Origins and track through the Lesser Antilles

Hurricane Dorian originated from a large tropical wave—an elongated trough of low air pressure—that departed from the western coast of Africa on August 19, 2019. ^[1] Around that time, much of the wave's convection or thunderstorm activity was located inland near Guinea and Senegal rather than close to its center. Convection in the northern portion of the wave was limited by an abundance of Saharan dust in the region. ^[2] As the wave traveled westward across the low latitudes of the Atlantic, it lost most of its convection before developing into a low-pressure area on August 22. Despite being located in an area of moderate Wind shear, the system continued to increase in organization. ^[1] The National Hurricane Center (NHC) initially predicted slow development on August 23 as the system continued westward at 15 mph (24 km/h), ^[3] however, it quickly organized into a tropical depression at 06:00 UTC on August 24, while approximately 805 mi (1,295 km) east-southeast of Barbados. ^[1] Early in the depression's existence, its southeastern outflow was restricted due to moderate easterly vertical wind shear. ^[4] The system was upgraded into Tropical Storm Dorian at 18:00 UTC after developing a 10 mi (15 km) wide eye-like feature at its mid-levels and banding features that wrapped around it. ^{[5] [1]} Despite being located in an environment of warm sea surface temperatures, Dorian only gradually strengthened over the next few days as a result of wind shear as well as abundant mid-level dry air. ^{[6] [1]} Initially, the tropical storm mainly produced short lived bursts of convection near its center, ^[6] however, Dorian's convective and internal structures became more organized as the wind shear relaxed on August 25. ^{[7] [8]} The tropical storm developed a central dense overcast as a result, however, a United States Air Force reconnaissance aircraft found Dorian to be quite disorganized by the time it arrived at the Windward Islands. ^[1] The tropical cyclone continued westward under the influence of a subtropical ridge to its north. ^[9] It made landfall over Barbados around 01:30 UTC on August 27 with maximum sustained winds of 50 mph (85 km/h), entering the Caribbean Sea. ^[1] At that time, composite radar showed the system lacked a strong inner core. ^[10] The tropical storm made its second landfall in St. Lucia around 11:00 UTC at the same intensity; the mountainous terrain of the island disrupted the low-levels of the system. ^[1] Soon after, Dorian's center re-developed farther north and convection fluctuated as the storm continued to be battered by dry air and interacted with the Leeward and Windward islands. Dorian briefly developed a 10 mi (15 km) wide eye on radar on a couple occasions, but it quickly eroded due to the storm's instability. Despite the dry air, the system's upper-level outflow had expanded and become more symmetrical. ^[11]

Although Dorian was initially forecast to make landfall in Hispaniola and subsequently weaken or dissipate over the island, the system's track shifted considerably eastward by August 28. ^{[12] [7]} After the aforementioned center reformation, the track was shifted due to Dorian's more northward position. The new forecast predicted landfall in Puerto Rico as well as a stronger Dorian, ^[11] however, as the storm took on a more northwesterly direction as a result of a weakness in the ridge caused by a mid- to upper-level low located to the north of Hispaniola, the track shifted to the eastern edge of Puerto Rico. ^[12] Dorian's structure began to improve on August 28, with banding features becoming more prominent and a partial eyewall forming. ^{[1] [13]} Dorian made landfall over St. Croix in the United States Virgin Islands at 15:30 UTC as it simultaneously strengthened into a Category 1 hurricane on the Saffir-Simpson scale. The cyclone made another landfall just a few hours later, at 18:00 UTC, over St. Thomas with sustained winds of 80 mph (130 km/h). Meanwhile, the cloud pattern of Dorian was continuing to increase in organization, with an eye becoming apparent on satellite imagery. The hurricane progressed northwestward under the influence of flow between an upper-level low over the Straits of Florida and the Atlantic subtropical ridge. ^[1] Additional strengthening was forecast as the storm would remain in a favorable environment with an increasingly moist mid-level, sea temperatures near or over 84 °F (29 °C), and low wind shear. Even though the storm had strengthened and developed a more defined eye, ^[14] some dry air penetrated the eyewall early on August 29, the result of which was a cloud- and rain-filled eye. Compounding on the effects of the dry air, wind shear temporarily increased to 10–15 mph (16–24 km/h), ^[15] before relaxing once again. Later in the day, a NOAA Hurricane Hunter aircraft indicated that concentric eyewalls were present, indicating that an eyewall replacement cycle had commenced. Despite this, the aircraft discovered that the central pressure had fallen, but did not find any stronger winds as Dorian continued to track towards the northwest. ^[16]