances of the threat, the facilities of the port and the capabilities of his vessel and crew.
(U) A preliminary evaluation of the balance of these risks along the U.S. Gulf of Mexico and Atlantic coasts is illustrated in Figure I-1. This evaluation relies upon examining four factors:
|(1) (U)||Local history of hurricane encounters. The risk of a particular port encountering a hurricane depends upon strong seasonal and geographical influences. The heavy solid line of Figure I-1 shows for all seasons how geographical factors concentrate the risk of hurricane encounter at large-scale coastal promontories. The "probability of encounter" here refers to the probability of a tropical cyclone of hurricane intensity passing within 70 n mi of the coast. It is expressed as the % probability to the left of the figure and in terms of return period on the right.|
|(2) (U)||Local predictability of hurricane movement. The risk of misjudging a hurricane threat at the point in time when preparations by large vessels to leave a port should be started (typically 48 hours ahead of destructive force winds or at Hurricane Condition III) is influenced by the size of the forecast error. The dashed line in Figure I-1 shows a pronounced maximum near Philadelphia. This maximum is associated with the special problems of predicting the movement of recurving storms. A minimum value near Key West is associated with storms of lower speed of advance and greater constancy of movement.|
|(3) (U)||Local shelter and security of berths. The risk of a vessel sustaining damage in port in the event of a hurricane strike depends upon the suitability of berthing facilities available to her and the shelter they offer. This cannot be assessed fully on the broad scale but Figure I-1 gives an indication of where topographical shelter may be available. (It has been assumed that shelter may be available if terrain of at least 100 ft elevation is located near the coast.)|
|(4) (U)||Local speed of advance of tropical cyclones. The risk of a vessel sustaining damage at sea increases abruptly as the speed of advance of the storm rises towards the maximum speed capability of the vessel - particularly when the effects of heavy weather on vessel speed are considered. A large range of speeds of advance of tropical cyclones creates the additional possibility than even the well-prepared mariner will be trapped in a late departure dilemma in which insufficient sea room can be gained to exercise evasion tactics successfully. The cross-hatched band of storm speeds in Figure I-1 extends from the mean speed of advance of near-coastal storms to their top 5% extreme speeds.|
(U) Large changes in the balance of these four factors affecting the leave/stay decision are evident along the U.S. Gulf and Atlantic coasts. However, in the absence of any well sheltered natural harbors, the hurricane haven qualities of ports along these coasts can only be rated in shades of gray. Such a "gray" rating is especially applicable to the case of Charleston - located on Figure I-1 approximately two thirds along the baseline distance from Mexico to Canada. None of the special coastal features which are highlighted along the baseline (e.g., topographical shelter), apply to Charleston. The continuous curve displaying "Frequency of Hurricane Threat" gives a moderate value of 15% for Charleston on the left-hand scale or a hurricane threat return period of 6 1/2 years on the outer right-hand scale. The broken curve of mean forecast error at Hurricane Condition III gives a moderate value of 220 nm on the inner right-hand scale. Finally, the cross-hatched band displaying speed of advance for near-coastal hurricanes, gives relatively low values for both the mean and extreme speeds of advance of hurricanes affecting Charleston. These values from the left-hand scale are 9 1/2 and 18 kt respectively. They suggest a relatively low risk of damage at sea after evasion from Charleston provided sortie is executed as early as possible.
(U) The particular combination of leave/stay factors in the Mayport area should lead to a very low frequency of both justifiable sorties and sorties conducted because of uncertainties about the tropical cyclone threat. Furthermore, all sorties should carry a low risk of unsuccessful evasion at sea because of the relatively low speeds of advance of near-coastal storms in this area. Ports in the Mayport area therefore have the potential of being fairly good hurricane havens despite their conspicuous lack of shelter, because of the rarity of serious hurricane threats and the prospect of a safe escape to sea when needed.
(U) New York and certain New England ports clearly have the potential to offer good hurricane haven qualities because of the low risk of a hurricane threat in conjunction with the possibility of topographical shelter. Note that the risk of misjudging the threat, due to the large forecast errors associated with storms threatening landfall in this area, is considerable. Furthermore, the risk of sustaining damage in attempting to evade at sea is increased by the high speed of advance of threatening storms. This combination of circumstances should encourage mariners at ports of this coastal region, to regard evasion at sea as a last resort, having exhausted all possibilities of safeguarding their vessels from a hurricane strike at protected berths or anchorages.
