The 1991 Atlantic hurricane season was a below-average yet destructive Atlantic hurricane season, mainly due to Hurricane Bob and the Perfect Storm. There were 12 tropical cyclones this season, 8 of which became named storms. Among them, 4 become hurricanes, of which 2 became major hurricanes (Category 3 to 5 strength on the Saffir–Simpson scale). The season officially began on June 1, and ended on November 30.

Ana developed on July 2 off the southeast United States and dissipated without causing significant effects. Two other tropical storms in the season – Danny and Erika – did not significantly affect land. Danny dissipated east of the Lesser Antilles, and Erika passed through the Azores before becoming extratropical. In addition, there were four non-developing tropical depressions. The second depression of the season struck Mexico with significant accompanying rains.

The most significant storm of the season was Hurricane Bob, which at the time was among the ten costliest United States hurricanes. After brushing the Outer Banks of North Carolina and Long Island in New York, the hurricane made landfall on Rhode Island. It caused $1.5 billion in damage (1991 USD), mostly in Massachusetts, and 17 fatalities. The strongest hurricane of the season was Claudette, which reached peak winds of near Bermuda. It passed near the island but did not cause any damage. Fabian was the only tropical storm to move over or near Cuba or Florida, producing heavy rainfall but no damage. Hurricane Grace, the final named storm of the season, provided the energy that led to the development of a powerful nor'easter known as the Perfect Storm. Originating from an extratropical storm, the Perfect Storm intensified while moving westward toward New England, leaving $200 million in damage and causing coastal damage from Puerto Rico to Florida and northward through Canada. It later transitioned into a hurricane over the Gulf Stream, finally dissipating over Nova Scotia on November 2.

Seasonal forecasts +Predictions of tropical activity in the 1991 seasonSourceDateNamedstormsHurricanesMajorhurricanesRefCSU April <10 <6 Unknown WRC April 9–10 6 5 CSU June 8 4 1 CSU August 7 3 Unknown Record high activity 30 15 7 (Tie)Record low activity 1 0 (tie) 0 Actual activity 8 4 2

Before the start of the season, hurricane expert William M. Gray released his forecast for the year's activity, a yearly practice that he began in 1984. In early April, Gray anticipated a "mild" season with fewer than ten tropical storms, of which less than six would become hurricanes. Later that month, the Weather Research Center forecast ten named storms and six hurricanes, of which five would become major hurricanes. while three would hit the United States. In early June, Gray released an updated report that predicted the formation of eight tropical storms, four hurricanes, and one major hurricane. The revised June total was very close to the actual season activity, with the exception of forecasting one fewer major hurricane. However, a later revision in August incorrectly anticipated less activity, when Gray predicted seven storms and three hurricanes.

Seasonal summary For a chronological guide, see Timeline of the 1991 Atlantic hurricane season

Overall activity in 1991 was below normal. This was partially due to decreased tropical cyclogenesis from African tropical waves, which are troughs that move across the ocean with associated convection. In most seasons, the majority of storms develop from tropical waves. Of the season's twelve tropical cyclones, only five originated from tropical waves; in addition, only three of the eight tropical storms were from tropical waves, and none had the characteristics of a Cape Verde-type hurricane. From late April to late November, there were 73 tropical waves that exited the west coast of Africa. The total was higher than average, although many of them were poorly defined and had little thunderstorm activity. The waves traversed the Atlantic Ocean further south than normal, typically not becoming convectively active until moving across northern South America. Cyclogenesis was also suppressed by higher than normal wind shear, as well as low rainfall amounts across the Sahel. There were also no tropical storms in the Gulf of Mexico for only the third time in the 20th century, after 1927 and 1962. The season produced twelve tropical depressions, which was the lowest in five years. The eight tropical storms was the lowest amount in four years. Four of the storms developed into hurricanes, although for the first time in over 24 years, none of the hurricanes originated from tropical waves.

The season's activity was reflected with a low cumulative accumulated cyclone energy (ACE) rating of 36. ACE is, broadly speaking, a measure of the power of the hurricane multiplied by the length of time it existed, so storms that last a long time, as well as particularly strong hurricanes, have high ACEs. ACE is only calculated for full advisories on tropical systems at or exceeding or tropical storm strength. Although officially, subtropical cyclones are excluded from the total, the figure above includes periods when storms were in a subtropical phase.

Systems Tropical Storm Ana

The first storm of the season was Ana, which originated from a cold-core low that persisted east of Jacksonville, Florida, by June 25. The system moved in a clockwise motion around an anticyclone located over Florida. The cold-core low gradually developed to the surface, and on June 29, a low pressure area formed within a surface trough over the Bahamas. It moved westward across southern Florida, dropping heavy rainfall along its path. Punta Gorda recorded a statewide peak of of precipitation. The low moved northwestward and later curved northeastward, exiting into the Atlantic Ocean near Saint Augustine by early on July 2. Although it was initially disorganized as it moved offshore, the convection quickly developed in organization, and by 1800 UTC that day it had developed into Tropical Depression One about south of Charleston, South Carolina.

