What caused the storm in the perfect storm movie?

The 1991 Perfect Storm, also known as the The No-Name Storm (especially in the years immediately after it took place) and the Halloween Gale, was a nor'easter that absorbed Hurricane Grace and ultimately evolved back into a small unnamed hurricane late in its life cycle. The initial area of low pressure developed off Atlantic Canada on October 29. Forced southward by a ridge to its north, it reached its peak intensity as a large and powerful cyclone. The storm lashed the east coast of the United States with high waves and coastal flooding before turning to the southwest and weakening. Moving over warmer waters, the system transitioned into a subtropical cyclone before becoming a tropical storm. It executed a loop off the Mid-Atlantic states and turned toward the northeast. On November 1 the system evolved into a full-fledged hurricane with peak winds of 75 miles per hour (120 km/h), although the National Hurricane Center left it unnamed to avoid confusion amid media interest in the predecessor extratropical storm. It later received the name "the Perfect Storm" (playing off the common expression) after a conversation between Boston National Weather Service forecaster Robert Case and author Sebastian Junger. The system was the fourth hurricane and final tropical cyclone in the 1991 Atlantic hurricane season. The tropical system weakened, striking Nova Scotia as a tropical storm before dissipating.

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