PRELIMINARY REPORT
This preliminary report has been prepared in order to provide as much meteorological information as can be made readily available to all those who have a need for background on the meteorological aspects of the 1998 Sydney to Hobart Yacht Race disaster including especially those organisations which need the information to assist in their own assessments of performance during the event.
The report is in two parts. The first consists of an extended Executive Summary that aims to meet the need for a concise overview of the meteorological events and services associated with the 1998 Sydney to Hobart Yacht Race. It includes its own set of figures and attachments. These are repeated in the main body of the report but the numbering is different. Those in the Executive Summary have the prefix "E" to distinguish them from the information appearing in the main body of the report.
The second part of the document is the main body of the report. It is aimed at the more technical reader who requires detailed information on particular aspects of the meteorological situation and/or wishes to examine more closely the services provided for the yacht Race by the Bureau of Meteorology during the period from about 23 December through to the end of search and rescue operations on 29 December 1998.
The report has been prepared in the Bureau’s Head Office with input from many officers. The lead authors were Ms Clare Richards of the Bureau’s Services Policy Branch and Mr John Mottram, an external Meteorological Consultant (and formerly Principal of the Bureau of Meteorology Training Centre), with special assistance from Mr John Bally, Mr Kenn Batt and Mr Geoff Feren from the Bureau’s Regional Offices in Tasmania, Sydney and Melbourne, respectively and Dr John McBride from the Bureau of Meteorology Research Centre.
A final more comprehensive report on all meteorological and related aspects of the events surrounding the 1998 Sydney to Hobart Yacht Race will be prepared over the next few months.
Introduction
Of the 115 yachts that set sail at 1pm on 26 December 1998 in the Sydney to Hobart Yacht Race (the Race), only 44 reached their destination. The destruction caused by a storm encountered by the fleet triggered a massive search and rescue operation involving numerous personnel from organisations such as the Australian Maritime Safety Authority (AMSA), the Royal Australian Navy (the Navy), the Royal Australian Air Force (RAAF) and Police. Even so, it resulted in the abandonment of several yachts and the death of six people. It was the most disastrous event in the 54-year history of this yachting classic.
2. The yachts encountered very severe wind and sea conditions before most were half way into their approximately 630 nautical mile journey down the southeast coast of Australia (Fig E1). The worst weather to hit the fleet occurred off the southern NSW coast and in eastern Bass Strait. The Bureau of Meteorology had issued a gale warning for the southern NSW coast 4 hours in advance of the start of the Race and upgraded this to storm warnings for the southern NSW coast and eastern Bass Strait area about one hour into the Race (see Attachment E1).
Figure E1: Map of southeastern Australia showing the key coastal locations adjacent to the Sydney to Hobart yacht race course, and the areas of forecast responsibility for the Sydney, Melbourne and Hobat Regional Forecasting Centres. Note that for the special yacht race forecasts, Hobart's area of responsibility is the area from 38oS to Hobart.
3. This Executive Summary outlines the evolution of the associated weather systems and highlights key meteorological and oceanographic aspects of the event. The advance briefing, and the forecasts and warnings that were provided by the Bureau, are also described. Some of the terminology used in this report is, necessarily, technical and relevant explanatory notes are provided in Attachment E2.
Evolution of weather conditions
4. After a very warm Christmas Day with a north to northeasterly airflow over much of southeastern Australia, the surface weather pattern for 9pm on 25 December 1998 showed a significant cold front crossing the Great Australian Bight towards Tasmania. A prefrontal trough was already located over western Victoria (Fig E2). These features combined during the next day (Boxing Day) and the resultant system intensified as it moved eastwards. The surface weather pattern at 9am on 26 December (Fig E3), four hours before the start of the Race, showed it would commence in conditions conducive to a fast race with a favouring current, reportedly running at 4 knots, and north to northeast winds of generally 25 to 35 knots prevailing off the New South Wales southern coast.
Fig E2. Surface pressure analysis 9pm 25 December 1998
Isobars at 2 hPa intervals (for balelling see Attachment E2)
Fig E3. Surface pressure analysis 9am 26 December 1998
Isobars at 2 hPa intervals (for labelling see Attachment E2)
5. By early morning on 27 December 1998, an upper air jet stream and a substantial cold air mass had moved rapidly northeastwards over Victoria producing unseasonable snow falls on the Australian Alps in the wake of the surface cold front. In association with this injection of a deep layer of cold air over Victoria, a new small-scale low had begun to form and intensify rapidly over Bass Strait to the south of Wilsons Promontory. This was occurring at the boundary of the interacting very cold and warm air masses which, with the positioning of a strongly curving jet stream in the upper atmosphere, had become a favourable location for the formation of a rapidly intensifying low (as shown schematically in Fig E4).