(U) Two ports with a high risk of encountering a hurricane threat - Key West and Morehead City - show a large contrast in the remaining factors affecting the leave/stay decision. The threat at Key West appears relatively predictable and easy to evade at sea. However, the combination of low threat predictability and the relatively high speed of advance of near-coastal storms affecting North Carolina, marks Morehead City as a less secure port to occupy during the hurricane season than Key West, and one from which evasion at sea carries a higher risk of damage.
(U) The U.S. Gulf of Mexico coast displays a balance of hurricane haven factors lying between the extremes discussed so far. However, the reduced flexibility in evasion options created by the shape of the Gulf of Mexico biases the leave/stay decision in favor of an early departure, which effectively reduces the predictability of the threat at the time of sortie decision. The large range of possible speeds of advance of tropical cyclones affecting the New Orleans to Pensacola sector of the coast, should encourage even earlier departure. The net effect is that ports in this sector of the U.S. Gulf of Mexico coast, should be considered to be as insecure as the conspicuously "high risk" ports typified by Key West and Morehead City. Local factors in the Gulf of Mexico further diminish the security of many ports. For example, the strong impact of storm surge along much of the Gulf coast which, in places, leads to closure of ports due to sudden silting of their long, dredged approach channels. The Texas coast may also be prone to a highly destructive local augmentation of a hurricane's winds immediately after its landfall. The case of Celia's landfall near Corpus Christi in 1970 reveals this effect, which Fujita (1980) (U) has ascribed to the result of the hurricane's interaction with dry, desert air.
(U) Finally, ports which are well set back from the coast on major tidal rivers may be so well isolated from the effects of landfalling hurricanes that, even if they do not offer topographical shelter, they may be considered to be good hurricane havens. For example, the indicated hurricane threat frequency in Figure I-1 for Philadelphia lies below the "coastal" value. Even so, this does not convey the full extent of this port's isolation from the threat because the effects of surface friction and overland dissipation on reducing the strength of the hurricane's windfield have not been considered. In fact, both Philadelphia and Baltimore show good promise as hurricane havens.
(U) The above approach to the leave/stay decision emphasizes the importance of coming to terms with the probable error in tropical cyclone movement forecasts. Unfortunately, this error is highly variable - even for a specific forecast - interval and location - and furthermore, it is not symmetrically distributed around the forecast position of the storm. Therefore, the application of the forecast error data in Para. 2.4.2 to a specific threat situation could only provide the most rudimentary indication of the probability of destructive weather at the port.
(U) For this reason, the U.S. Navy operates the "Hurricane Wind and Strike Probability" service for each of its major North Atlantic ports and near-coastal USAF bases - a service which allows for the error associated with each tropical cyclone forecast and determines the risk of destructive winds and strike for these locations quantitatively. This is the ideal tool for setting Hurricane Conditions on a rational basis up to 3 days ahead of possible strike (see User's Manual (NEPRF, 1981) for details). At 3 days ahead and beyond, the climatological Near Pass Probability maps included with each port evaluation in this Handbook, can provide advance warning of possible encounter (within 180 n mi) up to a week ahead. For this purpose, a plot of actual and forecast positions of the tropical cyclone should be made on the map appropriate to the time of year (e.g., at Gulfport, Mississippi in September, (U) Figure XI-8 in Section XI of the Handbook should be used). As soon as the position of the tropical cyclone approaches the 3-4 day time line, attention could be diverted to the USN Strike Probability forecast (at Gulfport, MS use the product supplied to Keesler AFB).
(U) Note that the Wind and Strike Probability forecast does not reduce the error in the original forecast and therefore, does not reduce the degree of overwarning which is needed to provide a safeguard against that error. In fact, the % probability threshold values suggested in the User's Manual for setting Hurricane Conditions (see Para. 2.6) imply a higher degree of overwarning than is employed in the coastal warnings issued to the public via the Hurricane Warning Offices. Furthermore, no account is taken in the wind and strike probability forecasts of the effects of shelter or of the dramatic effect which a hurricane's direction of approach can have on its impact at a particular port. For example, both Mayport and Norfolk have experienced numerous threats from storms approaching overland. Few of these have merited ships leaving harbor. However, objective methods for setting Hurricane Conditions on the basis of the forecast "open ocean" winds, would have supported many unnecessary sorties as a result of ignoring the effects of increased friction on the surface wind field. These local considerations are addressed in detail for each port evaluation in the Handbook. The penalty for abandoning a well-rounded evaluation of each hurricane threat, in favor of a purely "objective" approach based upon certain probabilities of strike and 50-kt winds, will be a large increase in unnecessary sorties. Instead, a current tropical cyclone threat should be monitored with the best objective aids available, but also with a keen awareness of the character of the "worst case" threat and the likely impact of lesser threats. For example, at the better hurricane havens, the rare direct landfalling storms are inevitably the "worst case" threats and usually possess conspicuously different track features (e.g., Hurricane Dora of 1964 at Mayport, the 1933 Hurricane at Norfolk and the 1938 New England Hurricane at Newport and New London).
