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

Abstract

The 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.

Changes in Extreme Daily Rainfall Characteristics in South Africa: 1921 – 2020

“…Contingency tables have been used in various meteorological studies (Ihara et al, 2007;Fowler et al, 2010;Maldonado et al, 2013;Mittermaier et al, 2022). Therefore, as an initial approach, this method was used to examine individual stations' data with respect to changes in the general distribution of daily rainfall intensity.…”

Convective‐scale numerical weather prediction and warnings over Lake Victoria: Part I—Evaluating a lightning diagnostic

“…In this study, the forecasts initialized at 18 UTC were evaluated. The full forecast is 54 h long, but in line with part 2 of this study, the verification is focused on the definition of 'Day 1', which spans the 24 h period between 06 and 06 UTC, in line with the issuance of warnings (see companion paper Mittermaier et al, 2021), covered by forecast ranges of t + 13 h to t + 36 h. Forecasts were considered in two ways: hourly fields and as a 'maximum-in-the-day' field, extracting the maximum number of lightning flashes in each grid box within a 24-h period from the 24 underpinning hourly fields.…”

“…As stated earlier, the forecast itself is 54 h in length, initialized at 18 UTC, thus covering two full days. HIGHWAY and part 2 of this paper (see Mittermaier et al, 2021) are primarily interested in the lead times between t + 13 h and t + 36 h. This is the period that corresponds with the warnings that are issued, which are valid for 24 h from 06 UTC to 06 UTC. Although the focus of the verification statistics is also for the t + 13 h to t + 36 h period, it is interesting to see how the model behaves before and after this time to understand aspects of forecast conservation, for example.…”

“…A typical airmass storm will have IC: CG or z-ratio (Prentice & Mackerras, 1977) of about 4:1. Having an awareness of the high proportion of CG lightning strokes is important for the interpretation of model output to produce severe weather warnings in the region, a topic that is explored in Mittermaier et al (2021). Looking further to the future, Meteosat Third Generation (MTG) will carry a new Lightning Imager (LI) covering the whole of Africa.…”

“…The model resolution is 0.04° (4.4 km) with the domain stretching from 26.5° E to 36.5° E and 5° S to 2.5° N providing a 250 × 187 grid squares. Note a map of the terrain is available in part 2 of this paper (Mittermaier et al, 2021, fig. 2)…”