(U) The Hurricane Warning Service is provided through the cooperation of the Department of Commerce, the Department of Defense and the Department of Transportation. The National Hurricane Center, Miami, Florida is responsible for collating data including the results of aircraft, radar and satellite surveillance, and for developing and issuing hurricane warnings and forecasts for the North Atlantic Ocean including the Caribbean Sea and Gulf of Mexico. The initial warning for each new tropical or subtropical cyclone is issued in consultation with the Naval Eastern Oceanography Center, Norfolk, Virginia.
(U) The principal product of the Hurricane Warning Service is the Hurricane Advisory Message; the format and content of which is illustrated in Figure I-2. AVIATION, MARINE and MILITARY Hurricane Advisories all include the first six sections. MARINE and MILITARY advisories carry the additional section on storm tides and precipitation. The MILITARY advisory has a supplementary section giving 48- and 72-hour extended outlook forecasts. The extended outlook is offered for Tropical Storms, Hurricanes and for Tropical Depressions which are forecast to become Tropical Storms within 24 hours.
(U) The Naval Eastern Oceanography Center, Norfolk, using the MILITARY ADVISORY as guidance, issues messages to U.S. Navy interests titled "Hurricane Warnings." Advisories and Navy Warnings are issued on formation of a tropical or subtropical depression and subsequently at six-hourly intervals at 0400Z, 1000Z, 1600Z and 2200Z.
(U) Additional Special Advisories/Navy Warnings are issued in the event of significant changes in intensity or any changes in motion which significantly affect the threat to coastal areas or U.S. Navy units.
(U) Advisories/Navy Warnings for any particular cyclone will continue until its dissipation or until it adopts the characteristics of - or becomes assimilated by - a frontal or extratropical cyclone.
(U) Identification of an advisory with a particular cyclone is achieved by numbering each new depression consecutively, e.g., TD1, TD2. A check on missing messages is achieved by observing the sequential number series for advisories on each depression, e.g., Advisory Number 1 on TD1, Advisory Number 2 on TD1. When a tropical depression intensifies to storm strength, it is NAMED and the Advisory Number reverts to 1 and starts all over again, e.g., the next Advisory would be designated Advisory Number 1 on Tropical Storm ANITA. Subtropical depressions are dealt with similarly except that those which intensify to become subtropical storms are numbered consecutively instead of being named.
(U) MILITARY advisories are disseminated to DoD users via the Automated Weather Network at Carswell AFB, Texas. NAVEASTOCEAN Navy Warnings are issued via AUTODIN and Channel 8 of the Fleet Multi-Channel Broadcast.
(U) MARINE advisories are broadcast to high-seas shipping according to details found in "Worldwide Marine Weather Broadcast" published by the U.S. Navy and National Weather Service or other maritime weather broadcast lists for the western North Atlantic.
Generic term referring to a (counterclockwise) rotating closed circulation (N. Hemisphere) irrespective of intensity or type.
When applied to TROPICAL cyclones, these refer to the following three stages of development and intensity:
(1) (U) Tropical Depression (TD) - A tropical cyclone in which the maximum sustained surface wind (1-minute mean) is 33 kt (38 mph) or less.
(2) (U) Tropical Storm (NAMED) - A tropical cyclone in which the maximum sustained surface wind (l-minute mean) ranges from 34 kt (39 mph) to 63 kt (73 mph) inclusive.
(3) (U) Hurricane (NAMED) - A tropical cyclone in which the maximum sustained surface wind (l-minute mean) is 64 kt (74 mph) or more.
(U) When applied to SUBTROPICAL cyclones, these terms also refer to two stages in development and intensity:
(1) (U) Subtropical Depression - Wind limits as for (1) (U) above (Tropical Depression).
(2) (U) Subtropical Storm - LOWER wind limit as for (2) (U) above (Tropical Storm) but NO UPPER LIMIT.
The first two adjectives are not used in their normal geographical sense. Tropical Cyclones may develop over both tropical and subtropical water while Subtropical Cyclones develop over subtropical water only.
(U) The meteorological distinction - made possible by satellite surveillance - is that subtropical cyclones possess a hybrid character lying between the Tropical Cyclone and the Extratropical Cyclone.
