Cyclone Alessia

Map plotting the track and the intensity of the storm, according to the Saffir–Simpson scale

On 20 November 2013, an area of low pressure, accompanied by persistent deep convection, developed over the southern Indian Ocean, roughly 1,465 km (910 mi) north of RAAF Learmonth in Western Australia. At the mid-levels, the circulation appeared more broad in nature, with notable rainbands wrapping into the low. Environmental conditions favored tropical cyclogenesis, with moderate wind shear, high sea surface temperatures, and prominent outflow that extended into the mid-latitude westerlies.[1] By this time, the Bureau of Meteorology (BOM) noted this system as a tropical low just outside their area of responsibility.[2] Owing to increasing organization over the following day, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert for the system late on 21 November.[3][nb 3] Subsequently, the JTWC began issuing advisories on the system under the identifier Tropical Cyclone 02S on 22 November, estimating it to have attained gale-force winds. Situated along the edge of a near-equatorial ridge, the storm tracked generally east and was forecast to maintain this movement for much of its existence.[5] Later on 21 November, the BOM classified the system as a Category 1 tropical cyclone on the Australian cyclone intensity scale and assigned it the name Alessia, at which time it was located 390 km (240 mi) north of Broome. A smaller-than-average cyclone, Alessia featured a core of deep convection but no notable banding features, making Dvorak satellite intensity estimates difficult.[6][7]

The storm, having a maximum diameter of no more than 335 km (210 mi) with gale-force winds covering less than half that distance, maintained a very small central dense overcast as it approached the Kimberley region.[8] Around 0600 UTC on 23 November, the JTWC estimated Alessia to have attained one-minute sustained winds of 85 km/h (50 mph).[9] Throughout 23 November, a slight northerly component took place in the track, with Alessia remaining offshore as it brushed the Kimberley region. However, dry air from over Australia was drawn into the system, causing a dramatic decrease in convection.[10] At this time, Alessia became the first storm since Tropical Cyclone Anika in 2008 to affect Western Australia during November.[6] Later that day, the storm passed over Troughton Island where gale-force winds were measured.[6] Moving over the Joseph Bonaparte Gulf at the southern end of the Timor Sea early on 24 November, a burst in convection over Alessia's center indicated it was maintaining its intensity as a minimal tropical cyclone.[11] Continued reorganization of the cyclone's structure took place as it accelerated east toward the Top End region, with defined banding apparent on satellite imagary and a tight circulation center seen on local radar.[12] Alessia made landfall at 0930 UTC on 24 November near the Daly River Mouth, south of Darwin, Northern Territory, before rapidly weakening to a tropical low. Alessia's movement onshore marked the first time since Cyclone Joan in 1975 that a tropical cyclone affected the Northern Territory during November.[6] In light of the system moving overland, the JTWC issues their final advisory on the cyclone.[13] Moving steadily eastward across Top End, Alessia remained weak but its proximity to water allowed significant bursts of convection to develop during this time.[14]

Early on 26 November, the former cyclone emerged over the Gulf of Carpentaria near Blue Mud Bay. Once offshore, a break in the upper-level ridge previously steering the storm eastward caused a southward shift in the track, bringing it directly over Groote Eylandt for several hours.[6][15] Gale-force winds soon redeveloped over the southeastern quadrant of Alessia; however, it was not re-designated as a tropical cyclone until 27 November when the winds extended halfway around the circulation.[16] As Alessia re-intensified over the southwestern Gulf, its movement became more southeasterly, taking the circulation parallel to the coast.[15] With an anticyclone over the Northern Territory providing modest outflow for the cyclone, it continued to intensify; the JTWC also re-initiated advisories on the storm by 0600 UTC on 27 November.[17] Hours later, Alessia reached its peak strength with winds of 85 km/h (50 mph) and a barometric pressure estimated at 991 mbar (hPa; 29.27 inHg).[6] Simultaneously, it moved ashore near Wollogorang, roughly 130 km (80 mi) east of Borroloola. Increasing shear and land interaction caused the low- and mid-level circulations to separate, with the later continuing eastward over water.[18] The storm soon weakened to a tropical low and its movement became erratic.[6] Its remnants persisted for several days in the same general region, executing a clockwise loop before moving back over the Gulf of Carpentaria on 28 November. While environmental conditions favored redevelopment once again and deep convection had formed over the system, its low-level circulation remained poorly organized.[14] Failing to regenerate, Alessia moved eastward before doubling back to the west, passing near the Sir Edward Pellew Group of Islands on 30 November.[19] It was last noted on 1 December over the Top End region.[20][21]

• 2013–14 Australian region cyclone season
• Cyclone Helen (2008)