When must I stop practice because of thunder, and where should players shelter?

Stop as soon as you can hear thunder. Environment and Climate Change Canada says: "If you can hear thunder, you are within striking distance of lightning."

• Go to an enclosed building with wiring and plumbing, or an all-metal vehicle (convertible tops do not count).
• Lightning can strike more than 40 km from the storm clouds, so keep an eye on the weather at all times.
• Terrain and buildings can block the sound of thunder, so not hearing thunder does not mean you are safe.