Late last week, torrential rain in Nairobi, Kenya, led to severe flooding. Heavy thunderstorms on Friday, in combination with poor drainage systems in parts of the city, led to at least eight flooding deaths and two deaths linked to electrocution, while more than 70 vehicles became trapped or stranded.
The Kenya meteorological department had issued a moderate to heavy rainfall warning for much of the country from Tuesday 3 March to Monday 9 March, with the heaviest rainfall expected between Wednesday and Saturday.
Areas in Nairobi such as Westlands, Dagoretti, Roysambu and Embakasi were expected to be hit by some of the heaviest downpours, with daily totals forecast to reach between 30mm and 70mm.
In reality, within the space of 24 hours, 6am on Friday to 6am on Saturday, 112mm of rain was recorded at a station in Nairobi, considerably more than estimated. For context, the average amount of rainfall recorded by this station for the whole of March is 92.2mm, meaning more than 120% of the average monthly rainfall fell within a single day.
Events such as this are rare but not unheard of. In May 2015, 189.1mm was recorded within 24 hours, marking this station’s all time daily rainfall record. March and April 2018 were also exceptionally wet months for this station, but rainfall was more spread out, rather than concentrated within a few days. However, in April 2018 alone, 495mm of rain was recorded, which is almost five times the monthly average.
Because Kenya is situated just south of the equator, it has a “bimodal seasonality”, meaning it has two distinct rainy periods in the year, rather than one. These seasons are driven by the migration of the intertropical convergence zone (ITCZ), a band of low pressure which encircles the Earth, as it follows the sun across the equator twice a year. As the ITCZ shifts northwards through the region between March and May, it triggers the “long rains” which is currently bringing the heavy downpours to the region. Then, between October and December, another wet period is triggered in Kenya by the southward migration of the ITCZ, resulting in what is known as the “short rains”.
