1991 Perfect Storm - Meteorological History

Meteorological History

The Perfect Storm originated from a cold front that exited the east coast of the United States. On October 28, the front spawned an extratropical low to the east of Nova Scotia. Around that time, a ridge extended from the Appalachian Mountains northeastward to Greenland, with a strong high pressure center over eastern Canada. The blocking ridge forced the extratropical low to track toward the southeast and later to the west. Hurricane Grace was swept aloft by its cold front into the warm conveyor belt circulation of the deep cyclone on October 29. The cyclone significantly strengthened as a result of the temperature contrast between the cold air to the northwest and the warmth and humidity from the remnants of Hurricane Grace. The low pressure system continued deepening as it drifted toward the United States. It had an unusual retrograde motion for a nor'easter, beginning a set of meteorological circumstances that occur only once every 50 to 100 years. Most nor'easters affect New England from the southwest.

While situated about 390 miles (630 km) south of Halifax, Nova Scotia, the storm attained its peak intensity with winds of up to 70 mph (110 km/h). The nor'easter reached peak intensity at approximately 12:00 UTC on October 30 with its lowest pressure of 972 millibars. The interaction between the extratropical storm and the high pressure system to its north created a significant pressure gradient, which created large waves and strong winds. Between the southern New England coast and the storm's center, the gradient was 70 mbar (2.1 inHg). A buoy located 264 miles (425 km) south of Halifax reported a wave height of 100.7 feet (30.7 m) on October 30. This became the highest recorded wave height on the Scotian Shelf, which is the oceanic shelf off the coast of Nova Scotia. East of Cape Cod, a NOAA buoy located at 41°06′N 66°36′W / 41.1°N 66.6°W / 41.1; -66.6 reported maximum sustained winds of 56 mph (90 km/h) with gusts to 75 mph (121 km/h), and a significant wave height (average height of the highest waves) of 39 feet (12 m) around 15:00 UTC on October 30. Another buoy, located at 40°30′N 69°30′W / 40.5°N 69.5°W / 40.5; -69.5, reported maximum sustained winds of 61 mph (98 km/h) with gusts to 72 mph (116 km/h) and a significant wave height of 31 feet (9.4 m) near 00:00 UTC on October 31.

Upon peaking in intensity, the nor'easter turned southward and gradually weakened; by November 1, its pressure had risen to 998 millibars (29.5 inHg). The low moved over warm waters of the Gulf Stream, where bands of convection around the center began to organize. Around this time, the system attained subtropical characteristics. On November 1, while the storm was moving in a counter-clockwise loop, a tropical cyclone had been identified at the center of the larger low. (Although these conditions are rare, Hurricane Karl during 1980 formed within a larger non-tropical weather system.)

By around 14:00 UTC on November 1, an eye feature was forming, and the tropical cyclone reached its peak intensity with maximum sustained winds of 75 mph (121 km/h); these estimates, combined with reports from an Air Force Reserve Unit flight into the storm and confirmation that a warm-core center was present, indicated that the system had become a Category 1 hurricane on the Saffir–Simpson Hurricane Scale. The hurricane accelerated toward the northeast and quickly weakened back into a tropical storm. It made landfall near Halifax, Nova Scotia, at 14:00 UTC on November 2 with sustained winds of 45 mph (72 km/h). While the storm was approaching the coast, weather radars depicted curved rainbands on the western side of the system. After crossing over Prince Edward Island, it fully dissipated late on November 4.