Extreme Rainfall and Flooding over Central Kenya Including Nairobi City during the Long-Rains Season 2018: Causes, Predictability, and Potential for Early Warning and Actions

Rutger Dankers,Abubakr Salih,Mary Kilavi,Maurine Ambani,Martin Todd,Richard Graham,Titley Helen,Joanne Robbins,Dave MacLeod

doi:10.3390/atmos9120472

Abstract

The Long-Rains wet season of March–May (MAM) over Kenya in 2018 was one of the wettest on record. This paper examines the nature, causes, impacts, and predictability of the rainfall events, and considers the implications for flood risk management. The exceptionally high monthly rainfall totals in March and April resulted from several multi-day heavy rainfall episodes, rather than from distinct extreme daily events. Three intra-seasonal rainfall events in particular resulted in extensive flooding with the loss of lives and livelihoods, a significant displacement of people, major disruption to essential services, and damage to infrastructure. The rainfall events appear to be associated with the combined effects of active Madden–Julian Oscillation (MJO) events in MJO phases 2–4, and at shorter timescales, tropical cyclone events over the southwest Indian Ocean. These combine to drive an anomalous westerly low-level circulation over Kenya and the surrounding region, which likely leads to moisture convergence and enhanced convection. We assessed how predictable such events over a range of forecast lead times. Long-lead seasonal forecast products for MAM 2018 showed little indication of an enhanced likelihood of heavy rain over most of Kenya, which is consistent with the low predictability of MAM Long-Rains at seasonal lead times. At shorter lead times of a few weeks, the seasonal and extended-range forecasts provided a clear signal of extreme rainfall, which is likely associated with skill in MJO prediction. Short lead weather forecasts from multiple models also highlighted enhanced risk. The flood response actions during the MAM 2018 events are reviewed. Implications of our results for forecasting and flood preparedness systems include: (i) Potential exists for the integration of sub-seasonal and short-term weather prediction to support flood risk management and preparedness action in Kenya, notwithstanding the particular challenge of forecasting at small scales. (ii) We suggest that forecasting agencies provide greater clarity on the difference in potentially useful forecast lead times between the two wet seasons in Kenya and East Africa. For the MAM Long-Rains, the utility of sub-seasonal to short-term forecasts should be emphasized; while at seasonal timescales, skill is currently low, and there is the challenge of exploiting new research identifying the primary drivers of variability. In contrast, greater seasonal predictability of the Short-Rains in the October–December season means that greater potential exists for early warning and preparedness over longer lead times. (iii) There is a need for well-developed and functional forecast-based action systems for heavy rain and flood risk management in Kenya, especially with the relatively short windows for anticipatory action during MAM.

Highlights

This paper aims to explain and quantify the nature, impacts, and large-scale climate drivers of the extreme rainfall over Kenya during March and April 2018, which led to widespread flooding across
It is clear that a major regional scale anomaly centered over Kenya was experienced during the study period
Three unusually intense intra-seasonal rainfall events were notable, with return periods estimated to be up about six to 20+ years, depending on the time period considered. These events resulted in extensive flooding with over 140 deaths, significant loss of livelihoods, the displacement of people, major disruption of essential services, and damage to infrastructure

Summary

Introduction

In the past few years, the region has experienced both significant drought and flood events. Notable recent flooding includes that during March–May 2018 over much of Kenya, which is the subject of this paper. Regarding flooding in particular, based on the Emergency Events Database (EM-DAT), in recent decades, Kenya experienced major flood events roughly every two years on average, typically affecting about 70,000 people per event [3]. Kenya recorded 17 major flood events between 1964–2004. Flood events with high impact occurred in 1961, 1997–1998, 2006, 2012, and 2018 [3]. The 1997–1998 event cost at least USD $870 million [4]—the equivalent of approximately 11% of the country’s gross domestic product (GDP) [5]—from damages to water systems, roads, communications, and buildings; the costs of treatment for waterborne diseases; and crop loss, which affected more than half a million people [6]

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