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A “Tempestade Perfeita” Características e Evolução 25 outubro - 2 novembro 1991. Lecture 15 A Case Study. The following description of the event is taken from the National Climatic Data Center (NCDC) web site at:
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A Case Study
The following description of the event is taken from the National Climatic Data Center (NCDC) web site at:
An enormous extratropical low is creating havoc along the entire Eastern Atlantic Seaboard in this infrared image at 1200 UTC (0700 EST) on 30 October 1991. Labeled the "perfect storm" by the National Weather Service, the storm sank the sword-fishing boat Andrea Gail, whose story became the basis for the best-selling novel "The Perfect Storm" by Sebastian Junger. A little-known and bizarre ending came to this monster, which came to be known as the Halloween Storm.
As the low pressure continued to deepen on 29 October, Grace became only a secondary contributor to the phenomenal sea conditions which developed over the Western North Atlantic during the next few days. At 1800 UTC on the 29th, the vigorous cold front from the extratropical low undercut and quickly destroyed Grace\'s low-level circulation east of Bermuda (Note the red and yellow area east of Charleston, SC in Figure 1).
Extratropical low center
Remnants of Grace
Instability clouds due to cold air advecting over warm waters
Figure 1 Hurricane Grace being absorbed by extratropical low. IR: 18 UTC 29 October 1991
The remnant mid- and upper-level moisture from Grace became caught up in the outer part of the extratropical storm center\'s circulation, far from the storm\'s center. By the next day these remnants had become indistinguishable. The center of the extratropical low drifted southeastward and then southwestward, while deepening. It reached peak intensity of 972 mb and maximum sustained winds of 60 knots at 1200 UTC on 30 October, when it was located about 340 nautical miles south of Halifax, Nova Scotia. After reaching peak intensity on 30 October, the low retrograded southwestward on 31 October (Note swirl off Delmarva Peninsula in Figure 2), and then southward as the central pressure rose to about 998 mb by 0000 UTC on 1 November.
Figure 2. Weakening Halloween Storm. IR: 12 UTC 31 October 1991
North Carolina\'s coast was lashed with occasional winds of 35 to 45 mph for five consecutive days. In New England on 30-31 October, wind gusts of above hurricane force pounded the Massachusetts coastline. Representative peak gusts included: 78 mph at Chatham NWS, 74 mph at Thatcher Island, 68 mph at Marblehead, 64 mph at Blue Hill Observatory (all in Massachusetts) and 63 mph at Newport, RI. Even more damaging were the heavy surf and coastal flooding caused by the tremendous seas and high tides caused by the long over-water fetch length and duration of the storm. Waves 10 to 30 feet high were common from North Carolina to Nova Scotia. High tides reached from three to seven feet above average. In New Jersey, the greatest tidal departures of winter storms of record occurred during this event, with tide heights exceeded only by the Great Atlantic Hurricane of 1944. In Delaware, Maryland, and Virginia, the highest water levels were comparable to those of the nor\'easter of March 1962. A record high tide of 7.8 feet occurred at Ocean City, MD on the 30th, which eclipsed the old record of 7.5 feet recorded during the March 1962 storm. In Massachusetts, 25-foot waves reached the shoreline atop high tides already 4 feet above average. At Boston, the tide reached 14.1 feet above mean low water or about 1 foot less than the tides associated with the "Blizzard of 1978." Elsewhere treacherous swells, surf, and associated coastal flooding occurred along portions of the Atlantic shoreline extending from Puerto Rico and the Dominican Republic, to the Bahamas, along the U.S. and Canada and in Bermuda.
Figure 3. Subtropical Storm. Vis: 18 UTC 31 October
By 0600 UT 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 (See Figure 4). Later it intensified to a hurricane.
Figure 4. Tropical Storm Formation. IR: 06 UTC 1 November
Deep 500-hPa trough over western North America contributed to strengthening 500-hPa ridge and strong surface high pressure system over eastern Canada. A trough deepened over the Maritime Provinces of Canada.
Deep 500-hPa trough over western North America contributed to strengthening 500-hPa ridge and strong surface high pressure system over eastern Canada. A trough deepened over the North Atlantic, near the Maritime Provinces of Canada. Hurricane Grace was beginning to move northeastward in response to the developing trough to the north.
A high amplitude flow pattern over North America featured a deep trough over western portions of the continent and a strong 500-hPa ridge and strong surface high pressure system over eastern Canada. A trough continued to deepen over the North Atlantic south of the Canadian Maritime Provinces.
Hurricane Grace being absorbed into the extratropical cyclone farther north.
A trough and extratropical surface low continued to deepen over the North Atlantic south of the Canadian Maritime Provinces.
Remnants of Hurricane Grace, as it loses its tropical identity
The 500-hPa trough over the North Atlantic become cut-off from the main flow (farther north), and the extratropical surface low reaches maximum intensity south of the Canadian Maritime Provinces.
The 500-hPa trough over the North Atlantic is completely cut-off from the main flow, and the 500-hPa low and surface cyclone drift slowly west-southwestward.
The 500-hPa low and surface cyclone continue to drift slowly west-southwestward.
The 500-hPa low and surface cyclone continue to drift slowly west-southwestward. Surface low begins to display tropical characteristics.
The 500-hPa low and surface cyclone remain nearly stationary. Surface low becomes a hurricane (unnamed).
Enhanced VIS 1906 UTC 1 November 1991
Region of storm development
Period of Storm
Slow eastward phase propagation
Veja imagens de satélite IR nos slides seguintes.
A evolução sinótica desta tempestade é mostrada no slide seguinte.
Nesse dia (27 de março), não há nenhuma indicação clara de uma estrutura de núcleo quente nessa tempestade.
Nota: a resolução baixa dos dados de reanálise pode não ser capaz de reproduzir Catarina.