Characteristics of Extra Tropical Transition

1 August 2004

By 1800 UTC 31 July, when the low centre was located about 175 NM east of Jacksonville, the system had enough convective organization to be classified as a tropical depression.

As the depression approached a break in the subtropical ridge early on 1 August, its forward motion slowed, and the cyclone remained nearly stationary for the next day or so about 115 NM east-southeast of Savannah. The depression remained poorly organized initially, due to northeasterly shear and an environment characterized by subsidence and dry air. However, an upper-level trough was approaching from the west, and in advance of this trough the northeasterly flow over the cyclone began to relax. During this transition the depression was able to strengthen, and it became a tropical storm at 1800 UTC 1 August.

Alex began to move northeastward early on 2 August, taking a track that would slowly approach the coastline of the Carolinas over the next 36 hours. The northeasterly shear continued to diminish during the day as upper-level south westerlies approached. The deep convection, which had previously been confined to the southwest quadrant of the circulation, was now able to organize in bands to the east of the centre.

Alex strengthened, becoming a hurricane near 0600 UTC 3 August, when it was centered about 65 NM south-southeast of Cape Fear.

The hurricane's maximum sustained winds reached 85 KT (category 2 on the Saffir-Simpson Hurricane Scale) at 1200 UTC, and the minimum pressure fell to 972 mb at 1800 UTC. Alex made its closest approach to land near 1700 UTC, with its centre located about 9 NM southeast of Cape Hatteras, while the western eye wall of the hurricane raked the Outer Banks with sustained Category 1 hurricane force winds.

After passing the Outer Banks, Alex turned away from land and accelerated as it became embedded in a deep layer of west-southwesterly flow.

Alex strengthened and became a major hurricane (Category 3) at 0000 UTC 5 August, with winds of 105 KT and a minimum pressure of 957 mb. At this time Alex was at 38.5° N (385 NM south-southwest of Halifax, Nova Scotia), moving east-northeastward at 20-25 KT, and over waters just above 26°C - not factors normally associated with major hurricanes. Only Hurricane Ellen of 1973 attained major hurricane status farther north. While the basic environmental current surrounding Alex was low in shear, the cause of this unexpected strengthening remains unknown.

By late on 5 August Alex had moved north of the Gulf Stream over sub-20°C waters and was weakening rapidly.

6 August 2004

1200 UTC 6 August 2004

By midday Alex was moving briskly across the Atlantic at approximately 40 KT.