If thunder is heard or lightning is seen, all outdoor activity should be suspended immediately, and players, coaches, and spectators should seek safe shelter. Of course, if the forecast is for continued thunder/lightning, a simple cancellation of practice or game and leaving the field is the easiest and safest method to provide safety to the players, coaches and spectators. A game or practice should not resume until 30 minutes after the last clap of thunder or flash of lightning has been seen or heard. 

WYSA's recommendation is that if there is a severe weather advisory still in place later in the afternoon relating to thunder and lightning, and bad weather is already apparent, then practices or games should be cancelled by the team coach/manager as soon as possible.

Elaboration:

Immediate Suspension:

• When thunder is heard or lightning is seen, even if the storm appears distant, all outdoor activity, including games and practices, must be immediately suspended. 

Safe Shelter:

• Players, coaches, and spectators should be directed to a safe location, such as a building with wiring and plumbing, or a metal-roofed vehicle. 

30-Minute Rule:

• After the last flash of lightning or clap of thunder, a 30-minute waiting period is required before any outdoor activity can resume. This is because lightning can still strike even after the immediate thunderstorm has passed. 

Decision-Making:

• Ultimately, the referee or designated field official (coach or team manager) should have the final say on suspending or restarting practices or games due to lightning. 

"Flash-to-Bang" Rule:

• The "flash-to-bang" method (counting the seconds between a lightning flash and the thunder) can be used to estimate the distance of the lightning and assess the need to suspend activities. 

Blue Sky Warning:

• Blue sky and absence of rain do not mean that lightning is safe. Lightning can strike even outside of the rain shaft. 

Thunderstorms Return:

• Thunderstorms can reverse course and return over an area that they have just passed, so it's important to maintain a 30-minute waiting period after the final flash or clap. 

Canada Soccer has a further document outlining the above, and you can find this  here.