Image at left:  An unnamed hurricane at the center of the dying Halloween Storm of October, 1991 has reached its peak intensity in this visible GOES 7 image from 1801 UTC on November 1, 1991. The storm was packing sustained winds of 65 knots and the minimum central pressure was 980 millibars. The counterclockwise swirl of the larger cyclone is still very evident in this view of the northwest Atlantic and Eastern U.S. The storm was moving rapidly northeastward and hit Nova Scotia as a weakened tropical storm the next day. It was the eighth hurricane (and 34th tropical cyclone of either tropical storm or hurricane intensity) which remained unnamed since the official naming of Atlantic tropical cyclones began in 1950.

A large and very powerful extratropical cyclone pounded the Western Atlantic in the closing days of October 1991. This storm was so severe that it became known as the Halloween Storm and was called a "perfect storm" by the National Weather Service. One of it's many casualties was the swordfishing boat the Andrea Gail, which sank with all hands on board and became the basis for Sebastain Junger's novel "The Perfect Storm."

The Halloween Storm had weakened and drifted southward to near 36.7 N, 71.5 W by 18:00 UTC on October 31. (See Figure 1). At that time, the central pressure had risen to 996 mb and winds had decreased to 40 knots. With the low centered over warm waters of the Gulfstream, subtropical characteristics were acquired. By this time, there was no longer a well-defined baroclinic zone across the area (i.e., temperatures were nearly uniform in all directions), but the center was not yet under the central dense overcast feature of tropical low pressure centers.

Subtropical Phase - Oct 31 18:01 Figure 1

Tropical Storm - Nov 1 06:01 Figure 2

By 0600 UTC on November 1, the storm was in the process of making a counter-clockwise loop and central convection had increased to the point where a tropical cyclone (of tropical storm intensity) could be identified at the center (See Figure 2). By nine hours later, at 1400 UTC, an eye was forming and the storm was near hurricane intensity. Maximum intensity of the storm was reached at 1800 UTC (See Main Gallery image above) when a minimum central pressure of 980 millibars with sustained winds of 65 knots was reached. An Air Force Reserve Unit aircraft investigated the hurricane around 0000 UTC on November 2 and found maximum flight level (850 mb) winds of 86 knots, a 4 degree C rise in air temperature at the center, and an estimated sea level pressure of 981 millibars. The radius of maximum winds was about 30 nautical miles. (In contrast, and typical of older massive extratropical storms, the Halloween storm had a more uniform area of gale force winds extending well over 300 n mi from its center with no clearly defined wind radius.) With the warm core finding and other data, this flight confirmed that the center of the Halloween Storm was now a hurricane!

Though the formation of a hurricane in the center of a large extratropical low is unusual, it has happened several times before. Hurricane Karl formed in the center of a deep layer non-tropical cyclone in the central Atlantic on November 25, 1980 and was of hurricane strength until November 27, 1980. By their very nature, the centers of deep layer cyclones are areas of small temperature gradients and light vertical wind shear. Given sufficient heating from the sea surface below, tropical cyclone formation within the larger low pressure center is possible.

Nova Scotia Landfall - Nov 2 14:01 Figure 4

Zoom-in View of Landfall - Nov 2 14:01 Figure 5

After its formation, the unnamed hurricane began speeding to the northeast. It passed over the same area where the extratropical cyclone had passed two days earlier (before the formation of the hurricane). By 0600 UTC November 2, six hours after the reconnaissance flight, the storm had weakened to tropical storm status and central pressure had risen to 988 millibars. Even so, the SFL Atlas, A Bahamian ship 110 miles southwest of the center, reported winds of 45 knots. Accelerating to the northeast, the storm made landfall near Halifax, Nova Scotia as a tropical storm around 1400 UTC (See Figures 4 and 5). The lowest reported pressure on land was 998.1 millibars at Shearwater, near Halifax, and the Canadian Coast Guard station at Chebucto Head (8 miles south of Shearwater) reported sustained winds of 40 knots with gusts to 45 knots. Radar showed curved rain bands on the western side of the system, which weakened as they approached the coast. Bedford reported only 0.27 inches and Shearwater a mere 0.23. No damage was reported from the unnamed tropical cyclone and it dissipated 10 hours after landfall.