(U) In considering the security of other Atlantic ports or even when making a decision on leaving or staying at a port under the pressure of a hurricane threat, the separate contemplation of the following factors will be helpful:
(U) (1) (U) Frequency of hurricane threats at the port. Estimate this from the storm frequency at the coast as indicated by the data of Para. 3. Use the appropriate seasonal map.
(U) (2) (U) Predictability of the hurricane threat. Determine the local accuracy of 24- and 48-hour forecasts from Figures I-3 and I-4 and use it to determine the risk of encounter as described below (Para. 1.3.2).
(U) (3) (U) Countermeasures available in port. Consider likely approach directions of threat and compare the security of alternative alongside berths, moorings or anchorages. Consider the possibility of steaming at anchor. Wave and tidal effects will then have least effect, cable strains will be minimized and uncertainties about the strength of piers or moorings are eliminated.
(U) (4) (U) Evasion hazards. Compare likely speed of advance of storm with ship's speed. Compare direction of storm movement in relation to shoal grounds and lee shores. Estimate latest safe departure time for each evasion route.
(U) The balance between hurricane threat frequency (1) (U) and the suitability of port facilities for safeguarding the vessel against damage (3) (U) will determine the mariner's stance towards the threat. Threat frequency varies between wide limits (see Para. 3). For example, all ports along the southern shores of the Caribbean Sea from Venezuela to Costa Rica are relatively secure in all seasons. Further north however, little solace is available except for the embayed western shore of Haiti southeast of Windward Passage.
(U) Use the initial position of the storm when extracting forecast error data from Figure I-3 or I-4. As long as the port's position lies outside these circles, the chances of encountering destructive weather remain below 10%. Note that for Caribbean storms, this method will overestimate the probability of their landfall along the southern shore from Venezuela to Costa Rica.
(U) Given the uncertainties created by large errors in the tropical cyclone movement forecasts, it is sufficient to judge the probable impact of a hurricane threat from the following simple guidelines:
(U) (1) (U) Storms threatening to make a direct landfall from the ocean within 50 n mi of the port are many times more destructive than storms approaching overland or storms paralleling the coast.
(U) (2) (U) Starting at gale force winds (34 kt), the force on a moored vessel nearly doubles for every 15 kt extra wind speed up to hurricane force (64 kt) and then more slowly after that.
(U) (3) (U) Tropical cyclones of hurricane intensity carry the added threat of storm tides which typically rise between 5 and 20 ft above normal. Note that this rise in sea level may cause otherwise sheltered berthing areas to become exposed to destructive wave action - especially if the harbor is only protected from the open ocean by low-lying reefs or sandbanks.
(U) (4) (U) The destructive effects of winds, seas and storm tides are most prominent in the right-front quadrant of a storm looking along its direction of movement. This is particularly noticeable to the right of the storm's point of landfall up to a distance of 70 n mi (measured at 90º to the direction of its track).
(U) A sudden unexpected change in the speed or direction of movement of a tropical cyclone, or a change in its intensity, may call for a hasty departure from port in deteriorating weather.
(U) However limitations in manpower onboard, port tug facilities or the state of readiness of the ship's machinery will increase the risk of the the vessel being damaged during departure. Furthermore, the chances of gaining sufficient sea room in heavy weather to avoid damage after leaving port, are also decreased.
(U) The odds for preventing serious damage to the vessel in these circumstances, swing in favor of using the resources available to secure the ship firmly to her berth. These measures should include laying anchors into the channel or basin to hold her away from the pier or wharf face. This is particularly important in preventing damage to both vessel and pier in the event of storm tides flooding the wharf. These tidal effects will require lines to the pier to be tended until the hurricane threat is well passed. Certain merchant vessels may also consider ballasting down if the bottom at the berth is likely to be clear of obstacles.
(U) Under pressure of these circumstances, proceeding to anchor or moor is a less attractive alternative unless both the resources to accomplish the move safely and the assurance of an authenticated hurricane mooring or anchorage, are available.
(U) Both Navy and civil port authorities use the setting of Hurricane Conditions (see Para. 2.6 )to announce the recommended state of preparedness to counter an approaching hurricane threat. This announcement includes a statement of the expected timing and impact of the hurricane threat.
(U) Mariners should pace their preparations to counter an approaching threat according to the prevailing Hurricane Condition. Keeping well ahead will allow for any sudden, unexpected changes in the tropical cyclone's behavior.