Analysis and Prediction of High-Impact Weather over Lake Victoria in East Africa

The Lake Victoria Basin (LVB) has one of the highest frequencies of thunderstorms and lightning occurrence in the world. Numerous human fatalities, estimated to range from 3000-5000 annually, occur over the lake due to the severe weather. The strong downdrafts and outflows associated with these thunderstorms are believed to spread out over the water and produce high wave heights that capsize fishermen boats and ferry boats that transport hundreds of people across the water. The HIGH impact Weather lAke sYstem (HIGHWAY) Project is a three-year project funded by the United Kingdom Department for International Development (DFID), managed by the World Meteorological Organization (WMO), with participants from the East African Meteorological Services, NCAR and the UK Met Office (UKMO). The goals of HIGHWAY are to a) gather all available observations over LVB during the HIGHWAY Field Campaign run from March to December 2019 which includes the rainy and dry seasons, b) increase forecast tools for weather forecasters, and c) use enhanced observations, nowcasts and NWP products in the development of a Regional Early Warning System for LVB.

The Meteosat-11 satellite routinely scans the East African region and the imagery is available to the National Meteorological and Hydrological Services (NMHSs) that are responsible for providing forecasts and warnings for the lake. The NMHSs install and maintain Automated Weather Stations (AWSs) in LVB but at a coarse spatial resolution that is insufficient to resolve the land and lake breezes that initiate convection along the coastal regions. Currently there are no upper air stations running operationally, but these stations are undergoing repair as part of the HIGHWAY project. High-resolution total lightning data are available at a cost from private sector companies but are being shared free of charge with HIGHWAY participants during the Field Campaign phase of the project. The Tanzania Meteorological Agency’s S-band dual-polarimetric radar located in Mwanza along the south shore of the lake provides the highest resolution information on the thunderstorms and winds over the lake and is being used extensively to understand storm intensity and evolution. All of the NMHSs run locally or receive internationally different types of NWP models and products for their operational needs. In this presentation we will focus on the UKMO 4.4 km Tropical Africa model that is being run specifically for the HIGHWAY project.

Using radar, lightning and satellite observations, there are distinct patterns of thunderstorm evolution over the LVB. Local topography largely control the daily evolution of storms in the LVB. Afternoon and evening thunderstorms occur over the land as a result of lake breezes and diurnal heating over the slopes of the higher terrain surrounding the lake. Thunderstorms occur over the lake during the night and early morning because of air from the land converging over the relatively warm lake. Gust fronts from these storms over the water trigger additional convection. Existing thunderstorms will often track westward with the steering level winds, while new convection initiation continues to occur southward over the lake in concert with the propagation of storm gust fronts. In this presentation we will show examples of the high impact weather that occurs over the lake and compare these observations with the UKMO 4.4 km Tropical Africa model precipitation and wind forecasts.