A planned visit to another airfield can acquire surprising momentum. Friends are expecting you, lunch is booked and the aircraft is ready. A thunderstorm forecast is a good reason to slow that momentum and revisit the plan with current aviation information.
The rain is only part of the picture
The Bureau of Meteorology identifies wind shear, severe turbulence, hail, strong vertical air movements, lightning and reduced visibility among thunderstorm hazards. A storm’s gust front can extend some distance from the cloud. All thunderstorms can be hazardous to aviation, regardless of their size or apparent intensity.
That is why a patch that looks inviting in a photograph or on a screen is not, by itself, a sound basis for a route decision. This article deliberately offers no “safe gap” or universal stand-off distance: operational decisions belong with current official information, your training and the applicable procedures.
Ask the planning question early
Before committing to the day, identify which part of the itinerary could become difficult to change. Is there pressure to get the aircraft home? Will somebody rely on you for transport? Could a planned stop become an overnight stay?
Make a ground-transport or cancellation arrangement while it still feels unnecessary. Tell the people meeting you that the flight is conditional. Those conversations are easier over breakfast than when you are already running late.
Use a cancelled day well
If you stay on the ground, compare the forecast with later observations as a learning exercise. Bring your questions to an instructor: what was uncertain, what changed, and which information mattered most? Avoid judging the original decision solely by a later photograph of blue sky. The useful question is whether the decision made sense with the information available at the time.
Further reading: Bureau of Meteorology: Thunderstorms (PDF).