The 1984 Atlantic hurricane season was the most active since 1971, though the season was below average in hurricanes and major hurricanes. It officially began on June 1, 1984, and lasted until November 30, 1984. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. Although the first tropical depression developed northeast of the Bahamas on June 11, no tropical cyclones intensified into a tropical or subtropical storm until August 19, an unusually late date. The final system, Hurricane Lili, dissipated near the north coast of the Dominican Republic on December 24. The 1984 season was an active one in terms of named storms, but most of them were weak and stayed at sea. Most of the cyclones tracked through the northwest subtropical Atlantic west of the 50th meridian to near the Eastern coast of the United States between mid-August and early October.
The most intense tropical cyclone of the season was Hurricane Diana, which peaked as a Category 4 hurricane on the Saffir–Simpson scale. Meandering erratically offshore the Carolinas, Diana weakened to a Category 1 hurricane before making landfall in North Carolina. Diana was the first hurricane to strike a nuclear power plant, but without incident. Hurricane Klaus, the most damaging storm of the season, caused $157 million (1984 USD) in damage in the Lesser Antilles, with a vast majority of that total in the British Virgin Islands. Also of note, Tropical Storm Fran became the second-deadliest cyclone in the history of the Cabo Verde Islands, killing 29 to 32 people. Overall, the systems of the 1984 season caused about $233.74 million in damage and 41-44 fatalities. Unusually, no hurricanes developed directly from tropical waves in 1984, which usually are the source of the strongest storms in an Atlantic hurricane season.
Seasonal forecasts
Forecasts of hurricane activity are issued before each hurricane season by noted hurricane experts such as Dr. William M. Gray and his associates at Colorado State University (CSU). A normal season, as defined by the National Oceanic and Atmospheric Administration (NOAA) in the period from 1981 to 2010, has approximately 12 named storms, with 6 of those reaching hurricane status. About 3 hurricanes strengthen into major hurricanes, which are tropical cyclones that reach at least Category 3 intensity on the Saffir–Simpson scale.
On July 23, 1983, forecasters at CSU predicted an above-average season in 1984 with a total of 11 tropical storms developing, 8 of which would reach hurricane status, under the assumption that sea-level pressures over the Caribbean Sea and Gulf of Mexico would be normal. Early in 1984, the Weather Research Center (WRC) forecast on the other hand called for a below-average season with seven named storms, with four of those strengthening into a hurricane. on May 24, 1984, forecasters at CSU revised their prediction downwards to a total of 10 tropical storms, 7 of which would reach hurricane status, as the easterly Quasi-biennial oscillation cycle had lasted longer than expected and the previously expected La Niña event in 1984 had yet to form and was now not expected until the autumn or winter, On the other hand, sea-level pressures over the Caribbean Sea and Gulf of Mexico were more favorable than expected. These numbers were unchanged in CSU's next season outlook, issued on July 30. None of these predictions included a forecast for the number of major hurricanes. Ultimately, the predictions issued by CSU proved to be too high for hurricanes and too low for tropical storms, with 13 subtropical or tropical storms forming in 1984 and 5 of those reaching hurricane status. CSU attributed the overforecast of hurricanes to the easterly QBO cycle not changing during the hurricane season as was expected and the sea-level pressures from August to October being less favorable than expected, and the underforecast of tropical storms to the unusual amount of formation in the subtropics from mid-latitude residual frontal activity.
