Mozambique has been rarely struck by strong hurricane in the past, including Eline in 2000, when 350 people died and 650,000 were displaced across the wider region.
National water resources authorities alert that at least 120 000 people are at risk of flooding and heavy rains in the coming days.
Center of La Reunion, who monitors tropical cyclones in the Mozambique channel and southwest Indian ocean, warned that sea level rises up to 4m could be expected in Beira on Thursday night, while storm surge levels between 5 and 6m were possible in the Pungwe river mouth.
A major port city on the coast of southeast Africa is bracing for a direct hit from a powerful tropical cyclone - in a situation the World Meteorological Organization has called a "potential worst case scenario".
"It [Idai] is on its way to Beira, it's going to make landfall tonight", South African Weather Service forecaster Jan Vermeulen told News24 on Thursday.
Flooding from Idai's path inland has already killed 122 people in Mozambique and Malawi, making it the deadliest weather disaster this year. The deluge will also be in the eastern parts of Zimbabwe.
If a cyclone hits further north, it usually draws moisture toward it, leaving much dryer air over South Africa's northern parts.
The Mozambican government's emergency services have yet to give an update on the situation.
Over the ocean, the winds whipped up waves in excess of 10m, but these should reduce as the storm weakens.
South African officials are working with their counterparts in Mozambique through the National Joint Operations Centre to share information on the storm and its likely impact.
"Our intervention includes 70 personnel (aquatic rescue, rescue technicians, rescue divers, advance life support paramedics and doctors), 22 vehicles (4×4), 10 power boats, 4 jet skis, 14 inflatable, non-motorised two-man rescue boats, a range of rescue equipment and access to an eight-seater plane", the organisation's founder Imtiaz Sooliman said in a brief statement.
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