(U) Subtropical Cyclone features of practical importance to the mariner are as follows:
(U) (1) (U) They are frequently short lived and dissipate without developing beyond the depression stage.
(U) (2) (U) Those which intensify beyond the depression stage occasionally change character to become Tropical Storms. In fact, subtropical storms which intensify to hurricane strength usually adopt tropical characteristics and are then designated as Hurricanes.
(U) (3) (U) Some Subtropical Cyclones are less compact and less intense towards the center than their tropical counterparts and may exhibit a belt of maximum winds as far as 100 miles from their center (compared with a radius to maximum winds in tropical cyclones typically close to 20 miles).
(U) The Extratropical Cyclone is the much larger scale, usually less intense, frontal cyclone of middle latitudes. These cyclones lie outside the scope of the Hurricane Warning Service - although many Tropical and a few Subtropical Cyclones adopt Extratropical characteristics or merge with existing Extratropical cyclones before dissipating, if they move sufficiently far north to encounter cold air.
(U) TROPICAL WAVE/TROPICAL DISTURBANCE
These terms are not normally employed in the Hurricane Advisories but may appear in related products of the Hurricane Warning Service such as the Tropical Cyclone Discussion and the Tropical Weather Outlook.
(U) The Tropical Wave is a minor cyclonic disturbance in easterly tradewinds which could develop into a Tropical Depression but lacks evidence of a closed circulation.
(U) Tropical Disturbance is a generic term which includes all of the foregoing, i.e., Tropical Wave, Cyclone, Depression, Storm, Hurricane and Subtropical Depression or Storm.
(U) A clear distinction must be made between "actual" information and the more speculative "forecast" information in the Hurricane Advisory Message (see Figure I-2).
(U) (1) (U) "Actual" information on the location and present movement of the cyclone is now of outstanding reliability even when the cyclone is well offshore because satellite images are available every 30 minutes from Geostationary Satellite (GOES) surveillance for all sea areas affected by the North Atlantic Hurricane. The average initial positioning error in routine Hurricane Advisories for the 10-year period 1970-1979 was only 20 n mi (Neumann, 1980). Satellite surveillance also permits the estimation of tropical cyclone intensity (Dvorak, 1975).
(U) If a tropical cyclone is threatening landfall along the United States coast, further improvement in the "actual" data contained in the Hurricane Advisory is provided by aircraft surveillance and also by land-based radar. Hourly updates of the actual position of any tropical cyclone within 200 n mi of land-based radar, are issued by the National Hurricane Center to the public.
(U) (2) (U) "Forecast" information on the location, movement and intensity of tropical cyclones, in comparison with "actual" data, is distinctly inaccurate. In fact, the forecasting of tropical cyclone movement alone is a formidable problem as it depends upon the interaction between several essentially independent scales and levels of atmospheric motion over a vast - mainly oceanic area. Even in the relatively well-populated Caribbean area, the network of vital upper air observing stations is sparse and in recent years, is showing signs of deteriorating. Despite these difficulties, improvements in satellite surveillance and forecast techniques have maintained a small but continued improvement in forecast accuracy.
|Average forecast position errors escalate rapidly as the forecast interval increases:|
|AVERAGE POSITION ERROR:||51||109||244||377||(NAUTICAL MILES)|
|(For period 1970 - 1979; from Neumann and Pelissier, 1981)|
(U) In fact, these averages reflect a serious weakness in movement forecasting the limited ability to predict recurvature and the subsequent tracks and speeds of recurving storms (i.e., those which change from a westerly track to a northeasterly one - often aligned with the east coast of the U.S.). This weakness leads to considerable regional inequality in forecast errors. Figures I-3 and I-4 show the regional distribution of average errors in 24- and 48-hour forecasts, respectively. Minimum errors appear in the Caribbean and Gulf of Mexico. The large errors associated with recurving storms become disproportionately large north of Florida beyond a forecast interval of 48 hours. Consequently, the 48- and 72-hour forecasts are considered to be unsuitable for dissemination to the public.
(U) To the commanding officer of a ship in harbor, the threat posed by a hurricane is more forcefully expressed by its chances of making a landfall nearby than by the chances of a near overland pass or a near pass offshore.