Seasonal summary
The 1984 Atlantic hurricane season officially began on June 1 and ended on November 30. On June 11, the first tropical depression developed northeast of the Bahamas. However, no tropical cyclones attained tropical or subtropical storm status until August 19, an unusually late date. Of the 20 cyclones that formed in the Atlantic basin in 1984, 13 strengthened into a tropical or subtropical storm, slightly above-average and the highest number in a season since 1971. However, a below-average number intensified into a hurricane or major hurricane, five and three, respectively. Six storms during the season had subtropical characteristics at some point in their track, those being Subtropical Storm One, Tropical Storm Cesar, Hurricane Hortense, Hurricane Josephine, Hurricane Klaus, and Hurricane Lili. Unusually, no hurricanes developed from tropical waves in 1984, which usually are the source of the strongest storms in an Atlantic hurricane season. The last storm of the season, Hurricane Lili, dissipated on December 24 near the north coast of the Dominican Republic. Collectively, the cyclones of the 1984 season caused about $233.74 million in damage and 41-44 fatalities.
Tropical cyclogenesis began with two short-lived tropical depressions in June, the first of which developed on June 11. After the second one dissipated on June 20, Tropical Depression Three did not form until July 24, more than a month later. August featured five cyclones, a tropical depression, Subtropical Storm One, and tropical storms Arthur, Bertha, and Cesar. Seven systems developed in September, including a tropical depression and six named storms, Diana, Edouard, Fran, Gustav, Hortense, and Isidore. Diana became the most intense tropical cyclone of the season, peaking as a Category 4 hurricane with maximum sustained winds of 130 mph (215 km/h) and a minimum atmospheric pressure of 949 mbar (28.0 inHg). A tropical depression and Hurricane Josephine formed in October. Similarly, another tropical depression and Hurricane Klaus developed in November. The season's final system, Hurricane Lili, developed on December 12 and dissipated on December 24.
The season's activity was reflected with a cumulative accumulated cyclone energy (ACE) rating of 84, which is classified as "near normal". 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. It is only calculated for full advisories on tropical systems at or exceeding 39 mph (63 km/h), which is the threshold for tropical storm strength.
Systems
Tropical Depression One
By June 11, an upper-level low caused thunderstorm development off the Florida coast, which caused the formation of a tropical depression. Moving westward, the depression moved into St. Augustine, causing a total of 5.02 in (128 mm) of rainfall at Jacksonville Beach, Florida, as its main thunderstorm activity was concentrated north of the center. It dissipated as a tropical cyclone on June 14 while moving through the Florida panhandle. The small remnant low continued moving westward inland of the Gulf coast, causing occasional redevelopment of thunderstorm activity as the system moved into Louisiana, before both the thunderstorm activity and low-pressure area dissipated by June 17.
Tropical Depression Two
An upper-level low-pressure area traversing the southern Gulf of Mexico spawned convective activity over the Isthmus of Tehuantepec on June 16. This convective area waxed and waned somewhat in intensity, until becoming a larger disturbance on June 18. A surface low soon formed, and around 12:00 UTC that day, the system developed into a tropical depression over the Bay of Campeche. With vertical wind shear preventing significant further intensification, the depression made landfall near Tampico, Tamaulipas, with winds of 35 mph (55 km/h). The depression quickly dissipated over the mountainous terrain of eastern Mexico. The cyclone and its precursor dropped heavy rainfall in some areas, including a peak total of 11.43 in (290 mm) of precipitation in San Lucas Ojitlán, Oaxaca.
Tropical Depression Three
A tropical depression formed about 800 mi (1,285 km) east of the Windward Islands on July 24. Moving west-northwestward, the depression passed between Martinique and Saint Lucia early on the following day. On the latter, the storm dropped up to 8 in (200 mm) of precipitation. The Castries River overflowed its banks, washing away three homes in the eastern section of Castries. Two commercial fisherman were reported missing. Barbados recorded up to 6 in (150 mm) of rainfall in association with the system. The depression entered the Caribbean Sea and failed to intensify further, dissipating about halfway between the Dominican Republic and Venezuela late on July 26.
