You can choose to have your Watch awaken to the app that was last open by opening the Watch's Settings and tapping GENERAL > ACTIVATE ON WRIST RAISE. Make sure that WRIST RAISE is turned on, then scroll down and select LAST USED APP. This will cause ForeFlight to automatically open when you raise your wrist if it was open when the Watch went to sleep.
Image 1. LAST USED APP in SETTINGS > GENERAL > ACTIVATE ON WRIST RAISE
You can also activate this setting from your iPhone: tap the My Watch app, go to GENERAL > ACTIVATE ON WRIST RAISE, then choose RESUME PREVIOUS ACTIVITY.
Image 2. RESUME PREVIOUS ACTIVITY in My Watch app > GENERAL > ACTIVATE ON WRIST RAISE