(U) Forecast aids which specifically address the landfall event are as follows:
(U) 2.5.1 Coastal Warnings for tropical storms and hurricanes threatening to cross the coast of the U.S. are issued to the public by the National Hurricane Center through the local Hurricane Warning Offices. They specify the coastal extent of the warning in order that defenses against damage and perhaps evacuation, can be implemented. Two levels of warning are employed: the "Hurricane Watch" is a preliminary alert that a hurricane may threaten a specified portion of the coast and is issued approximately 36 hours before landfall could occur. The second level is the "Hurricane Warning" which indicate that hurricane conditions are expected within 24 hours along a specified length of coastline - usually lying within the coastal area for which a Hurricane Watch had previously been issued. The Hurricane Warning is usually issued between 12 and 24 hours in advance of landfall. This service is aimed at providing the best compromise between timeliness and accuracy for civil defense purposes and therefore its warnings may be too late to allow ocean-going vessels to get underway and complete a successful evasion in open water. In the period 1970-1979, Hurricane Warnings were issued with an average lead time of 19 hours for the 23 tropical storms or hurricanes which made a direct landfall along the United States coast. The average landfall error of 39 n mi for landfall forecasts during this period is impressively low (Neumann and Pelissier, 1981).
(U) Cautious optimism is needed in assessing the accuracy of landfall forecasts because it depends critically upon the angle between the storm's approach and the coastline. Perpendicularly landfalling storms will usually show the least error - a characteristic of most landfalling storms in the Gulf of Mexico where movement forecast errors are also small. Given that at least 3/4 of all U.S. landfalling hurricanes occur in the Gulf of Mexico (a figure borne out for the 1970-1979 period cited above), it is clear that in the average landfall error figures, the well-forecast Gulf of Mexico cases overpower the minority of ill-forecast landfall cases in other regions (e.g., Tropical Storm Heidi crossed the coast at Bangor, Maine in 1971, 130 n mi from the forecast landfall point). In general, large errors in landfall forecasts can be expected from Miami to Maine with the worst combination of circumstances occurring in the north.
(U) 2.5.2 U.S. Navy Strike Probability Forecast Service. This service which has been in operational use in the western Pacific since 1979, and in the North Atlantic since 1981, is aimed specifically at the mariner, both at sea and in harbor, who is faced with a tropical cyclone threat. It offers a dramatic improvement on the established Navy practice of drawing "danger areas" based upon the sum of two distances: The forecast radius of 30-kt winds; and a fixed average position error determined solely by the forecast interval. The "danger area" method provides no quantitative indication of the risk of say, encountering 30-kt winds at the "danger area" boundary because it takes no account of the fact that errors in some forecasts are much larger than others.
(U) The Strike Probability forecasts uses a statistical analysis which estimates the error of each individual tropical cyclone forecast and from it, calculates the % probability of a specific location being struck by the cyclone at each forecast interval out to 72 hours. In its latest form denoted "Wind and Strike Probability Forecast," the % probability of 30- and 50-kt winds is also computed.
(U) Two versions of the service are available: one which applies to a moving datum is employed ashore as a tool in the Optimum Track Ship Routing Service and is also employed aboard aircraft carriers; the second applies to a fixed datum to assess the Tropical Cyclone threat at key Navy locations and coastal USAF bases. Figures I-5 and I-6 show shore locations serviced in the North Atlantic area.
(U) 2.5.3 Near Pass Probability. The maps of Near Pass Probability included with each port evaluation in the Handbook are for providing advance warning of a tropical threat when it is still beyond the range of real-time forecasts such as the Hurricane Advisories or Navy Strike Probability forecasts. They are therefore of value when a tropical cyclone is more than 3 days' and up to 6 days' movement from a port, but are less skillful than real-time forecasts at 72-hours range and less.
(U) The setting of Hurricane Conditions of readiness is carried out at Navy and civil ports in consultation with meteorologists. The procedure serves mostly as a landfall forecast - usually based on the Military Advisory message and therefore extending to 72 hours before expected landfall - and also as a framework for linking a staged schedule of hurricane countermeasures with specific levels in a mounting hurricane threat. Along the U.S. coast, Hurricane Conditions will be set by Navy or Coast Guard authorities according to similar rules. Timings implied by specific Hurricane Conditions may vary, because some Coast Guard authorities observe additional intermediate stages in their schedule at 36 and 18 hours. These correspond with the National Weather Service coastal warnings.
