A rather unusual event, the formation of a tropical cyclone of hurricane intensity within an extratropical low pressure system, was observed in the latter stage of the 1991 Atlantic hurricane season.
a. Synoptic History
The unnamed hurricane was preceded by two disparate meteorological events over the western Atlantic: Hurricane Grace, and a strong extratropical storm.
On 28 October, a baroclinic, i.e. extratropical, cyclone developed along a cold front which had moved off the northeast coast of the United States. Surface synoptic analyses indicate that the low was initially located a few hundred miles east of the coast of Nova Scotia at 1800 UTC on that date. The associated deep-layer circulation soon became a dominant feature over the extreme western Atlantic. Hurricane Grace was swept eastward by the flow over the southern periphery of this system. As the extratropical low was deepening, a vigorous cold front, trailing south-southwestward from it, overtook Grace at 1800 UTC on 29 October. The low-level circulation of Grace was destroyed quickly thereafter.
The extratropical low continued to strengthen, while drifting southeastward, then southwestward. By 30 October, the low was moving westward and it reached its peak intensity as an extratropical storm around 1200 UTC when it was located about 340 n mi south of Halifax Nova Scotia. Its minimum central pressure was about 972 mb and estimated maximum sustained winds were near 60 knots. At this time the storm was causing phenomenal seas and strong winds along the U.S. east coast, and considerable but local damage along much of the western Atlantic shoreline due to tidal flooding and wave action. Peak wave heights over the open Atlantic of 101 feet and 80 feet were measured by a buoy and a ship (whose name is not known at this time), respectively.
After reaching peak intensity as an extratropical system, the low turned southwestward, then southward and the central pressure gradually rose, to about 998 mb by 0000 UTC on 1 November. However, this southward motion brought the low center over a section of the Gulf Stream, with sea surface temperatures near 26°C. With the low moving over warmer waters, convection began increasing in bands around the center. It is estimated that subtropical characteristics were acquired at 1800 UTC on 31 October, because, by that time: 1) there was no longer a well-defined baroclinic zone across the area; and 2) although there were curved bands of convective clouds around the center of the low, that center was not yet underneath a central dense overcast feature. By 0600 UTC on 1 November, central convection had increased to the point where a tropical cyclone (estimated to be of tropical storm intensity) could be identified within the central area of the low. Visible satellite pictures around 1500 UTC showed that an eye was forming and thus the inner system was near hurricane intensity at that time.
The cyclone had turned southeastward, then eastward and northeastward as it executed a counterclockwise loop. An Air Force Reserve Unit aircraft confirmed that the system was already of hurricane intensity when they investigated it around 0000 UTC on 2 November. That mission found maximum flightlevel (850 mb) winds of 86 knots, a 4°C air temperature rise in the center and an extrapolated central surface pressure of 981 mb. The radius of maximum winds was about 30 n mi, in contrast to the structure of the extratropical storm, which (according to synoptic reports and SSM/I data) had a more uniform area of gale force winds extending well over 300 n mi from its center with no clearly defined maximum wind radius.
Although the formation of a tropical cyclone in the center of a non-tropical low is rather unusual, it is not unprecedented. Hurricane Karl in November of 1980 developed within the center of a deep layer nontropical cyclone over the eastern Atlantic. A few other analogous cases can be cited. Since the central portion of these deep layer cyclones is a region of small temperature gradient and hence light vertical wind shear, then, given the presence of sufficiently warm sea surface temperatures, tropical cyclone development is allowable.
The cyclone accelerated northeastward, crossing the path previously traversed by the extratropical storm, and made landfall in Nova Scotia very near Halifax, around 1400 UTC on 2 November, as a rapidly weakening tropical storm. Dissipation occurred just to the north of Nova Scotia, about 10 hours after landfall.
b. Meteorological Statistics
Table 1 lists the best track positions and intensities every six hours for the extratropical low and the unnamed hurricane. This track is based on surface synoptic analyses using observations from ships and land stations, satellite images and Air Force Reserve unit aircraft reconnaissance fixes. Figure 1 shows the best track positions and Figure 2 and Figure 3 show the best track pressure and wind curves as a function of time, along with the observations upon which the curves are based. Maximum intensity (of the tropical cyclone) is shown to have been reached at 1800 UTC on I November, with a minimum central pressure (980 mb) just slightly lower than that given by reconnaissance 6 hours later. This is justified by the fact that the eye was better defined on satellite imagery near the earlier time. Several vessels passed close to the extratropical storm center on 30 October and reported winds of 50-60 knots. One ship (unidentified), noted in the previous section to have reported seas to 80 feet, also reported winds to 80 knots at 1200 UTC on 30 October while located several hundred miles northwest of the storm center. Comparison of this report with neighboring ship observations indicates that this wind speed is probably too high. A NOAA buoy, identification number 44011 located at 41.1°N 66.6°W, reported maximum sustained winds of 49 knots with gusts to 65 knots and a significant wave height of 39 feet near 1500 UTC on 30 October. Another NOAA buoy, number 44008 located at 40.5°N 69.5°W, reported maximum sustained winds of 53 knots with gusts to 63 knots and a significant wave height of 31 feet near 0000 UTC on 31 October. It is important to reiterate that all of the aforementioned strong wind speeds and high wave heights were associated with the extratropical stage of the system, not with the hurricane which formed later.