Figure E4: A schematic of the atmospheric conditions suitable for the rapid development of an intense low pressure system in eastern Bass Strait.
6. The central pressures in the low, which formed to the south of Wilsons Promontory, dropped rapidly to near 980 hPa. The low initially moved northeastwards but shifted to a more easterly path at a speed near 25 knots by late on the morning of Sunday 27 December. At midday, the low passed to the south of Gabo Island, maintaining its intensity. The path of the low is shown in Fig E5.
Figure E5: The track of the low pressure system which developed on 27 December 1998 in eastern Bass Strait. Times indicated are in Eastern Daylight Saving Time (EDST) using the 24 hour clock.
7. In association with the low pressure development, west to southwest winds with mean speeds reaching storm force developed and extended over eastern Bass Strait and southern NSW coastal waters. The surface analysis at 3pm Sunday 27 December 1998, when the storm force winds were likely to have been at or near their peak, is shown in Fig E6. A satellite photograph, illustrating the very tight spiral structure of the low pressure system at about the time of this analysis, is shown in Fig E7.
Fig E6. Surface pressure analysis for 3pm 27 December 1998
Isobars 2 hPa intervals (for labelling see Attachment E2)
Fig E7. Infra Red (IR) satellite image taken at approximately 3pm on 27 December 1998
Satellite photograph courtesy of Japan Meteorological Agency (JMA)
8. By 9am the following day, 28 December, the surface analysis showed the low moving rapidly away towards the southwest of New Zealand (Fig E8) and winds and seas were beginning to moderate for the continuing search and rescue operations.
Fig E8. Surface pressure analysis for 9am 28 December 1998
Isobars at 2 hPa intervals (for labelling see Attachment E2)
Wind speeds
9. A preliminary reanalysis of the situation, taking account of all readily available information, including reports obtained to date from yacht crews, observations from the ESSO Kingfish B Platform in eastern Bass Strait, coastal station reports and information from the Navy’s race relay ship Young Endeavour, as well as the Bureau’s normal observation network, strongly suggests that the highest mean winds (for definition, see Attachment E2.) over open waters in eastern Bass Strait and off the southern NSW coast were about 55 to 60 knots. It should be noted that gusts and squalls of considerably higher wind speeds would almost certainly have been experienced by the yachts for short periods of time as mean winds of this magnitude (55 to 60 knots) could be expected to produce gusts of 70 to 75 knots or more on a fairly regular basis. The gusty nature of the winds in Eastern Bass Strait during the critical period of the Race is illustrated in the recording of observed winds at ESSO’s Kingfish B Platform (Fig E9). A more detailed survey of yacht reports is underway to establish the range of speeds experienced.
Fig E9. The 10 minute mean winds and maximum gusts observed at the Esso Kingfish B platform in eastern Bass Strait (location as given in fig. E1), with schematic representation of gust envelope.
10. The reanalysis of events has taken into account that mean winds of up to 79 knots were observed at Wilsons Promontory Lighthouse. The reanalysis has, however, concluded that these winds were not representative of surface conditions, without some correction, because the observations at Wilsons Promontory are measured at an elevation of about 100 metres and are additionally prone to local effects induced by the surrounding topography. According to studies of boundary layer effects, the wind speed recorded at Wilsons Promontory could be as much as 20 to 25 knots higher than that at an elevation of 10 metres.
Wave heights
11. Although instrument recordings of wind, sea state and ocean current are very sparse, it is apparent that the favourable conditions early in the Race deteriorated rapidly as the yachts encountered the storm force westerly winds over southern NSW coastal waters and Bass Strait during Sunday.
12. Significant wave heights of 6 to 7 metres and maximum wave heights of 11 to 12 metres were recorded during Sunday at ESSO Kingfish B Platform (Fig E10). The Young Endeavour reported seas of 5.5 metres and a swell of 6 metres (combined wave height 8 metres) while located approximately 30 nautical miles east of Gabo Island at 11pm on 27 December 1998. From the limited number of observations received from the yachts so far, many appear to have experienced significant wave heights in the 5 to 8 metre range and maximum waves approximately double these heights.
Fig E10. Significant and Maximum Wave Heights (metres) transcribed from recording instruments located at the Esso Kingfish B Platform, eastern Bass Strait 27 - 28 December 1998 (location as given in Fig 1)
Services provided and forecasts issued
13. Relevant services provided by the Bureau of Meteorology in the period leading up to and encompassing the yacht race were the usual basic marine weather services, including regular forecasts for the high seas and coastal waters (See Fig E1 for definition of forecast areas), a full 24 hour weather watch, and warnings as required. In addition, the Cruising Yacht Club of Australia (CYCA) sought an additional briefing and some race specific forecasts which were supplied in the public interest. Arrangements with the CYCA also enabled race officials to contact the Bureau’s Senior Forecaster at any time.
