Severe Tropical Cyclone Nora was a strong tropical cyclone that affected Far North Queensland and the northeastern Northern Territory, both in Australia, during March 2018. The ninth named storm and third severe tropical cyclone of the 2017–18 Australian region cyclone season, Nora developed from a tropical low which formed near the Torres Strait on 19 March. The system initially moved quickly to the west-northwest, and then began tracking slowly southwestwards over the Arafura Sea while gradually developing. A turn to the east on 22 March brought the tropical low into a favourable environment for strengthening, and the system reached tropical cyclone intensity later that day. Nora then underwent a period of rapid intensification as it moved southeastwards into the Gulf of Carpentaria. The storm peaked on 23 March as a high-end Category 3 severe tropical cyclone with sustained winds of and a minimum barometric pressure of 958 hPa (28.29 inHg). Nora made landfall north of Pormpuraaw at about 13:00 UTC on 24 March as a minimal Category 3 system. Nora weakened steadily as it tracked southwards along the coast, and was downgraded to a tropical low the following day. Nora's remnants meandered over land for several days before moving back over the Gulf of Carpentaria and dissipating on 28 March.
Nora was responsible for major impacts across large parts of Far North Queensland. Strong winds knocked down many trees and power lines in the towns of Pormpuraaw, Kowanyama and Mapoon, blocking roads and cutting power to more than 500 homes. Damage was caused to houses, council buildings, personal property and other buildings. Very heavy rainfall occurred across most of Cape York Peninsula and Far North Queensland. Many communities became isolated by floodwaters after roads were either blocked or damaged. Extreme rainfall fell in parts of Queensland's east coast, including in just 24 hours in Port Douglas. Flash flooding occurred in Cairns as a result of the intense rainfall, including in car parks, shopping centres and hotels. The torrential rain caused landslides that blocked highways in the region. Many people had to be rescued by emergency services after becoming trapped by floodwaters. Agricultural losses in crops and livestock, as well as damage to infrastructure, were also sustained when farms were flooded. The total economic impact of Severe Tropical Cyclone Nora is estimated to have exceeded AU$32.5 million (US$25 million).
Meteorological history
Several atmospheric features and climate drivers which are typically conducive to tropical cyclogenesis were present in Australian longitudes during March 2018. In early- to mid-March, a monsoon trough developed to the north of Australia, stretching from the eastern Indonesian archipelago to the northern Coral Sea. Although associated convective activity decreased somewhat as the monsoon trough began to weaken, this was offset by the interaction of significant cross-equatorial flow from the Northern Hemisphere with a large area of strong southeasterly wind from the Australian continent. The convergence of these winds provided ample rotational energy for the development of any low-pressure systems in the region. Reinforced by the effects of a westwards-propagating Rossby wave, the southeasterly wind flow contributed to enhanced atmospheric circulation over the tropical waters. In addition, the presence of a weak but discernible pulse of the Madden-Julian Oscillation in the Maritime Continent supported the generation of cloudy weather and increased atmospheric instability in the region.
Monsoonal activity began to restrengthen later in the month, and on 19 March the Australian Bureau of Meteorology (BOM) noted that a tropical low had developed in the vicinity of the Torres Strait, embedded within the monsoon trough. The newly formed tropical low moved quickly to the west-northwest along the southwestern coast of New Guinea, and was centred just off the coast of Indonesia's Pulau Dolak island by 00:00 UTC on 20 March. The system then began to track slowly southwestwards across the eastern Arafura Sea while its central barometric pressure steadily deepened. At 18:30 UTC, the Joint Typhoon Warning Center (JTWC) issued a tropical cyclone formation alert, and indicated that the likelihood of rapid intensification over the following days was high. Alt URL On 22 March, the tropical low assumed a course towards the east, bringing it into an environment which was increasingly favourable for further development. Fuelled by sea-surface temperatures of approximately 29 °C (84 °F), the low-level circulation centre quickly consolidated as convective banding wrapped into the developing system. Alt URL The JTWC indicated that the system had reached tropical storm strength on the Saffir-Simpson hurricane wind scale at 06:00 UTC, and gave it the designation Tropical Cyclone 16P. Six hours later, the system was officially upgraded to a Category 1 tropical cyclone and was assigned the name Nora by the BOM.
As Nora passed slowly to the north of the Wessel Islands in a weak steering environment, the already supportive environmental conditions became extremely favourable for strengthening. A co-located anticyclone in the upper troposphere provided excellent poleward and equatorward outflow channels for the system in an environment of low vertical wind shear, and sea surface temperatures and ocean heat content had both become favourably high. Alt URL Nora began to rapidly intensify, reaching Category 2 strength on the Australian scale at 00:00 UTC on 23 March as a primitive eye began to emerge on visible-light satellite imagery. Alt URL Shortly thereafter, Nora turned southeastwards under the influence of a strengthening high-pressure ridge to the east. Alt URL The BOM upgraded the storm to a Category 3 severe tropical cyclone at 09:00 UTC, just after one-minute sustained winds had reached the equivalent of minimal hurricane intensity. Over the following three hours, Nora underwent explosive intensification, with maximum ten-minute sustained winds increasing by an average of every hour. Nora peaked in intensity as it entered the Gulf of Carpentaria, reaching high-end Category 3 severe tropical cyclone status on the Australian scale. The BOM estimated ten-minute sustained winds to be at , gusting to , with a minimum central pressure of 960 hPa (28.35 inHg). Concurrently, the JTWC indicated that one-minute sustained winds had reached , equivalent to a Category 3 major hurricane on the Saffir-Simpson hurricane wind scale.