(U) Navy instructions for setting Hurricane Conditions are based upon the following schedule (Dept. of the Navy, 1974):
Hurricane Conditions IV: Trend indicates a possible threat of destructive winds of force indicated within 72 hours. Review hazardous and destructive weather implementation plans.
Hurricane Condition III: Destructive winds of force indicated are possible within 48 hours. Take preliminary precautions.
Hurricane Condition II: Destructive winds of force indicated are anticipated within 24 hours. Take precautions that will permit establishment of an appropriate state of readiness on short notice.
Hurricane Condition I: Destructive winds of force indicated are anticipated within 12 hours or less.
(U) Considerable enlargement of the precautions demanded at each stage is given in both Navy and civil Hurricane Preparedness plans according to local circumstances. An additional, low state of preparedness, designated Hurricane Condition V may, in certain areas, be set automatically at the beginning of the Atlantic hurricane season (1 June) and rescinded at the end of the season (1 December). At ports listed in the Handbook, the climatological Near-Pass Probability maps provide the possibility of setting Hurricane Condition V on the basis of a threat which is specifically directed towards the port. Suitable criteria for setting Hurricane Condition V on the basis of the Near-Pass Probability maps are as follows:
|"Hurricane Condition V should be set when:|
|EITHER:||(1) (U)||Any tropical cyclone (irrespective of its intensity) forms within or moves inside the 3% probability envelope. If its position inside this envelope lies inside the 4 1/2 - 6 day time line, higher conditions of readiness may have to be considered (see below).|
|OR:||(2) (U)||Any tropical cyclone (irrespective of its intensity) forms within or moves within a radius of 360 n mi from the port, even though outside the 3% probability envelope...."|
(U) If the tropical cyclone continues to move towards the 3-4 day time line within the 3% probability envelope or if it threatens to continue closing its range within 360 n mi even though it lies outside the 3% envelope, the setting of Hurricane Condition IV should be considered. However, at this stage, attention should be diverted towards real-time forecasts.
(U) There are two problems in determining whether the storm will have sufficient impact at the port to justify setting higher conditions of readiness: The large errors associated with tropical cyclone forecasts; and the influence of local factors which affect the impact of storms.
(U) The Navy Wind and Strike Probability forecast is the recommended approach towards the first problem. Note that it does not reduce the error in the original forecast and therefore does not reduce the degree of overwarning needed to compensate for that error. As a starting point, the Users Manual (NEPRF, 1981) suggests the following threshold values of "time integrated probability" for strike, at which each Hurricane Condition should be set:
|Hurricane Threshold Value of "Time-Integrated |
______Condition Probability" of Strike______
|IV||Greater than or equal to: 5% within 72 hours|
|III||Greater than or equal to: 10% within 48 hours|
|II||Greater than or equal to: 20% within 24 hours|
|I||Greater than or equal to: 30% within 12 hours|
(U) It is further recommended that these objective criteria for setting higher conditions of readiness, be regarded as minimum criteria. Further consideration should be given to the individual circumstances of the current threat before revising the prevailing state of readiness. Otherwise a high degree of over-warning will be perpetrated. Details of these local considerations are supplied for each port listed in the Handbook. An example illustrating the influence of local factors on the setting of Hurricane Conditions without employing the Strike Probability forecast, appears in the Appendix to Section IV of the Handbook entitled "Proposed Rationale for Setting Hurricane Conditions at Key West." Given the added facility of the Strike Probability forecast, a simpler set of criteria can be devised in which the threshold values of Wind or Strike Probability at which Hurricane Conditions are set, are adjusted according to the local factors affecting the impact of a hurricane threat. For example, at Mayport, Jacksonville and King's Bay, lower threshold values for strike or 50-kt winds should be demanded of storms threatening to parallel the east coast after swinging northward from the Antilles (e.g., Hurricane David, 1979) than for storms approaching overland from the Gulf of Mexico. Still lower threshold values of strike probability should alert these ports to possible danger in the rare case of storms with more northerly courses which threaten to make direct landfall along this section of the coast (e.g., Hurricane Dora, 1964).