The 2004 Planet Z Atlantic hurricane season was the period in 2004 during which tropical cyclones formed in the Atlantic Ocean on Planet Z. The season officially started on June 1 and ended on November 30, dates which conventionally delimit the period of each year when most tropical systems form in the basin. However, the season effectively started in mid-May due to the early formation of Hurricane Alan. The season's activity was well above the long-term average; 19 named storms formed, with 13 becoming hurricanes and 6 becoming major hurricanes (Category 3 or higher).
The 2004 season was a disastrous one, primarily due to Hurricanes Danielle, Jane, Sandy, and Toby. Danielle struck St. Vincent, Jamaica, and eastern Texas at Category 3, 4, and 5 intensity, respectively, killing 187 people and causing $27 billion (2004 USD; $31.5 billion 2011 USD) in damage; also, when it peaked with winds of 190 mph (305 km/h) and a minimum central pressure of 891 mbar (hPa), it became the third strongest hurricane ever recorded in the Atlantic basin on Planet Z, only behind Percy of 1985 and Homer of 1995. Hurricane Jane crawled over Puerto Rico and Hispaniola, resulting in catastrophic flooding which killed 952 people across the two islands. Hurricane Sandy proved to be the Canary Islands' worst storm in decades when flooding and mudslides caused 233 fatalities — 189 of which were in a single mudslide. Lastly, Hurricane Toby was an even greater disaster for Haiti than Jane was, slaughtering nearly 4,000 people there when it stalled over the northern part of the country for two full days.
In addition to the four aforementioned storms, numerous other notable storms occurred this year. Hurricane Alan prompted gratuitous use of profanity on part of National Hurricane Center (NHC) forecasters when it reached major hurricane status in May, while Hurricane Charley killed 16 people and caused $980 million (2004 USD; $1.14 billion 2011 USD) on its track from the southern Bahamas and into northeastern Mexico. Tropical Storm Gary survived the trek across Central America, eventually becoming Category 3 Hurricane Irma in the northeastern Pacific Ocean; Hurricane Nina quickly became a Category 2 storm and made landfall in northeastern Mexico at peak intensity; and Tropical Storm Orville, after killing 10 people in Florida and neighboring states due to flooding, merged with a non-tropical low to cause high winds, flooding rains, and 33 fatalities in New England and the Mid-Atlantic states.
Midway through the second week of May, a low pressure center developed approximately 600 miles northeast of the Leeward Islands. Conditions in the area were abnormally conducive for tropical cyclone development for the month of May, and at around midday on May 14, the low acquired sufficient organization to be classified as a tropical depression. Despite sea surface temperatures (SST) of 24°C (75°F), which is quite a bit below the 26.5°C/79.7°F usually required for tropical cyclone development, the depression took advantage of atmospheric moisture and light vertical wind shear; at midnight UTC on May 15, 12 hours after formation, the depression strengthened into a tropical storm, thus earning it the name Alan.
Forecasters expected Alan to remain a weak storm, and to dissipate quickly. However, Alan defied these forecasts and strengthened at a steady pace, becoming the season's first hurricane 24 hours after becoming a tropical storm. At this time, Alan entered an area of higher-than-normal atmospheric pressure; as such, when Alan strengthened from 65 mph (100 km/h) to 75 mph (120 km/h) to become a hurricane, its barometric pressure rose from 999 mbar to 1001 mbar. Alan's barometric pressure would remain higher than normal for the remainder of the storm's existence; this was evidenced by the fact that, when Alan peaked as a major hurricane at around midnight UTC on May 17, the hurricane's 115 mph (185 km/h) winds were partnered with a barometric pressure of 982 mbar – a value which is typical of a strong Category 1 hurricane. After reaching its peak intensity, Alan weakened even faster than it intensified, dropping to tropical storm strength in only 18 hours, and weakening to a tropical depression 12 hours after that. At around midday on May 18, Alan had lost enough convection and organization to be declassified as a tropical cyclone, and its remnants dissipated completely later that day.
Alan was a meteorological enigma. Throughout its life, Alan was consistently forecast to weaken and dissipate quickly. When Alan became a major hurricane early on May 17, forecasters responded in alarm; forecaster Ivan Montague Unstable went so far as to post "What the f*ck" as the lead-in for the advisory, resulting in his termination from the NHC. A more level-headed forecaster expressed surprise at Alan's abnormally high barometric pressure, commenting that "while it is not unusual for tropical cyclones to have above average minimum central pressures, the concept of a major hurricane with the minimum central pressure of a Category 1 storm is obscene; even the most extreme cases, the minimum central pressures of such storms have never been more than 10–15 mbar above average."
Unstable's family – furious with his firing – demanded that the name Alan was retired the following spring. The request was denied; the response to the Unstable family was a letter whose sole contents were the word "NO." in large, red, capital letters.
An early-season Cape Verde-type storm, Brittany was forecast to become a Category 2 hurricane (with one model taking the storm to Category 4 intensity), but did not fulfill these forecasts due to vertical shear and dry air, instead peaking as a moderate tropical storm. As it never approached land, there were no reports of damage or fatalities due to Brittany.