14. A Bureau meteorologist provided a special briefing session for the competitors and organisers of the Race on 24 December 1998. The approximately 250 people who attended this meeting were made aware of the possibility of hazardous weather conditions during the Race although the eventual nature and strength of the development were not foreseen at this time. However, soon after 9am on 26 December 1998, about 4 hours before the start of the Race, the Bureau issued a gale warning for the New South Wales coastal waters south from Broken Bay. The gale warning was disseminated through normal channels (eg facsimile, coastal radio and the Internet) and competitors and organisers were also able to access this information at the Bureau’s pre-race briefing stand.
15. The Bureau’s New South Wales (NSW) Regional Forecasting Centre (RFC) upgraded the gale warning to a storm warning for the area south of Merimbula at 2.14pm on 26 December 1998, just over one hour into the Race. A copy of this warning is included in Attachment E1. The warning was disseminated via the normal channels and, in addition, a Bureau officer from the NSW RFC contacted the Australian Maritime Safety Authority, Eden Royal Volunteer Coastal Patrol and the Sydney to Hobart Race media centre before issuing an updated special Race forecast at 2.50pm on 26 December 1998. The updated forecast (see Attachment E1), which included advice that a storm warning was current south of Merimbula, was sent to the Young Endeavour which was responsible for relaying forecasts and warnings to the competitors during the official radio schedules. According to the limited number of reports from crews that have been received so far, it appears that in addition to the radio schedules some yachts received the details of the warning from other means (ie coastal radio broadcasts). The results of a CYCA survey will provide more detailed information on how and when the crews were made aware of the storm warning.
16. An initial investigation of the relevant meteorological analyses and prognoses has shown that output from the Bureau’s high resolution computer model at 1pm on 26 December was an important factor in triggering forecasters to issue the storm warning. The alerting computer forecast output, valid for 11pm on Sunday 27 December 1998, is shown in Figs E11 and E12.
Figure E11: Computer generated prognosis of Mean Sea Level (surface) Pressure for 11pm Sunday 27 December 1998, which became available to forecasters at 1pm on 26 December 1998.
Figure E12: Computer generated prognosis of wind speed in knots (contours) and direction (arrows) for 11pm Sunday 27 december 1998, which became available to forecasters at 1pm on 26 December 1998.
17. After consultations between the Bureau’s Victorian and NSW RFCs, storm warnings were promptly issued for coastal waters in eastern Victoria and southern NSW (Attachment E1). Warnings for the high seas were issued soon after. The warnings were issued some 18 to 21 hours ahead of the onset of the storm force winds over the Race area.
18. The special Race forecast service provided to the Cruising Yacht Club of Australia has, by agreement between the Bureau and race organisers, always been performed by the New South Wales and Tasmanian RFCs. During the event, these offices liaise together and with the Victorian RFC which is responsible for routine forecasts in eastern Bass Strait, forecasts for the high seas in the Tasman Sea and for high seas warnings south of latitude 38 degrees south (See Fig E1).
19. During the search and rescue operations, the Bureau provided particular forecast products on five occasions to AMSA to assist in search planning and rescue efforts. The products originated from the Bureau’s Victorian RFC (one on 27 and three on 28 December 1998) and from its Meteorological Office in Canberra (one issued on 28 December 1998). To help define the search areas, the Victorian RFC also provided AMSA with additional information on winds (speed and direction) likely to have already occurred over the search and rescue area and to have influenced the movement of disabled yachts.
20. Storm warnings for both the coastal waters and high seas referred to winds of up to 45 to 55 knots. By international convention these are mean wind speeds. It is known that wind gusts cause large temporary fluctuations about this mean and that maximum gusts of up to 40% above the forecast mean wind are to be expected. Therefore, with a forecast of 45 to 55 knots, regular gusts in excess of 70 knots were to be expected.
21. Storm warnings for the coastal waters mentioned waves of 4 to 7 metres. These refer to significant wave height, which is the average height of the highest one third of all waves. Storm warnings for the high seas referred to rough (2.5 to 4 metres) to very rough (4 to 6 metres) seas and moderate (2 to 4 metres) to heavy (greater than 4 metres) swell which would result in a combined significant wave height of at least 7 metres. It should be noted that while forecasts and observations of waves are for the significant wave height, infrequent individual waves approaching twice that size must be expected to occur.
Summary
22. In summary, a preliminary investigation of the meteorological situation surrounding the Race and the services provided by the Bureau, indicates that:
23. A more complete description of the meteorological conditions, including the range of wind speeds and sea conditions, associated with the Race is expected to emerge as additional information comes to hand from other sources (eg surveys being undertaken by the CYCA, and from AMSA) and all available relevant information will be included in the Bureau’s final report.