Convective‐scale numerical weather prediction and warnings over Lake Victoria: Part II—Can model output support severe weather warning decision‐making?

Abstract

AbstractThe greater Lake Victoria region was the focus of the ‘HIGHWeather impact lAke sYstem’ project, with the aim of developing an early warning capability for hazardous weather. Strong thunderstorm outflows can cause boats to capsize and lead to drowning. Lightning is also a hazard to fishermen, and over land flash flooding from heavy rainfall is a significant hazard. Between March and October 2019, 16 regional warnings were issued, and 14 days had reported impacts. Only one of the issued warnings aligned with any reported impacts. The issued warnings were verified using three different methodologies and found to have a very low frequency bias, confirming that too few warnings are being issued, especially for precipitation. The skill of the warnings based on the probability of detection and symmetric extremal dependency index was also found to be low. Potential warnings days are derived from individual and multivariate combinations of numerical weather prediction forecasts of mean wind speed, precipitation and a new lightning diagnostic from the 4.4 km Tropical Africa configuration of the Met Office Unified Model. The methodology identified 5–7 times the issued number of warnings, including 50% of days with reported impacts and 25% of days with issued warnings. Overall, 70%–80% of the identified potential warning days appear to be justified, suggesting that for this period not enough warnings were issued. It is shown that a daily aggregate of the lightning diagnostic is a skilful addition to any guidance supporting a decision‐making process.