The National Hurricane Center (now called the Center for Tropical Prediction) began naming tropical storms and hurricanes in 1950. The hurricane which developed at the center of the dying Halloween Storm met all meteorological criteria to be designated as a hurricane, and a track for it is shown on the National Hurricane Center's 1991 "North Atlantic Hurricane Tracking Chart." So why did the storm remain unnamed?

At the time of the hurricane, news media attention was still focused on the massive damage from Maine to Florida caused by the slowly dying Halloween (or "Perfect") Storm. It was felt that naming the hurricane 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 of concern to maritime interests, it was decided to leave the storm unnamed. All associated warnings were handled through enhanced High Seas and Offshore and Coastal Waters Forecasts. The decision was made jointly by NOAA's National Meteorological Center, selected National Weather Service Forecast Offices, the U.S. Navy, and the Maritimes Weather Center of the Atmospheric Environment Service of Canada. The unnamed hurricane brought a bizarre ending to one of the most massive Atlantic storms of record.

Extensive extratropical cyclones such as the Halloween Storm usually cover larger areas than even the largest hurricanes or typhoons. Sometimes, as was the case with Hurricane Grace's interaction with the Halloween Storm, a hurricane will be absorbed by a larger extratropical storm and the hurricane's energy becomes a part of the larger storm's circulation. This sometimes leads to a dramatic energizing of the larger storm, often with disastrous consequences. Such was the case with Hurricane Hazel in October, 1954 which first struck the Carolinas as a Category 4 hurricane, and then merged with a low pressure center to become a monstrous extratropical storm which devastated Toronto, Canada causing 76 deaths. A cold front associated with the Halloween Storm overtook Grace at 1800 UTC on the 29th and the low pressure center absorbed the remnants of the hurricane. Similar absorption of hurricanes by extratropical storms is a common occurrence, and such is the end of many tropical cyclones. Still other tropical cyclones wander over land or over cooler ocean waters and slowly fade away or loose their tropical characteristics.

Unusual Endings to Extratropical Storms

Though hurricanes are often absorbed by major extratropical storms, a reverse process can occur, i.e., under the right conditions, a weak extratropical cyclone can remain over warm ocean water long enough that the storm becomes "warm core" and evolves into a tropical storm, and later possibly a hurricane. If this occurs, it is often in the early part of the hurricane season. A favored place is off the Carolina or Georgia coastlines in July or early August when weak low pressure centers sometimes stall offshore and later become tropical storms or hurricanes. The process can occur at any time, however. Hurricane Grace, which was absorbed by the Halloween Storm, had similar origins in that it was initially subtropical in character and formed primarily from a mid-level low pressure center which extended down to the warm ocean surface.

An even rarer event is the conversion of the center region of a major extratropical storm into one with tropical characteristics. As with most major storms, the Halloween Storm underwent a complete occlusion process by which surface frontal boundaries underneath the spinning vortex center disappear. The central portion of such a deep-layer cyclone at this time is relatively slow moving; the surface air temperature is relatively uniform with small temperature gradients; and there is light vertical wind shear. If the center of the low is sitting over a very warm ocean water, the air in the center of the low can eventually become warm core at all levels and a tropical storm can develop. This process occurred south of Newfoundland when the Halloween Storm drifted over the Gulfstream. Later it became a true hurricane in every sense of the word.

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.

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.

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.

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

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Table 1. Preliminary best track, extratropical low and unnamed hurricane, 28 October - 3 November 1991.

Date/Time (UTC)	Position		Pressure (mb)	Wind Speed (kt)	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
minimum pressure
01/1800	38.2	66.5	980	65	minimum pressure
Landfall
02/1400	44.6	63.5	998	45	Halifax, Nova Scotia

 

Rainfall Totals for the Perfect/Halloween Storm of 1991