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

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 Atmosphere 2018, 9, 472 2 of 30 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.

The effect of westerlies on East African rainfall and the associated role of tropical cyclones and the Madden–Julian Oscillation

“…Kilavi et al . () report that “Over the Kenya‐core region and indeed Kenya as a whole, 2018 saw the wettest MAM season over the 119‐year record of the Global Precipitation Climatology Centre (GPCC) data, and the 118 years of Centennial Trends (CenTrends) data.”. Their figure c shows that this was in fact the case for the majority of the Lake Victoria Basin.…”

“…Kilavi et al . () have also shown that both these datasets perform well over Kenya during this period compared to gauge station measurements. The three periods where rain rates are anomalous compared to the climatology are: days 0–5 (P1), days 14–19 (P2), and days 43–54 (P3).…”

“…These three periods broadly match the three periods identified as high rainfall over Kenya by Kilavi et al . (). Enhanced rainfall at the beginning of the season during P1 and P2 coincide with the presence of cyclones near Madagascar (tropical cyclones Dumazile and Eliakim ).…”

“…As illustrated in Section 4 and described by Kilavi et al . () and Mwangi et al . (), there are large‐scale and remote features of the atmosphere that possibly generate, but at least coexist with, a westerly moisture flux over East Africa.…”

“…Following the data and methods sections (Sections 2 and 3), we use a case‐study of the 2018 long rains (Section 4) to provide context of the interacting influence of westerlies, the MJO and tropical cyclones on East African rainfall. As mentioned, the long rains of 2018 were particularly wet, especially considering the decline in the long rains over the past decades (Kilavi et al ., ; Mwangi et al ., ). The aim of analysing the case‐study here is to clearly demonstrate the potential role, and interactions, of westerlies within the season.…”