The yellow alert has been activated by the Royal Meteorological Institute throughout the territory, it is even temporarily orange on the side of the Province of Liège. See also this summary of this alert, province by province:
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Tracking the evolution of thunderstorms: short-term forecasts
The stormy episode that we are going to experience will therefore be locally and potentially violent. What can we expect?
A. Numerous lightning strikes
B. Sustained rain in a short period of time
C. Hail locally
D. Gusts of wind at the height of the storm
Watch thunderstorms in real time with this animation: