page_title: Storm history of Hurricane Ivan
section_title: Formation and intensification
hierarchical_section_title: Storm history of Hurricane Ivan / Formation and intensification
caption_attribution_description: English: Tropical storm Ivan in the eastern Atlantic
context_page_description: The meteorological history of Hurricane Ivan, the longest tracked tropical cyclone of the 2004 Atlantic hurricane season, lasted from late August through late September. The hurricane developed from a tropical wave that moved off the coast of Africa on August 31. Moving westward from the effects of a ridge, favorable conditions allowed it to develop into Tropical Depression Nine on September 2 in the deep tropical Atlantic Ocean. The cyclone slowly intensified until September 5, when it underwent rapid deepening and reached Category 4 status on the Saffir-Simpson Hurricane Scale; at the time Ivan was the southernmost major hurricane on record.
  Ivan quickly weakened because of dry air, but it slowly reorganized itself, passing just south of Grenada as a major hurricane on September 7. The hurricane strengthened into a Category 5 hurricane while in the central Caribbean Sea. Over next the days its intensity changed largely due to eyewall replacement cycles, and Ivan passed just south of Jamaica, the Cayman Islands, and western Cuba with winds at or slightly below Category 5 status.
context_section_description: On August 31, a large tropical wave moved off the west coast of Africa. A tropical system along the wave axis contained a low pressure area as well as an impressive outflow pattern, though originally its convection was disorganized and limited. By September 1 a cyclonic circulation with a diameter of 690 mile (1115 km) was seen on satellite imagery, well to the southeast of the Cape Verde Islands, and several hurricane forecast models expected development and strengthening. As it moved quickly westward, the convection organized and developed into rainbands–bands of showers and thunderstorms that move in a spiral direction towards the center of the storm–and late on September 1 meteorologists began tracking the system using the Dvorak technique. Low amounts of wind shear and favorable outflow allowed strong deep convection to develop and persist near the center, and by 1800 UTC on September 2 the system developed into Tropical Depression Nine about 450 miles (730 km) southwest of Praia, Cape Verde.
  After being classified as a tropical cyclone, the depression was embedded within a deep easterly steering current provided by a ridge to its north. Expected to move across sea surface temperatures greater than 82 °F (28 °C), the cyclone was forecasted to slowly strengthen and within four days become a hurricane; the Geophysical Fluid Dynamics Laboratory predicted the depression to reach Category 4 status on the Saffir-Simpson Hurricane Scale within three days. Increased northeasterly wind shear shifted the center to the northeastern edge of the deep convection, and despite the shear and its relatively low latitude of 9.7° N, the depression strengthened to attain tropical storm status early on September 3; upon reaching the intensity, the National Hurricane Center classified the system as Tropical Storm Ivan.
  Tropical Storm Ivan slowly became more organized as wind shear decreased, and its outflow expanded in all quadrants. Satellite imagery late on September 3 showed a well-defined curved band wrapping around much of the circulation. The next day, the deep convection temporarily became disorganzied, before reorganizing and developing an eye feature. Convection strengthened further as the feature transitioned into an eye, and Ivan became a hurricane at around 0600 UTC on September 5. After reaching hurricane status, Ivan began to rapidly intensify with continued favorable conditions, and in an 18 hour period the pressure dropped 39 mbar (1.15 inHg) as the winds increased by 60 mph (95 km/h); early on September 6 Ivan reached its first peak intensity of 135 mph (215 km/h) while about 825 miles (1330 km) east of the island of Tobago in the southern Lesser Antilles.
  While at major hurricane status, Ivan kept very strong convection in its core with a well-defined eye. Operationally, the probability for further rapid strengthening was considered nearly nine times the average of a normal hurricane. Accordingly, Ivan was forecasted to pass near Barbados with winds of about 150 mph (240 km/h). Shortly after becoming a Category 4 hurricane, the outer convection of the hurricane became ragged. Hurricane Hunters found a Saharan Air Layer in the northern portion of the eye, leaving the eyewall eroded which caused a marked decrease in winds; by late on September 6 Ivan weakened to winds of 105 mph (165 km/h). The inner eyewall died out as a 23 mile (37 km) outer eyewall became took over, and at the same time, the overall organization of the hurricane improved. Ivan then re-gained major hurricane status as it approached the Lesser Antilles, and at 2130 UTC on September 7 the cyclone passed 7 miles (11 km) south-southwest of the southern tip of Grenada, its closest approach to the island. At the time, the hurricane maintained an eye diameter of 12 miles (19 km), with the northern portion of the eyewall producing strong winds on the island.
caption: Tropical Storm Ivan after formation in the eastern Atlantic
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