11.   The trees must have sturdy branches and must not be prone to shedding their leaves, as this will impede airport operations.

12.   They could not strike or impede military operations.

13.   Bad weather, including snow, is forecast until Sunday, which could impede the operation, which had been scheduled to conclude this weekend, the source said.

14.   Corn and soybeans were boosted slightly by forecasted rain in growing regions which could impede harvest operations.