From Coaches News 2018:
If thunderstorms are forecasted in any likelihood/ percentage, even if the sky is clear, practice as close to a shelter as you can so you can evacuate your team in case a storm develops or moves-in.
A shelter suitable for a thunderstorm is a building with roof and walls that is also grounded or an enclosed vehicle with all windows and doors shut.
Ensure all coaches are familiar with your Team Practice Venue Emergency Action Plan (now includes water stations, shelters and cellular data for lightning trackers)
If you have a consistent cellular data connection at your practice area, use My Lightning Tracker or Weatherbug cellular apps to alert you to nearby lightning
It is considered safe to resume/start practice after 30 minutes has passed since the last clap of thunder OR lightning is no longer detected within 10 miles of the practice site.