Transition of Hurricane Alex

By Nigel Emery, Jason Bradley, Ian Mills (Met Office), James L. Franklin (National Hurricane Center, NOAA) and Jochen Kerkmann (EUMETSAT)

Tropical Depression 'One' formed on 31 July 2004 in the western Atlantic. Moving slowly in a north-westerly direction and deepening, it became a Tropical Storm on the 1 August 2005. Turning to the north-east and intensifying further, it achieved hurricane status on the 3rd (with the name Alex), brushing the north-eastern United States at Cape Hatteras. 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 00:00 UTC on 5 August, with winds of 105kts and a minimum pressure of 957hPa.

At this time Alex was at 38.5° N (690 km south-southwest of Halifax, Nova Scotia), moving east-northeastward at 20–25kts, 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. Moving at 40–45kts, Alex weakened to a tropical storm after 06:00 UTC on 6 August and became extratropical a few hours later.

Tropical phase, 5 August 2004

Of the average 40% of tropical cyclones in the Atlantic Basin that undergo Extratropical Transition (ET) about half develop explosively into extra-tropical depressions. Hurricane Alex is a good example for such a development. During the transformation phase on 6 August 2004 the structure of Alex is at it's weakest and all characteristics of a tropical cyclone have disappeared (see images below). On the RGB images the associated cloud is becoming more fragmented and the tops are becoming warmer.

Also, due to the increasing influence of the 'westerlies' with a strong vertical wind shear, the high-level clouds drift away in a south-easterly direction while the low-level clouds (and the warm/moist air in the lower layers) move slowly in a north-easterly direction. This brings the low-level remnants of hurricane Alex (that have the structure of a so-called cloud-head, or baroclinic leaf) closer to an upper-level jet, which is visible in the right image by the reddish colour (see interpretation below the image). At this time all the elements are in place for explosive deepening: 1) an upper-level trough, associated with a pre-existing weak system to the north; 2) a baroclinic zone; 3) a jet stream.

Transformation phase, 6 August 2004

On 7 August 2005, Alex strengthened significantly and re-curved northwards towards Ireland (see images below). At this stage, it has all the appearance of a young, strongly developing winter-type cyclone with a marked warm conveyor belt and a large shield of warm frontal cloudiness. What is however unsual is the fact that this severe development takes place in the middle of the summer!

The red stripe to the rear side of the cyclone (see right image below) indicates that vorticity is being advected above the cyclone and that further deepening will take place. Indeed, in the next days ex-hurricane Alex brought a prolonged spell of unsettled weather and associated flooding to Ireland and the UK. The heavy and prolonged rain was linked to the slow moving occlusion and the tropical origin of the air. Wittering, for example, recorded 90.8 mm of rain in the 24 hours up to 09:00 hrs on the 10th and Pembrey Sands had 44.2mm in three hours on the 12th.

Re-intensification phase, 7 August 2004

Oceanographical aspects

The occurrence of heavy seas and large swell through the autumn, winter and early spring seasons is a regular feature of mid-latitude weather, well documented and readily appreciated. What can make the transition of a tropical cyclone important is that energy levels associated with these 'winter' conditions can occasionally, and suddenly, occur in the late summer. They typically occur during a spell of relatively quiet weather and often after a lengthy period of low or even flat sea states.

The holiday season is in full swing, coastal populations both on land and at sea are high, beaches are crowded but hazard awareness is often low. The acceleration of a transitioning tropical cyclone can allow the accompanying wind field to move and increase at a speed in phase with the growth of the ocean waves it is generating, variously called dynamic, trapped or phase linked fetch.

Swell generally travels along the shortest route between two points on the Earth's surface; great circles. The track of transitioning Atlantic tropical cyclones can occasionally follow a great circle for some distance. These are often the same great circles that radiate out from the coast of western Europe into the transitioning zone, which is one of the main reasons dynamic fetch waves affect this coastline.

Lundy wave buoy data

The Lundy wave diagrams (see Wave Height and Wave Period. Source: Met Office) graphically indicate how quickly the swell front from Alex arrived, in this case recorded in deep water at the Lundy wave buoy off the west coast of Devon. The actual wave heights seen on Fistral Beach, Newquay increased from approx. 0.5m to 1.5m between 16:00 and 18:00 UTC.

While the measured deep water wave heights from the Lundy buoy indicate only a slight increase in size at this time, the period increased from a modest nine seconds to a far more energetic 15 seconds, with the result that wave heights on the beach increased by a factor of three over the same time scale. This sort of increase can rapidly generate rip-currents, cause difficulties of exit from enclosed beaches, especially if it coincides with an incoming tide, and can pose a threat to pleasure craft or divers as the swell begins to break over formally 'hidden' reefs. More dramatic instances than that of Alex have certainly occurred. The swell from ex-hurricane Irene in 1998 increased from 1m to 4m within an hour, resulting in an air sea rescue operation in Cornwall. Fatalities around the UK coast could be linked to the arrival of 'swell fronts'.

Additional content

RGB WV6.2-WV7.3, IR9.7-IR10.8, WV6.2 in Mercator projection
Channel 05 (WV6.2) in Mercator projection
IR image with absolute topography 500 hPa (Source: Met Office)
IR image with isotachs 300 hPa (Source: Met Office)
IR image with absolute topography 500 hPa (Source: ZAMG)
IR image with isotachs 300 hPa (Source: ZAMG)