Shortly after reaching peak intensity, Nora began to undergo an eyewall replacement cycle, causing the intensification trend to cease. Despite the very warm waters in the Gulf of Carpentaria, diminishing upper-level outflow caused the system to begin weakening by 03:00 UTC on 24 March. Alt URL Drier air from Cape York Peninsula also became entrained into the circulation as Nora approached the coast, contributing to the storm's decline in intensity. Nora made landfall in Far North Queensland at about 13:00 UTC on 24 March as a low-end Category 3 system, approximately north of the town of Pormpuraaw. After moving ashore, the cyclone turned generally southwards, and tracked along the coastline of the Gulf Country. The system was downgraded to a tropical low by the BOM at 06:00 UTC the following day as it approached the mouth of the Gilbert River. The development of a competing steering environment associated with a strengthening ridge over central Australia caused Ex-Tropical Cyclone Nora to become quasi-stationary from 25 to 27 March. Alt URL The system's remnant low-pressure system re-emerged over the southern Gulf of Carpentaria on 28 March, but was unable to restrengthen. It dissipated near Mornington Island later that day.
Nora was the storm to make landfall on Queensland's Gulf of Carpentaria coast since Tropical Cyclone Abigail in 2001. It was also the strongest tropical cyclone in the Gulf of Carpentaria since Severe Tropical Cyclone Monica in 2006.
Preparations Tropical cyclones in the Gulf of Carpentaria can be relatively unpredictable in their motion, and are often difficult to forecast. As a result, tropical cyclone watches and warnings were issued for the entire Gulf coast of Queensland and the northeastern Top End when Nora intensified into a tropical cyclone. Early forecasts also predicted that Nora would stay offshore for much longer while tracking into the southeastern Gulf of Carpentaria. Forecasts indicated that the system could have strengthened into a powerful Category 4 severe tropical cyclone if this scenario were to have eventuated. Residents were warned that Nora had the potential to cause major impacts due to its predicted intensity, size and direction of motion. The BOM indicated that storm surge and large waves posed a potential threat to coastal communities in the path of the system, and that inundation of low-lying areas on the shoreline was possible. Residents were also warned that widespread heavy rainfall was anticipated, with isolated daily falls of up to possible in some areas.
Due to the potential for communities to become isolated by flooding, emergency electricity generators were installed near critical infrastructure and buildings such as schools, hospitals, accommodation facilities and some council buildings. Additional Queensland Police Service (QPS) officers and other emergency services personnel were also deployed to communities in Far North Queensland in order to assist with cyclone preparations and maintain safety during the cyclone. The Queensland Fire and Emergency Services (QFES) assembled several rapid response and swift water rescue teams in preparation for the cyclone. These teams were able to respond to calls for assistance from endangered residents by air within four hours, and were equipped to be self-sufficient for up to 48 hours if required. The local council in Croydon also installed retaining walls across roads in order to redirect the flow of floodwaters.
Despite the region being one of the most cyclone-prone areas of Australia, Mornington Island, located in the southeastern Gulf of Carpentaria, did not have a suitable cyclone shelter for residents to evacuate to. In the interest of safety, the local government offered an additional flight to Cairns for any residents who wished to evacuate. As a precaution, Queensland Health evacuated a total of 25 patients from medical facilities in Mornington Island, Pormpuraaw and Kowanyama, including pregnant women, dialysis patients and vulnerable elderly residents. An evacuation shelter was also prepared in Darwin, the Northern Territory's capital, to accommodate for potential evacuations from towns in the northeastern Top End. The Queensland Government's closed state schools in Pormpuraaw, Kowanyama, Karumba and Burketown prior to the arrival of the cyclone in order to protect students and teachers.
Impacts
Queensland
Nora was the first tropical cyclone to make landfall on Queensland's Gulf of Carpentaria coastline since Tropical Cyclone Oswald in 2013, and the first to do so at Category 3 or higher since Severe Tropical Cyclone Abigail in 2001. After making landfall, Nora tracked southwards along the coast, passing just to the east of Pormpuraaw at Category 3 intensity, and then west of Kowanyama as a Category 2 system. Neither town experienced the full strength of the cyclone; however, strong winds were still experienced in both communities. Kowanyama was subjected to sustained gale-force winds in the early morning of 25 March, and the local airport recorded a maximum wind gust of just before 18:00 UTC.