On 2 November, a Bahamian ship, the CFL Atlas, located about 110 miles southwest of the center of the tropical system, reported winds from 020° at 45 knots and pressure of 1006.5 mb at 0600 UTC.
By the time the system made landfall in Nova Scotia on 2 November, considerable weakening had taken place. The lowest reported pressure was 998.1 mb around 1430 UTC at Shearwater, near Halifax. According to the Maritimes Weather Service in Bedford, Nova Scotia, the Canadian Coast Guard at Chebucto Head (about 8 miles south of Shearwater) reported sustained winds from the southeast at 40 knots with gusts to 45 knots. An automated station at Beaver Island, located on the coast about 50 n mi to the eastnortheast of the landfall point, had maximum sustained winds from 180° at 36 knots with gusts to 41 knots and a minimum pressure of 1005.7 mb around 1600 UTC. It is conceivable that slightly higher winds could have occurred along the coast between this site and the landfall point.
Radar observations from Halifax airport showed some fairly well-defined curved rainbands over the western semicircle of the system, as it approached the area around 1000-1300 UTC on 2 November. These bands appeared to weaken with time, as the center neared the coast.
Only light rainfall was reported in association with the storm in the Halifax area, with totals of 0.27 inches at Bedford and 0.23 inches at Shearwater.
c. Casualty and Damage Statistics
It appears that all of the significant damage was attributable to the extratropical storm which preceded the unnamed hurricane. Further information concerning the effects caused by the extratropical storm will be contained in a forthcoming NOAA Natural Disaster Survey Report.
d. Forecast and Warning Critique
This hurricane was, to a great extent, a separate phenomenon from the strong extratropical storm which caused major coastal damage along the east coast from Florida through Canada, and even over portions of Atlantic shorelines of the Greater Antilles. By the time the tropical system had formed, the extratropical system was on the wane with conditions improving on the coasts. The damage that had been created by the extratropical system was continuing to receive major media attention at that stage and it was felt that naming the system (which clearly met all of the meteorological criteria to be designated as a hurricane) at that time would cause major confusion on the part of the media, Emergency Management officials and the public. Since the hurricane was expected to be short-lived and primarily a problem to marine interests, it was decided to handle all associated warnings in enhanced High Seas and Offshore and Coastal Waters Forecasts. This decision was made in consultation with the National Meteorological Center, the National Weather Service Forecast offices along the east coast, the U.S. Navy and the Atmospheric Environment Service Maritimes Weather Center in Canada. Based upon reports to date, this process provided all necessary warnings.
Figure 1. Best track positions for unnamed hurricane, 28 October - 3 November, 1991.
Figure 2. Best track minimum central pressure curve for unnamed hurricane, 28 October - 3 November, 1991
Figure 3. Best track maximum sustained 1-minute 10 meter wind speed curve for unnamed hurricane, 28 October - 3 November, 1991
Table 1. Preliminary best track, extratropical low and unnamed hurricane, 28 October - 3 November 1991. Date/Time
Stage Lat. (°N) Lon. (°W) 28/1800 44.0 59.0 1006 30 Extratropical 29/0000 43.0 57.5 999 40 " 0600 42.5 55.5 992 45 " 1200 41.0 56.0 990 50 " 1800 39.5 57.5 986 50 " 30/0000 39.0 59.5 981 55 " 0600 39.0 61.5 977 60 " 1200 39.0 63.5 972 60 " 1800 39.6 65.8 978 60 " 31/0000 40.0 68.5 982 55 " 0600 39.0 71.0 988 55 " 1200 37.7 71.5 992 50 " 1800 36.7 71.5 996 40 Subtropical Storm 01/0000 36.0 70.0 995 50 " 0600 36.2 68.5 993 55 Tropical Storm 1200 37.0 67.0 988 60 " 1800 38.2 66.5 980 65 Hurricane 02/0000 39.5 65.7 981 65 " 0600 41.6 64.7 988 60 Tropical Storm 1200 44.0 63.6 996 50 " 1800 46.3 62.6 1005 30 Tropical Depression 03/0000 48.5 61.0 Dissipated 01/1800 38.2 66.5 980 65 minimum pressure Landfall 02/1400 44.6 63.5 998 45 Halifax,