I see there is a chance of thunderstorms in the South of Joburg from 17:00 increasing in probability to 19:00
Maximum temperature expected to be 30 at 15:00
Also a chance of thunderstorms in the North of Joburg from 17:00 increasing in probability to 20:00 (so I assume moving up from the south and west)
Maximum temperature expected to be 32 at 15:00
There is a chance of thunderstorms in Pretoria from 17:00 - however the probability remains unchanged
Maximum temperature expected to be 33 at 15:00
From these forecasts I assume the storms are going to form in their normal places (Free State and NW Provence) and head north east towards Mpumalanga whilst crossing over JHB and Centurion possibly hitting Pretoria later in the evening
Current temperature trend for Johannesburg (Time in UTC)