As the depression moved northeastward parallel to the southeast United States coastline, it dropped light rainfall, although portions of Virginia recorded more than . Late on July 3, a buoy reported sustained winds of over a period of eight and a half minutes. As a result, the National Hurricane Center upgraded the depression to Tropical Storm Ana. The storm accelerated east-northeastward toward a stalled frontal zone, entering an area of increased wind shear. Despite these hostile conditions, Ana strengthened slightly, reaching peak winds of . Moving over cooler waters and interacting with the frontal zone, the circulation became broad as the thunderstorms diminished. On July 5, Ana became extratropical in the northern Atlantic Ocean about to the south of Cape Race.

Tropical Depression Two

A tropical wave emerged off the coast of Africa on June 20, and no significant development occurred until it became Tropical Depression Two in the western Gulf of Mexico on July 5. On its first advisory, a tropical storm watch was issued for from Baffin Bay, Texas, southward to Tampico, Tamaulipas. Nearing the coast of Mexico, the depression attained its peak intensity with winds of and a minimum pressure of . Failing to intensify further, Tropical Depression Two made landfall near La Pesca, Tamaulipas, on July 6. The National Hurricane Center issued the final advisory on July 7, although the circulation persisted until July 9 southwest of Texas. The depression had only minor impacts in Mexico and Texas, other than rainfall. Precipitation was heaviest in the state of San Luis Potosí, where the rainfall peaked at in Tamazunchale.

Hurricane Bob

Hurricane Bob originated from a decaying cold front, developing into a tropical depression early on August 16 near the Bahamas. It produced an area of organized convection, and the depression intensified into Tropical Storm Bob roughly 18 hours after forming. It gradually organized over the Gulf Stream, and based on reports from the Hurricane Hunters, Bob attained hurricane status on August 17. Shortly thereafter, the hurricane began to turn towards the north-northeast in response to a subtropical ridge over the Atlantic and the trough over the southeastern United States. After further intensification off the Carolinas, Bob reached peak winds of to the east of Virginia on August 19, making it a major hurricane. Significantly cooler sea surface temperatures resulted in weakening. After brushing Long Island, the center of Bob moved over Block Island, Rhode Island. About 40 minutes later it struck Newport, Rhode Island, with winds of , making it a Category 2 hurricane. It rapidly weakened to tropical storm intensity while moving through the remainder of New England, hitting Rockport, Maine, early on August 20. After crossing New Brunswick, Bob became extratropical in the Gulf of St. Lawrence and lasted another nine days before dissipating west of Portugal.

The hurricane first affected the Carolinas, spawning four confirmed and nine unconfirmed tornadoes in North Carolina. One person each died in North and South Carolina, and about 10% of houses in the Outer Banks sustained minor roof damage. As the storm moved up the coast, heavy rain fell on the western side of the center. High winds left 300,000 people without power on Long Island. In neighboring Connecticut, strong winds downed trees across the region, with damage heaviest in the southeastern portion near the coast. Damage was heaviest as Bob made its final landfall, with wind gusts of reported on Block Island, Rhode Island. The hurricane produced extensive beach erosion which destroyed coastal roads in the state. Monetary damage was greatest in Massachusetts, and along Bob's path through southeastern New England more than 60% of people were left without power. High tides and strong winds destroyed boats and houses along the Massachusetts coastline. The heaviest rainfall from the hurricane fell at the Portland International Jetport in Maine, where fell during its passage. Across the United States, damage totaled $1.5 billion (1991 USD), including over $1 billion in Massachusetts. The high damage total made Bob among the ten costliest U.S. hurricanes at the time. In addition, there were 15 fatalities in the country. In Canada, high waves killed two people. In Fredericton, New Brunswick, tropical storm-force winds downed trees and power lines.

Tropical Depression Four

One of the few vigorous tropical waves of the season emerged from the western coast of Africa with a large area of convection in late August. On August 24 it developed into a tropical depression near Cape Verde. Upon first forming, the depression had a circular area of convection near the center. It was initially well-organized, but the depression was not expected to intensify due to marginal water temperatures; tropical cyclones generally require warm waters to develop. By August 25, the system lost much of its deep convection, and on August 26 the depression dissipated to the west-southwest of the Cape Verde islands.

Tropical Depression Five

Around the same time as the previous system dissipated, another tropical wave moved off the coast of Africa on August 26. On August 28 it formed into a tropical depression about southwest of Cape Verde. Upon developing, the depression had a small area of convection with a spiral rainband, and the NHC anticipated slow strengthening to tropical storm status. With a ridge to the north, the depression maintained a general westward track. Ultimately, the depression failed to organize significantly. By August 29, it had a broad and poorly organized circulation with only scattered convection. Due to cool water temperatures, the system was unable to maintain deep convection, and on August 31 the depression degenerated into a tropical wave about east of the Lesser Antilles.

Hurricane Claudette