Cyclone Ann

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

Map key

Saffir–Simpson scale

  Tropical depression (≤38 mph, ≤62 km/h)
  Tropical storm (39–73 mph, 63–118 km/h)
  Category 1 (74–95 mph, 119–153 km/h)
  Category 2 (96–110 mph, 154–177 km/h)
  Category 3 (111–129 mph, 178–208 km/h)
  Category 4 (130–156 mph, 209–251 km/h)
  Category 5 (≥157 mph, ≥252 km/h)
  Unknown

Storm type

Tropical cyclone

Subtropical cyclone

Extratropical cyclone, remnant low, tropical disturbance, or monsoon depression

During late April and early May, a strong pulse of the Madden–Julian oscillation tracked eastwards across the tropical Indian Ocean and the Maritime Continent. Despite the traditional Australian wet season having concluded at the end of April, the pulse led to the redevelopment of monsoonal conditions to the north of Australia.[9] Additionally, the presence of the pulse led to environmental conditions which were more favourable for tropical cyclogenesis than normal for the cyclone off-season.[10] As the pulse continued into the Pacific Ocean,[11] the Australian Bureau of Meteorology (BOM) noted the formation of a weak tropical low embedded within a low-pressure trough on 7 May, located within the northwestern South Pacific cyclone region. The BOM assigned the system the identifier code 26U.[12]

The tropical low moved slowly towards the southwest while organisation gradually proceeded in a generally favourable environment. By 00:00 UTC on 8 May, the system was located just to the east of Honiara in the Solomon Islands.[13] After passing over the Solomon Islands, the developing low assumed a more southerly track under the influence of a mid-level high-pressure ridge located to the east.[2][12] Over the following few days, the tropical low skirted the boundary of the Australian cyclone region and South Pacific cyclone region, passing between the two basins three times in total.[14][15][16] By 10 May, disagreement emerged between meteorological agencies regarding the prospect of future development for the system. The BOM anticipated that further development into a tropical cyclone was very unlikely, citing an atmospheric environment that they expected to be unconducive for maintaining a significant tropical system.[17] Conversely, the United States' Joint Typhoon Warning Center (JTWC) analysed the environment to be supportive of tropical cyclogenesis overall, and forecast a gradual strengthening to tropical cyclone intensity over the following days, in accordance with numerical weather prediction model solutions.[18] Aided by warm sea surface temperatures and the favourable outflow provided by an anticyclone in the upper troposphere, deep convection continued to grow over the consolidating low-level circulation centre on 11 May.[19] The storm was upgraded to a Category 1 tropical cyclone on the Australian scale by the BOM at 18:00 UTC on 11 May, while located approximately 1,420 km (880 mi) east of Cairns.[2] Located just within the Australian region at this time, the newly formed cyclone was named Ann by the BOM. The JTWC indicated that Ann had become equivalent to a tropical storm on the Saffir-Simpson hurricane wind scale at this time.[20]

An eastwards-propagating trough in the upper troposphere began to erode the ridge which had been steering the strengthening system southwards. A new mid-level ridge to the south began to develop, causing Ann to accelerate west-northwestwards across the Coral Sea towards Queensland. Environmental conditions remained favourable for development, and the system quickly reached high-end Category 1 intensity at 00:00 UTC on 12 May.[21] Soon afterwards, however, competing environmental influences began to arrest the intensification trend. The cyclone was maintained at Category 1, although the BOM noted that this assessment may have been conservative, with the possibility that the system may have already reached marginal Category 2 intensity. Analysis of satellite imagery indicated that easterly vertical wind shear had begun to increase near the system, and that drier air had started to become entrained into the circulation from the southwest.[22][23][24] Ann reached peak intensity at 18:00 UTC on 12 May as a Category 2 tropical cyclone on the Australian scale, generating ten-minute sustained winds of 100 km/h (65 mph) and gusts to 140 km/h (85 mph), with a central barometric pressure of 990 hPa (29.23 inHg).[1] The JTWC reported that one-minute sustained winds reached an estimated 110 km/h (70 mph) at this time, equivalent to a strong tropical storm on the Saffir-Simpson hurricane wind scale.[3]

Ex-Tropical Cyclone Ann shortly before landfall near Lockhart River on 15 May, with the exposed low-level circulation centre clearly visible

Ann began to weaken on 13 May. Good poleward outflow provided by the trough had been mitigating the negative effects of other environmental conditions up until this point; however, this had now begun to diminish as the trough receded further eastwards. Dry air intrusion began to damage the storm, disrupting the central deep convection.[25][26] The BOM downgraded the cyclone to Category 1 at 06:00 UTC on 13 May as the central dense overcast shrunk to less than 20 nautical miles (37 km; 23 mi) in diameter.[27][28] The BOM once again noted the possibility of underestimating the true intensity of the cyclone due to the very small circulation.[29] The system's decay was evident on satellite imagery, which displayed the effects of dry air, marginal ocean heat content, and increasing wind shear from the previously favourable upper-level anticyclone, now located well to the east.[30][31] Gale-force winds soon became confined to the southwestern quadrant as the storm became devoid of central deep convection, leaving the low-level circulation centre exposed. With gales no longer extending more than halfway around the system, Ann was downgraded to a tropical low at 03:00 UTC on 14 May.[4][32][33] Located 375 km (233 mi) northeast of Cairns, Ex-Tropical Cyclone Ann continued quickly west-northwestwards towards the coast of Far North Queensland.[34] The tropical low made landfall near the town of Lockhart River on the east coast of Cape York Peninsula about 24 hours later, at 2:30 p.m. local time (04:30 UTC) on 15 May.[5]

The tropical low passed quickly over land and emerged over the northeastern Gulf of Carpentaria.[6] Despite the warm sea surface temperatures in the vicinity, unfavourable atmospheric conditions prevented reintensification to tropical cyclone strength.[20][35][36] Continuing on the same west-northwestwards track that it had maintained for the previous five days, the tropical low passed the northeastern coast of the Top End on 16 May while moving into the Arafura Sea. After persisting for several days as a remnant tropical low, Ex-Tropical Cyclone Ann eventually dissipated by 06:30 UTC on 18 May in the Banda Sea, while located near East Timor and Indonesia's southern Maluku Islands.[37][38][39]

• List of off-season Australian region tropical cyclones
• Severe Tropical Cyclone Rhonda (1997)—the strongest off-season tropical cyclone on record in the Australian region
• Severe Tropical Cyclone Donna (2017)—the strongest off-season tropical cyclone on record in the South Pacific region
• Tropical Cyclone Liua (2018)—the other off-season tropical cyclone in the 2018–19 South Pacific cyclone season
• Tropical Cyclone Penny (2018)—another storm that affected Lockhart River as a tropical low during the 2018–19 season
• Severe Tropical Cyclone Trevor (2019)—made landfall near Lockhart River as a high-end Category 3 severe tropical cyclone, during the 2018–19 season
• Extremely Severe Cyclonic Storm Fani—the MJO pulse which contributed to Ann's development also contributed to Fani's intensification

Wikimedia Commons has media related to Cyclone Ann.

• Australian Bureau of Meteorology
• Fiji Meteorological Service
• Joint Typhoon Warning Center