|Above: Tropical Cyclone Idai as seen by the Sentinel-3 satellite on Thursday afternoon, March 14, 2019. Image credit: Simon Proud.|
Extremely dangerous Tropical Cyclone Idai made landfall approximately 20 miles northeast of Beira, Mozambique on Thursday near 23 UTC (6 pm EDT). The final advisory before landfall (18Z Thursday) issued by the Joint Typhoon Warning Center (JTWC) had Idai as a Category 2 storm with 110 mph winds (1-minute average), making it the fourth strongest tropical cyclone on record to hit Mozambique. The agency issuing the official forecasts for the South Indian Ocean, RSMC-La Reunion, put Idai’s winds at 105 mph (10-minute average) at 18Z Thursday, which would roughly equate to a low-end Category 3 storm with 115 mph winds (1-minute average).
As Idai approached the coast, the stronger left eyewall of the storm moved over Beira at the time of high tide, potentially driving a catastrophic storm surge up to 13 feet above the high tide level into the city. Beira (population 530,000) is Mozambique’s fourth largest city, and second largest port. The city is very low-lying, with portions lying below sea level.
Idai will bring highly destructive winds and heavy rains of 1 - 2 feet as it pushes west at about 7 mph towards Zimbabwe through Friday night. Idai will likely die on Friday night over the high terrain along the Mozambique-Zimbabwe border, but its remnants will generate very heavy rain through the weekend. Idai’s heaviest rains may exceed two feet in the higher terrain along the Mozambique-Zimbabwe border, according to recent runs of the HWRF model. Idai's rains will likely cause catastrophic flooding in some regions.
Idai already the deadliest weather disaster of 2019, with 122 deaths
The disturbance that developed into Idai originated in Mozambique earlier in March, when it caused flooding in northern parts of the country and in neighboring southern Malawi. Cuamba, Mozambique received 11.14” of rain in just 12 hours on March 7. At least 66 were killed by flooding in Mozambique and another 56 lost their lives in Malawi. This makes Idai the deadliest weather disaster so far in 2019, ahead of the 80 people killed in January flooding in Indonesia, according to statistics from insurance broker Aon. Idai's floods have displaced 100,000 people and injured 688.
|Figure 1. Predicted surface winds (colors) at 21Z Thursday (5 pm EDT) March 14, 2019, from the 12Z Thursday run of the HWRF model. The model predicted that Tropical Cyclone Idai would be a Category 2 storm with 105 mph winds, making landfall near Beira, Mozambique. The clockwise flow of air around Idai brought a worst-case storm surge at high tide into the funnel-shaped bay on the west side of Beira, thanks to the strong southerly winds in the west eyewall of the storm. Image credit: tropicaltidbits.com.|
A storm tide of up to 13 feet above high tide may have occurred
Fortunately, Idai hit during some of the lowest tides of the month. In locations where Idai’s storm surge was 7 feet, the peak water levels would have been near what the normal high tide will bring on March 20, due to the phase of the moon. Anything over a 7-foot storm surge would have been trouble, and RSMC-La Reunion warned with their 18Z Thursday advisory that Beira could expect a storm surge of up to 13 feet (4 meters), with a much higher storm surge over 20 feet (6 meters) at the mouth of the Pungwe River, which forms the western boundary of the city. The bay that the river flows into is funnel-shaped and shallow, which very likely allowed a huge storm surge to build up, since the dangerous southern portion of the eyewall moved over it for several hours. The maximum storm tide (height of the water above the normal high tide mark) could have been as high as 13 feet (4 meters) on the western side of Beira, based on the 18Z forecast. A storm surge of this magnitude could have been catastrophic with high loss of life, if evacuations were not performed.
According to Chemane et al., 1997, Vulnerability of coastal resources to climate changes in Mozambique: a call for integrated coastal zone management, Beira is the most vulnerable city in the country to sea level rise and storm surges. A sea level rise of 1 meter will flood 40% of the town (42.5 square km), including the port and the airport. Beira’s port is the second busiest in the nation, and has long been a major trade point for exports coming in and out of Zimbabwe, Malawi, Zambia and other Southern African nations. According to a January article in Aljazeera.com, Beira is steadily sinking, and portions of the city now lie below sea level.
An active South Indian Ocean tropical cyclone season for major storms
According to the CSU Tropical Meteorology Project, the South Indian Ocean has seen 11 named storms, 8 hurricanes, and 7 major hurricanes during the 2018 – 2019 tropical cyclone season. An average season has 13 named storms, 7 hurricanes, and 3 major hurricanes by this point in the season. The most major hurricanes on record in one season is 9, set in 1980.
|Figure 2. Tracks of the three major hurricanes that have hit Mozambique from NOAA’s historical hurricanes database—Eline, Favio, and Jokwe—along with the tracks of the nation’s deadliest tropical cyclone (Nadia, 240 killed) and their most damaging storm (Domoina, $185 million in damage).|
Mozambique tropical cyclone history
NOAA’s historical hurricanes database lists ten tropical cyclones of hurricane strength and ten tropical storms that have hit Mozambique since 1934. Many storms that hit before the advent of reliable satellite imagery in the Indian Ocean in the early 1990s were likely missed, though. The strongest landfalling storm was Tropical Cyclone Eline, which hit southern Mozambique as a Category 4 storm with 130 mph winds on February 22, 2000. The only other major hurricanes to hit were Favio of 2006 (Category 3, 115 mph winds) and Jokwe of 2008 (Category 3, 115 mph winds). The last tropical cyclone to make landfall in Mozambique as a Category 1 or stronger storm was Dineo in mid-February 2017.
Highly destructive tropical cyclones are rare in Mozambique. According to EM-DAT, the international disaster database, the country’s most expensive storm was Tropical Cyclone Domoina, which made landfall in southern Mozambique near Inhambane as a tropical storm with 65 mph winds on January 28, 1984. Domoina killed 109 people and did $185 million (2019 dollars) in damage. Mozambique’s deadliest storm was Tropical Cyclone Nadia, which made landfall in northern Mozambique on March 24, 1994, as a Category 1 storm with 85 mph winds. Nadia killed 240 people.