Subtropical Storm One
A weak frontal trough generated a low-pressure system that organized into a subtropical depression north of Bermuda on August 18. The depression headed northeast and strengthened to a subtropical storm. It is believed to have merged with a front on August 21. The history of Subtropical Storm One is not entirely certain, as satellite images were largely unavailable due to a failure of the VISSR unit on GOES EAST (then GOES-5), and this system remained at the fringe of the GOES WEST and Meteosat throughout its existence. Wind gusts up to 65 mph (105 km/h) were reported on the southwest coast of Newfoundland. In addition, a weather office on the island reported rainfall at 2.1 in (53 mm).
Tropical Storm Arthur
A well-defined tropical wave emerged into the Atlantic from the west coast of Africa on August 23. Moving westward and later northwestward, the system remained to the south of a persistent shearing pattern that inhibited the development of several tropical waves. A reconnaissance aircraft flight indicated that a tropical depression formed late on August 28 roughly 700 mi (1,100 km) east of Trinidad. On the next day, another reconnaissance flight recorded tropical storm conditions, and thus, the depression intensified into Tropical Storm Arthur. The cyclone attained its peak intensity several hours later with maximum sustained winds of 50 mph (85 km/h) and a minimum barometric pressure of 1,004 mbar (29.6 inHg). Arthur was downgraded to a depression on September 1 after being negatively impacted by vertical wind shear, and dissipated on September 5 about halfway between the Bahamas and Bermuda. Despite its close proximity to the Lesser Antilles, Arthur caused no significant impact on land as it was a tropical depression at the time.
Tropical Storm Bertha
On August 26, a tropical wave emerged into the Atlantic from the west coast of Africa. Tracking westward, the wave developed into a tropical depression about 1,170 mi (1,885 km) west-southwest of the southwesternmost islands of Cabo Verde and in close proximity to the east of Arthur. A reconnaissance flight into the depression on August 31 indicated that it strengthened into Tropical Storm Bertha. Later that day, Bertha peaked as a minimal tropical storm with maximum sustained winds of 40 mph (65 km/h) and a minimum barometric pressure of 1,007 mbar (29.7 inHg). The system moved northwestward due to a weakening high pressure ridge to the north. Based on observations from reconnaissance flights on September 1, Bertha was downgraded to a tropical depression. On September 2, Bertha turned north-northeastward into response to an approaching cold front. The cold front then eroded the high pressure ridge, causing the cyclone to accelerate northeastward. Bertha later merged with the cold front on September 4.
Tropical Storm Cesar
The National Hurricane Center detected a low-level cloud circulation east of Jacksonville, Florida, on August 30. Moving north to northeastward due to a high pressure ridge to the east and a surface high pressure cell to the west, the system organized into a subtropical depression early on August 31 approximately 200 mi (320 km) northwest of Bermuda. Based on a ship report of sustained winds of 40 mph (65 km/h), the subtropical depression transitioned into a tropical system and intensified into a tropical storm, receiving the name Cesar. On September 2, Cesar peaked with winds of 60 mph (95 km/h) and a minimum atmospheric pressure of 994 mbar (29.4 inHg), but soon merged with an extratropical low east of Newfoundland.
Tropical Depression Seven
A tropical wave moved across Central America into the far eastern north Pacific Ocean by August 28. The system moved westward with no signs of development until September 1, when an upper-level low to its north across the Gulf of Mexico caused an area of thunderstorms to form just south of the Mexican coastline. An upper trough developed across the southern Plains of the United States, which slowly lured the northern portion of this increasingly large disturbance northward through the Mexican Isthmus. The southern portion moved westward, developing into Hurricane Marie. For a short while, Marie acted as a source of vertical wind shear from the west for this system, halting further development.
By September 6, the disturbance had emerged into the southwest Gulf of Mexico and consolidated into a smaller system which had enough organization to be classified as a tropical depression, the seventh of the season. The depression moved north-northwest into northeast Mexico on the afternoon of September 7, dissipating completely on September 8. Heavy rains fell over parts of Mexico, with a peak total of 34.65 in (880 mm) in Chicontepec de Tejeda, Veracruz. This precipitation contributed to ongoing floods, especially in Tamaulipas and Veracruz.